COVER - INTERVISIBILITY CALCULATION AND DISPLAY

MODULE COVER

Intervisibility Calculation and Display

Issue 5.2 - 22-February-1993
Copyright (C) 1989  Laser-Scan Ltd
Science Park, Milton Road, Cambridge, England CB4 4FY  tel: (0223) 420414
Document "COVER Reference"   Category "Reference"
Document Issue  1.0 D R Catlow  04-November-1985
                2.0     D R Catlow  14-August-1986
  3.0     D R Catlow              18-March-1987
  4.0 D R Catlow  23-November-1987
  5.0     D R Catlow  09-March-1989
  5.1     M W S Reid  20-July-1989
  5.2     J Barber  22-February-1993


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MODULE COVER

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FUNCTION

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FORMAT

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PROMPTS

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DESCRIPTION

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CALCULATION OF INTERVISIBILITY

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ENCODING OF COVER MAP DATA VALUES

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EARTH CURVATURE AND TERRESTRIAL REFRACTION

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USE OF SURFACE COVER HEIGHT INFORMATION

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USE OF A DIGITISING TABLE AND PUCK BUTTONS

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COVER COMMANDS

_! COMMAND

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_!

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FORMAT: _! [comment text]

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Command parameters:

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DESCRIPTION:

An exclamation mark is the standard TVES package comment delimiter. All text (and numbers) which lie to the right of a '!' character are excluded from command interpretation. Comments are useful for annotating command procedures used in batch processing etc.

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Messages: None.

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Examples:


COVER> FILEIN TEST !open the file<CR>
COVER> !Define a window<CR>
COVER> WINDOW 1 1 10 10<CR>
COVER>

CONE COMMAND

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CONE

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FORMAT:		CONE angle

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Command parameters:

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DESCRIPTION:

CONE is used to define the cone of vision of the observer selected using the DEFINE OBSERVER command.

If the cone angle is less than 180 degrees, the cone of vision is a 3 dimensional area, defining in combination with the distance of view, the x, y and z extremities of an observer's field of view.
For cone angles of 180 degrees or greater, the cone angle defines in combination with the distance of view, the x and y (planimetric) extremities of an observer's field of view.
Only the visibility status of nodes that lie within an observer's field of view are determined.
In the output cover map, those nodes that are outside the field of view of all observers are given a value of -1.

When an observer is first selected, the default cone of vision is 360 degrees (ie. a complete circular scan around the observer), however if either a TARGET or DIRECTION command is subsequently given, the default is redefined to be 90 degrees.
For a 360 degrees field of view, the DISTANCE command defines the radial distance. For cones of vision less than 360, the field of view is bisected by a line defining the direction of view. For example, if the current direction of view is along a line 45 degrees from North, and the cone 60 degrees, then the subtended cone is between 15 degrees and 75 degrees from North (ie 30 degrees either side of the direction of view).

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Messages:

The following error messages are specific to the CONE command:


*** ERROR *** Specifying command CONE
No observer has been selected using the DEFINE OBSERVER command

*** ERROR *** Specifying command CONE
Command requires 1 real argument

*** ERROR *** Specifying command CONE
Command argument should be in range 1.0 to 360.0

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Example:

COVER>DEFINE OBSERVER 1<CR>
COVER>POSITION 100 100<CR>
COVER>DISTANCE 70<CR>
COVER>SHOW OBSERVERS<CR>


Current observer settings (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  1          100      100         50.3        0.0      360.0      70.0


COVER>DIRECTION 45<CR>
COVER>SHOW OBSERVERS<CR>

Current observer settings (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  1          100      100         50.3       45.0       90.0      70.0


COVER>CONE 60<CR>
COVER>SHOW OBSERVERS<CR>

Current observer settings (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  1          100      100         50.3        0.0       60.0      70.0


COVER>

CREATE COMMAND

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CREATE

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FORMAT:		CREATE infile-spec outfile-spec

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Command parameters:

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DESCRIPTION:

CREATE is used to generate a new cover map using all or part of the cover information contained in an existing cover map.

The SELECT OBSERVER command controls what information is output to the cover map. Only visibility data relating to a selected observer is extracted from the input cover map, and written to the output cover map.
For example, if the input cover map was generated using observers 1 2 and 3, information relating to only observers 1 and 3 may be selected for output to the new cover map, by giving the command SELECT OBSERVERS 1 3 before using the CREATE command.
Information on the position and field of view of each selected observer read from the input cover map file header, is displayed on the terminal. The information is output in the current units of measurement.

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Messages: None

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Example:

COVER>SELECT OBSERVERS 1 3
COVER>CREATE TEST TEST1


Observer values (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  1          100      100         50.3        0.0      360.0      70.0
  3           45       30         27.3       23.6       90.0     123.6


COVER>

DEFINE OBSERVER COMMAND

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DEFINE OBSERVER

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FORMAT:		 OBSERVER observer_number

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Command parameters:

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DESCRIPTION:

The DEFINE OBSERVER command controls to which observer the position and field of view parameters are applied.
The POSITION, DIRECTION, DISTANCE, CONE, HEIGHT and TARGET commands act only on the observer currently selected by means of the DEFINE OBSERVER command.

If a digitising table and 16 button puck is available on the workstation, puck buttons 1 to 8 may be used to give simultaneous DEFINE OBSERVER and POSITION commands. Button 1 is used for observer 1, button 2 for observer 2 and so on.

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Messages:

The following error messages are specific to the DEFINE and DEFINE OBSERVER commands:


*** ERROR *** Defining OBSERVER POSITION using puck button
No map set up has been performed

*** ERROR *** Defining OBSERVER POSITION using puck button
Observer position must lie within DTM bounds

*** ERROR *** Specifying command DEFINE
Command qualifier is OBSERVER

*** ERROR *** Specifying command DEFINE OBSERVER
Command requires 1 integer argument

*** ERROR *** Specifying command DEFINE OBSERVER
Command requires an integer argument in the range 1 to 8

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Examples:

COVER>DEFINE OBSERVER 5
COVER>

DIRECTION COMMAND

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DIRECTION

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FORMAT:		    DIRECTION angle

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Command parameters:

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DESCRIPTION:

DIRECTION is used to specify the direction of view of the observer currently selected by means of the DEFINE OBSERVER command.
The direction of view is a line constructed between the observer and the central target point. It is specified as an angle measured clockwise from North.

Note that no direction need be specified if a 360 degrees scan around the observer is required.

The TARGET command is an alternative way of specifying target distance and direction of view.

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Messages:

The following error messages are specific to the DIRECTION command:


*** ERROR *** Specifying command DIRECTION
No observer has been selected using the DEFINE OBSERVER command

*** ERROR *** Specifying command DIRECTION
Command requires 1 real argument

*** ERROR *** Specifying command DIRECTION
Command argument should be in range 0.0 to 360.0

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Example:

COVER>DEFINE OBSERVER 1
COVER>POSITION 100 100
COVER>DIRECTION 225
COVER>SHOW OBSERVERS


Current observer settings (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  1          100      100         50.3       225.0      90.0       0.0


COVER>

DISABLE ABSOLUTE COMMAND

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DISABLE ABSOLUTE

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FORMAT: DISABLE ABSOLUTE

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Command parameters: None.

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DESCRIPTION:

DISABLE ABSOLUTE cancels a previous ENABLE ABSOLUTE command. If DISABLE ABSOLUTE is given, then coordinate values required by the POINT, POSITION, TARGET and WINDOW commands, supplied in metre or projection units, must be specified as an offset from the SW corner of the matrix.

The command also controls the format in which DTM header coordinate values, and observer and target details are displayed, when the SHOW FILEIN, SHOW OBSERVERS and SHOW TARGET commands are given.

By default the use of absolute coordinate values is enabled.

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Messages:

The following error message is specific to the DISABLE command:


*** ERROR *** Specifying command DISABLE
Command qualifiers are ABSOLUTE, ADD_TARGET, BILINEAR, CULTURE, DIAGNOSTICS,
EARTH_CURVATURE or TABLE

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Examples:


COVER> DISABLE ABSOLUTE<CR>
COVER>

DISABLE ADD_TARGET COMMAND

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DISABLE ADD_TARGET

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FORMAT: DISABLE ADD_TARGET

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Command parameters: None.

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DESCRIPTION:

The DISABLE ADD_TARGET command is used to cancel any previous ENABLE ADD_TARGET command.
The ENABLE ADD_TARGET command controls whether the height of a surface feature is added to the terrain height at a target point. It therefore controls whether the visibility of the top of a feature, or the visibility of the base of a feature at the target point is determined.
By default the ADD_TARGET option is disabled.

Use of the SHOW ENABLE command is recommended to check on the status of the option before generating a cover map or line of sight.

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Messages:

The following error message is specific to the DISABLE command:


*** ERROR *** Specifying command DISABLE
Command qualifiers are ABSOLUTE, ADD_TARGET, BILINEAR, CULTURE, DIAGNOSTICS,
EARTH_CURVATURE or TABLE

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Examples:


COVER> DISABLE ADD_TARGET
COVER>

DISABLE BILINEAR COMMAND

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DISABLE BILINEAR

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FORMAT: DISABLE BILINEAR

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Command parameters: None.

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DESCRIPTION:

DISABLE BILINEAR cancels a previous ENABLE BILINEAR command. If DISABLE BILINEAR is given, the observer height is assumed to be that of the nearest pixel. This is the default action.

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Messages:

The following error message is specific to the DISABLE command:


*** ERROR *** Specifying command DISABLE
Command qualifiers are ABSOLUTE, ADD_TARGET, BILINEAR, CULTURE, DIAGNOSTICS,
EARTH_CURVATURE or TABLE.

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Examples:


COVER> DISABLE BILINEAR<CR>
COVER>

DISABLE CULTURE COMMAND

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DISABLE CULTURE

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FORMAT:		DISABLE CULTURE 

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Command parameters: None.

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DESCRIPTION:

The DISABLE CULTURE command is used to disable the use of surface feature height information. It therefore cancels any previous ENABLE CULTURE command.

Use of the SHOW ENABLE command is recommended to check on the status of the option before generating a cover map or line of sight.

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Messages:

The following error message is specific to the DISABLE command:


*** ERROR *** Specifying command DISABLE
Command qualifiers are ABSOLUTE, ADD_TARGET, BILINEAR, CULTURE, DIAGNOSTICS,
EARTH_CURVATURE or TABLE

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Example:

COVER>DISABLE CULTURE
COVER>

DISABLE DIAGNOSTICS COMMAND

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DISABLE DIAGNOSTICS

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FORMAT: DISABLE DIAGNOSTICS

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Command parameters: None.

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DESCRIPTION:

DISABLE DIAGNOSTICS disables the output of diagnostic messages, and is therefore used to cancel any previous ENABLE DIAGNOSTICS command.
By default diagnostic printout, which is sent to SYS$OUTPUT, is enabled.

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Messages:

The following error message is specific to the DISABLE command:


*** ERROR *** Specifying command DISABLE
Command qualifiers are ABSOLUTE, ADD_TARGET, BILINEAR, CULTURE, DIAGNOSTICS,
EARTH_CURVATURE or TABLE

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Examples:


COVER> DISABLE DIAGNOSTICS<CR>
COVER>

DISABLE EARTH_CURVATURE COMMAND

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DISABLE EARTH_CURVATURE

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FORMAT:		 DISABLE EARTH_CURVATURE

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DESCRIPTION:

Disables the option to take into account earth curvature and terrestrial refraction when calculating visibility along a line of sight. The command is therefore used to cancel any previous ENABLE_CURVATURE command.

By default, no compensation is made for earth curvature or refraction in line of sight calculations.

Use of the SHOW ENABLE command is recommended to check on the status of the option before generating a cover map or line of sight.

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Messages:

The following error message is specific to the DISABLE command:


*** ERROR *** Specifying command DISABLE
Command qualifiers are ABSOLUTE, ADD_TARGET, BILINEAR, CULTURE, DIAGNOSTICS,
EARTH_CURVATURE or TABLE

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Examples:

COVER>DISABLE EARTH_CURVATURE
COVER>

DISABLE TABLE COMMAND

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DISABLE TABLE

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FORMAT:		DISABLE TABLE

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Command parameters: _None

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DESCRIPTION:

The DISABLE TABLE command turns off command and coordinate input from the puck button and digitising table. If table input is disabled then COVER will accept commands only from the terminal.

Input from the table can be reselected using the ENABLE TABLE command.

The SHOW ENABLE command may be used to check on the status of table input.

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Messages:

The following error messages are specific to the ENABLE command:


*** ERROR *** Specifying command ENABLE
Command qualifiers are ABSOLUTE, ADD_TARGET, BILINEAR, CULTURE, DIAGNOSTICS,
EARTH_CURVATURE or TABLE
 

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Example:

COVER>DISABLE TABLE
COVER>

DISPLAY COMMAND

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DISPLAY

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FORMAT:		 DISPLAY file-spec

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Command parameters:

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DESCRIPTION:

The DISPLAY command is used to display information contained in the specified cover map in colour coded form on a graphics screen.

Different colours are used to indicate how many observers can see a particular area of terrain. Black is used to indicate those parts of the terrain surface that are outside the field of view of all observers.
The RGB values for the colours are read from the colour table 'LSL$LOOKUP:COVERCOL.DAT'. The colour values may therefore be changed by editing this file.

Only information relating to those observers currently selected is displayed on the graphics screen. Using the SELECT OBSERVERS command it is therefore possible when a cover map has been generated using a number of observers, to view the visibility pattern associated with each observer in turn, or any combination of observers.
For this and other applications where a series of images are being generated from the same cover map, the REDISPLAY command which requires no file specification, should be used.

The image is scaled to fill the area of the screen reserved for cover map display. A legend is written on the left of the screen, and relates the display colours to the number of observers to whom the node is visible.
Text giving details of the position and field of view for each observer is written at the bottom of the screen. The UNITS command controls the units of measurement that is used when writing observer information.
If the cover map is larger than this display area, more of the screen will be used, but no legend or text will be included.

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Messages: None

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Example:

COVER>DISPLAY TEST
COVER>SELECT OBSERVERS 1 3<CR>
COVER>REDISPLAY
COVER>

DISTANCE COMMAND

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DISTANCE

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FORMAT:		 DISTANCE distance

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Command parameters:

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DESCRIPTION:

The DISTANCE command is used to specify the distance of the target point from the observer currently selected using the DEFINE OBSERVER command.

The distance is specified in the units of measurement that have been selected with the UNITS command, or set by default when the input DTM was selected. If matrix units are selected, the distance is calculated in terms of columns and rows; for all other units of measurement, the distance is measured in metres.
If a cone angle of 360 degrees has been specified for an observer, then the DISTANCE command is used to specify the radius of the circle surrounding the observer.

NOTE If no distance of view is specified for an observer, infinity is assumed.

The TARGET command is an alternative way of specifying target distance and direction of view.

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Messages:

The following error messages are specific to the DISTANCE command:


*** ERROR *** Specifying command DISTANCE
No observer has been selected using the DEFINE OBSERVER command

*** ERROR *** Specifying command DISTANCE
Command requires 1 real argument

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Examples:

COVER>DEFINE OBSERVER 1
COVER>POSITION 100 100
COVER>DISTANCE 400.32


Current observer settings (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  1          100      100         50.3        0.0       360.0     400.32


COVER>

ENABLE ABSOLUTE COMMAND

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ENABLE ABSOLUTE

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FORMAT: ENABLE ABSOLUTE

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Command parameters: None.

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DESCRIPTION:

If ENABLE ABSOLUTE is given, then coordinate values required by the POINT, POSITION, TARGET and WINDOW commands, supplied in metre or projection units, must be specified as absolute (rather than relative) coordinate values.

For example if the projection indicates U.K. National Grid, then the position of an observer should be specified as 6 figure National Grid coordinates.

By default values should be specified as absolute coordinates. This option can be disabled using the DISABLE ABSOLUTE command.

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Messages:

The following error message is specific to the ENABLE command:


*** ERROR *** Specifying command ENABLE
Command qualifiers are ABSOLUTE, ADD_TARGET, CULTURE, DIAGNOSTICS,
EARTH_CURVATURE or TABLE

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Examples:


COVER> ENABLE ABSOLUTE<CR>
COVER>

ENABLE ADD_TARGET COMMAND

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ENABLE ADD_TARGET

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FORMAT: ENABLE ADD_TARGET

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Command parameters: None.

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DESCRIPTION:

The ENABLE ADD_TARGET command controls whether a surface feature height value is added to the terrain height at a target point.
The command therefore controls whether the visibility of the top of a feature, rather than the visibility of the base of a feature at a target point, is computed.

The command should be used in conjunction with the ENABLE CULTURE command.
If the ENABLE CULTURE command has been given, then unless ENABLE ADD_TARGET is specified, the height of surface features is added to the terrain surface height at all nodes along a line of sight, but NOT at the target point itself.

Use of the SHOW ENABLE command is recommended to check on the status of the option before generating a cover map or line of sight.

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Messages:

The following messages are specific to the ENABLE and ENABLE ADD_TARGET commands:


*** ERROR *** Specifying command ENABLE
Command qualifiers are ABSOLUTE, ADD_TARGET, CULTURE, DIAGNOSTICS,
EARTH_CURVATURE or TABLE

*** WARNING *** CULTURE is currently not enabled

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Examples:


COVER> ENABLE ADD_TARGET
COVER>

ENABLE BILINEAR COMMAND

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ENABLE BILINEAR

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FORMAT: ENABLE BILINEAR

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Command parameters: None.

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DESCRIPTION:

If ENABLE BILINEAR is given, the height at the observer position is calculated from a bilinear interpolation performed on the four surrounding pixel heights, doing the subsequent visibility calculations with these more accurate values. (The default action is to set the observer height to that of the nearest pixel.)

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Messages:

The following error message is specific to the ENABLE command:


*** ERROR *** Specifying command ENABLE
Command qualifiers are ABSOLUTE, ADD_TARGET, BILINEAR, CULTURE, DIAGNOSTICS,
EARTH_CURVATURE or TABLE

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Examples:


COVER> ENABLE BILINEAR<CR>
COVER>

ENABLE CULTURE COMMAND

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ENABLE CULTURE

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FORMAT:		ENABLE CULTURE [file-spec]

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Command parameters:

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DESCRIPTION:

The ENABLE CULTURE command selects the use of surface feature height information when generating a cover map, or determining visibility along a line of sight.

This additional height information is read from a DTI file, which will typically contain height values relating to surface features such as woodland and large buildings. These features are often referred to as 'culture' in military environments, and 'clutter' when used in radio propagation studies.
If a file specification is supplied with the command, the height data is read from the specified DTI file.
If no file specification is given, the command will simply cancel any previous DISABLE CULTURE command, and height data read from a currently opened file will be used. An error message is generated if no such file is currently opened, and you will be prompted for a filename.

It is important that the x and y grid resolutions of the data in the surface feature DTI file, are the same as the elevation data in the input DTM.

If the ENABLE CULTURE command is given, the program will add a height read from the surface feature file on to the corresponding node in the DTM (ie. the surface feature height on to the terrain elevation value).
The ENABLE ADD_TARGET command controls whether this action is carried out for all nodes along a line of sight, including the target node, or for just those nodes between the observer and the target point.
The SET OFFSET command may be used to control the registration of the two files.

Use of the SHOW ENABLE command is recommended to check on the status of the option before generating a cover map or line of sight.

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Messages:

The following error messages are specific to the ENABLE and ENABLE CULTURE commands:


*** ERROR *** Specifying command ENABLE
Command qualifiers are ABSOLUTE, ADD_TARGET, CULTURE, DIAGNOSTICS,
EARTH_CURVATURE or TABLE

*** ERROR *** Specifying command ENABLE CULTURE
Matrix filename is missing

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Example:

COVER>ENABLE CULTURE TREES
COVER>DISABLE CULTURE
COVER>ENABLE CULTURE Use of height values in file LSL$DTI:TREES.MAT enabled
COVER>

ENABLE DIAGNOSTICS COMMAND

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ENABLE DIAGNOSTICS

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FORMAT: ENABLE DIAGNOSTICS

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Command parameters: None.

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DESCRIPTION:

ENABLE DIAGNOSTICS selects output of diagnostic messages.
The diagnostic printout consists of a series of messages indicating which operation is currently being performed by COVER, and the percentage progress.
The messages are sent to SYS$OUTPUT. Note that if you are using a hardcopy terminal no percentage progress figures are generated.

By default diagnostic printout is selected, and may be turned off using the command DISABLE DIAGNOSTICS

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Messages:

The following error message is specific to the ENABLE command:


*** ERROR *** Specifying command ENABLE
Command qualifiers are ABSOLUTE, ADD_TARGET, CULTURE, DIAGNOSTICS,
EARTH_CURVATURE or TABLE

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Examples:


COVER> ENABLE DIAGNOSTICS<CR>
COVER>

ENABLE EARTH_CURVATURE COMMAND

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ENABLE EARTH_CURVATURE

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FORMAT:		 ENABLE EARTH_CURVATURE

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DESCRIPTION:

Enables the option to take into account earth curvature and terrestrial refraction when calculating visibility along a line of sight.

The effect of earth curvature and terrestrial refraction on a line of sight becomes increasingly significant as the distance between the observer and the target point increases.
Earth curvature causes points to be observed at lower angles as distance increases, and points to disappear below the horizon. The equation


		e = d*d / 2R

is used to compute earth curvature (e), where d is the distance between the observer and target point in kilometres, and R is the radius of the earth (taken as 6378 kms). For a distance of 10 kilometres, the value of e will be approximately 7.84 metres.

Rays of light passing through the earth's atmosphere in any direction other than vertical are bent from a straight path. This refraction takes place in a direction towards the earth's surface under normal atmospheric and temperature conditions. As a result of refraction, points tend to be observed at a higher elevation as distance from the viewpoint increases. The equation


		r = (d*d) * k/2R

is used to compute the effect of refraction (r), where d is the distance between the observer and target point in kilometres, R is the radius of the earth, and k is the coefficient of refraction. By default COVER uses a coefficient value based on a 4/3 Earth of 0.125 This may be redefined by the user using the command SET REFRACTION_COEFFICIENT. For a distance of 10 kilometres the value of r is approximately 0.98 metres.

If both earth curvature and refraction are taken into account, the height of a point 10 kilometres from the observer is therefore adjusted downwards by 6.86 metres.

By default, no compensation is made for earth curvature or refraction in the line of sight calculations.

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Messages:

The following error message is specific to the ENABLE command:


*** ERROR *** Specifying command ENABLE
Command qualifiers are ABSOLUTE, ADD_TARGET, CULTURE, DIAGNOSTICS,
EARTH_CURVATURE or TABLE

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Examples:

COVER>ENABLE EARTH_CURVATURE
COVER>

ENABLE TABLE COMMAND

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ENABLE TABLE

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FORMAT:		ENABLE TABLE

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Command parameters: _None

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DESCRIPTION:

The ENABLE TABLE command initialises the Table Monitor to allow the input of commands from a table puck and digitising table.

The introduction contains details on table initialisation, and on how commands may be input using a table puck.

The DISABLE TABLE command may be used to turn off input from the digitising table.

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Messages:

The following error messages are specific to the ENABLE and ENABLE TABLE commands:


*** ERROR *** Specifying command ENABLE
Command qualifiers are ABSOLUTE, ADD_TARGET, CULTURE, DIAGNOSTICS,
EARTH_CURVATURE or TABLE
 
*** ERROR *** Specifying command ENABLE TABLE
Unable to initialise the table monitor

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Example:

COVER>ENABLE TABLE
COVER>

EXIT COMMAND

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EXIT

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FORMAT:		 EXIT

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Command parameters: None.

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DESCRIPTION:

The EXIT command is used to exit from COVER.
(pressing the Ctrl and Z keys together) may also be used to exit from the program.

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Messages: None.

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Examples:

COVER>EXIT
COVER>

FILEIN COMMAND

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FILEIN

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FORMAT:		 FILEIN file-spec

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Command parameters:

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DESCRIPTION:

This command opens and maps into memory a DTI file, containing the terrain elevation data to be used in the intervisibility calculations. Details derived from the header of the file are displayed on the terminal to confirm that the file has been successfully opened.

If the logical name LSL$MATRIX_DEFAULT_WINDOW is defined with a value of "1", then a default area of interest defined in matrix units of bottom left hand corner 1,1 and top right hand corner 200,200 is set when the file is opened. If the logical name is absent or has any other value, or if the input DTI file has fewer than 200 columns or 200 rows, then a default window equivalent to the total matrix is set.

After opening the DTM, the program looks for a second DTI file with the same name but with the extension MAT. This second file is optional, and is used to hold data on the height of surface features such as woodland that are present in the area covered by the DTM.
If a second DTI file is found, the option to additionally use the height values in this file is automatically enabled. This is equivalent to giving the command ENABLE CULTURE.
If no second DTI file is found, the option to use the height of surface features is disabled. The option may be subsequently enabled by giving an ENABLE CULTURE command with a DTI file specification.

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Messages:

The following error message is specific to the FILEIN command:


*** ERROR *** Specifying command FILEIN
Input DTI filename is missing

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Example:

COVER>FILEIN TEST


LSL$DTI:TEST.DTI	
Header: LSLA	 Data: WORD

Units are DTI Matrix Values

Matrix Coverage    SW:    1     1    NE:   301   251
Matrix Window      SW:    1     1    NE:   200   200
Matrix Interval	    E:	  1	      N:     1
Value Range	     :      0   to    851


COVER>

FILEOUT COMMAND

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FILEOUT

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FORMAT:		 FILEOUT file-spec

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Command parameters:

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DESCRIPTION:

This command is used to specify the name of the output cover map. The input file must have previously been defined with the FILEIN command.

COVER map generation is initiated with the GO command.

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Messages:

The following error message is specific to the FILEOUT command:


*** ERROR *** Specifying command FILEOUT.
The input DTM is undefined.

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Example:

COVER>FILEOUT TEST File LSL$DTI:TEST.VIS selected for output.
COVER>

GO COMMAND

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GO

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FORMAT:		GO

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Command parameters:None

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DESCRIPTION:

This command is used to initiatate intervisibility calculation.

The viewing parameters for at least one observer must have been previously defined, the FILEIN command must have been given to read in a DTM, and a FILEOUT command must have been used to specify an output visibility file.
When parameters for a number of observers have been defined, the SELECT OBSERVER command may be used to restrict calculation to specific observers.
The areas of the terrain which are visible and hidden, are calculated for each observer in turn. If diagnostics are enabled, a summary of the percentage of the terrain within the area of interest that is either visible, hidden or outside an observer's field of view, is displayed after an observer has been processed.

The COVER> prompt is displayed when all calculations have been completed, and the output cover map containing the encoded intervisibility data is closed.
Observer viewing parameters and observer selections remain current until redefined by the user. Similarly, the input DTM remains open, and current area of interest values in force, until either a new DTM is selected or the program is terminated.

If the ENABLE EARTH_CURVATURE command has been given, compensation for earth curvature and terrestrial refraction will be made. Similarly if the ENABLE CULTURE command has been given, the program will take into account the height of any intervening surface features when determining visibility.

The generated cover map may be displayed on a graphics device as a colour coded image using the DISPLAY command.

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Messages:

The following error messages are specific to the GO command:


*** ERROR *** Specifying command GO
The input DTM is undefined

*** ERROR *** Specifying command GO
No observer has been selected using the DEFINE OBSERVER command

*** ERROR *** Specifying command GO
output visibility matrix has not been defined

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Example:

COVER>ENABLE DIAGNOSTICS<CR>
COVER>GO


+-----------------------------------------------------------+
|                                                           |
|               Determining areas visible to                |
|                       Observer  1                         |
|                                                           |
+-----------------------------------------------------------+

Observer 1   Visible  13%   Not Visible  32%   Outside  55%


COVER>

HEIGHT COMMAND

--------------------------------------------------------------------------------
HEIGHT

--------------------------------------------------------------------------------

FORMAT:		 HEIGHT height

--------------------------------------------------------------------------------
Command parameters:

--------------------------------------------------------------------------------
DESCRIPTION:

HEIGHT allows the observer currently selected by the DEFINE OBSERVER command to be positioned above the terrain surface.

The command is interpreted differently depending on whether the observer is positioned within or outside the DTM area.
If the observer has been positioned within the geographical bounds of the DTM using the POSITION command, the value supplied with the HEIGHT command is added to the terrain surface height at the observer position.
If the observer is positioned outside the geographical bounds of the DTM, the command defines the height of the observer above sea-level.

The SHOW OBSERVERS command should be used to check that the height of an observer has been correctly defined.

--------------------------------------------------------------------------------
Messages:

The following error messages are specific to the HEIGHT command:


*** ERROR *** Specifying command HEIGHT
No observer has been selected using the DEFINE OBSERVER command

*** ERROR *** Specifying command HEIGHT
Command requires 1 real argument

--------------------------------------------------------------------------------
Examples:

COVER>DEFINE OBSERVER 1
COVER>POSITION 100 100
COVER>SHOW OBSERVERS


Current observer settings (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  1          100      100         50.3        0.0      360.0       0.0


COVER>HEIGHT 2.6
COVER>SHOW OBSERVERS

Current observer settings (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  1          100      100         52.9        0.0      360.0       0.0


COVER>POSITION -10 -10
COVER>HEIGHT 100.7
COVER>SHOW OBSERVERS

Current observer settings (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  1          -10      -10        100.7        0.0      360.0       0.0


COVER>

HELP COMMAND

--------------------------------------------------------------------------------
HELP

--------------------------------------------------------------------------------

FORMAT:		 HELP [command]

--------------------------------------------------------------------------------
Command parameters:

--------------------------------------------------------------------------------
DESCRIPTION:

A brief description is given of the function and format of the specified command.
If no parameter is supplied then a list of all commands available is given.

--------------------------------------------------------------------------------
Messages: None.

--------------------------------------------------------------------------------
Examples:

COVER>HELP DISTANCE
COVER>

POINT COMMAND

--------------------------------------------------------------------------------
POINT

--------------------------------------------------------------------------------

FORMAT:		 POINT x_coordinate y_coordinate

--------------------------------------------------------------------------------
Command parameters:

The format of the command parameters is controlled by the UNITS command.

Note that in all cases, the input values are adjusted to the nearest column and row values.

--------------------------------------------------------------------------------
DESCRIPTION:

The POINT command is used to define the position, and compute the visibility of a single target point.

On giving the command, the program immediately computes the visibility of this point to all currently selected observers, and prints a summary on the terminal. The summary tabulates to how many and to which observers the point is visible or hidden.

The target point must lie inside the geographical bounds of the DTM.

If a digitising table is available, and a map has been registered to the DTM using the SETUP MAP command, puck Button A may be used to digitise the position of the point.

If the ENABLE EARTH_CURVATURE command has been given, compensation for earth curvature and terrestrial refraction will be made. Similarly if the ENABLE CULTURE command has been given, the program will take into account the height of any intervening surface features when determining the visibility of the point.

The command is used to determine visibility along a simple line of sight. It may also be used to determine visibility along a linear feature, such as a road, if the command is repeatedly given to digitise a series of points along the map feature.

--------------------------------------------------------------------------------
Messages:

The following error messages are specific to the POINT command:


*** ERROR *** Specifying command POINT from the puck button
No map set up has been performed

*** ERROR *** Specifying command POINT
The input DTM is undefined

*** ERROR *** Specifying command POINT
Point must lie within DTM bounds

*** ERROR *** Specifying command POINT
No observer has been selected using the DEFINE OBSERVER command

*** ERROR *** Specifying command POINT
Command requires 2 x y coordinate values

*** ERROR *** Specifying command POINT
Latitude and longitude values supplied in wrong format

--------------------------------------------------------------------------------
Examples:

COVER>DEFINE OBSERVER 1
COVER>POSITION 10 10
COVER>DEFINE OBSERVER 2
COVER>POSITION 70 50
COVER>POINT 32 45


Point at position 32 45 (height 345) is:
    Visible to 1 observer (observer: 1)
NOT Visible to 1 observer (observer: 2)


COVER>

POSITION COMMAND

--------------------------------------------------------------------------------
POSITION

--------------------------------------------------------------------------------

FORMAT:		 POSITION x_coordinate y_coordinate

--------------------------------------------------------------------------------
Command parameters:

The format of the command parameters is controlled by the UNITS command.

Note that in all cases, the input values are adjusted to the nearest column and row values.

--------------------------------------------------------------------------------
DESCRIPTION:

The POSITION command is used to define the geographical location of the observer currently selected by means of the DEFINE OBSERVER command.

An observer may be positioned inside or outside the geographical bounds of the DTM.
If the observer is positioned inside the DTM, the height of the surface at the observer position is determined from the DTM. The HEIGHT command may be used, if you wish to position the observer a given distance above the terrain surface.
If the observer is positioned outside the DTM, the height of the terrain surface cannot be determined, and you should use the HEIGHT command to set the absolute height of the observer.

If a digitising table is available, and a map has been registered to the DTM using the SETUP MAP command, puck button 0 may be used to digitise the position of the observer. When using this option, the observer may only be positioned within the geographical bounds of the DTM.

--------------------------------------------------------------------------------
Messages:

The following error messages are specific to POSITION command:


*** ERROR *** Specifying command POSITION using puck button
No map set up has been performed

*** ERROR *** Specifying command POSITION using puck button
Digitised point must lie within DTM bounds

*** ERROR *** Specifying command POSITION
The input DTM is undefined

*** ERROR *** Specifying command POSITION
No observer has been selected using the DEFINE OBSERVER command

*** ERROR *** Specifying command POSITION
Command requires 2 x y coordinate values

*** ERROR *** Specifying command POSITION
Latitude and longitude values supplied in wrong format

--------------------------------------------------------------------------------
Examples:

COVER>DEFINE OBSERVER 1
COVER>POSITION 100 100
COVER>SHOW OBSERVERS


Current observer settings (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  1          100      100         50.3        0.0      360.0       0.0


COVER>UNITS LATLONG
COVER>DEFINE OBSERVER 1
COVER>POSITION 42 00 00N 2 58 40E
COVER>SHOW OBSERVERS

Current observer settings (Units are Degrees, Minutes, Seconds)

Observer       Position          Height    Direction    Cone    Distance
  1       42 00 00N  2 58 40E     73.2        0.0      360.0       0.0


COVER>

REDISPLAY COMMAND

--------------------------------------------------------------------------------
REDISPLAY

--------------------------------------------------------------------------------

FORMAT:		 REDISPLAY

--------------------------------------------------------------------------------
Command parameters: _None

--------------------------------------------------------------------------------
DESCRIPTION:

The REDISPLAY command is used to display information contained in a currently opened cover map in colour coded form on a graphics screen.

The command acts in the same way as the DISPLAY command. For example, only information relating to those observers currently selected is displayed on the graphics screen. Unlike the DISPLAY command however, REDISPLAY does not require a cover map file specification. REDISPLAY should therefore be used in situations where a series of images are being generated from the same cover map eg. when displaying the visibility pattern of each observer in turn.

In the example below, DISPLAY is used initially to open and display the information in a cover map, and REDISPLAY is subsequently used to display data from the same cover map.

--------------------------------------------------------------------------------
Messages:

The following error message is specific to the REDISPLAY command:


*** ERROR *** Specifying command REDISPLAY
No cover map is currently opened for display

--------------------------------------------------------------------------------
Example:

COVER>DISPLAY TEST
COVER>SELECT OBSERVERS 1 3<CR>
COVER>REDISPLAY
COVER>

SELECT ALL COMMAND

--------------------------------------------------------------------------------
SELECT ALL

--------------------------------------------------------------------------------

FORMAT:	        SELECT ALL

--------------------------------------------------------------------------------
Command parameters: _None

--------------------------------------------------------------------------------
DESCRIPTION:

The SELECT ALL command is used to select all observers when generating a cover map using the commands GO or CREATE, or when computing a line of sight using the POINT command. The command also ensures that information relating to all observers is included when displaying a cover map on a graphics device using the DISPLAY or REDISPLAY commands, and when decoding information using the WRITE command.
The command overrides any observer selections made using the SELECT OBSERVER command.

Current observer selections may be examined using the command SHOW OBSERVERS.

--------------------------------------------------------------------------------
Messages:

The following error message is specific to the SELECT command:


*** ERROR *** Specifying command SELECT
Command qualifiers are OBSERVER or ALL


--------------------------------------------------------------------------------
Examples:

COVER>SELECT ALL
COVER>

SELECT OBSERVERS COMMAND

--------------------------------------------------------------------------------
SELECT OBSERVERS

--------------------------------------------------------------------------------

FORMAT:	        SELECT OBSERVERS observer_number [...]

--------------------------------------------------------------------------------
Command parameters:

--------------------------------------------------------------------------------
DESCRIPTION:

The SELECT OBSERVER command controls which observers are used when generating a cover map using the commands GO or CREATE, or computing a line of sight using the POINT command. The command also controls whether intervisibility information relating to a particular observer is included when displaying a cover map on a graphics device using the DISPLAY or REDISPLAY commands, and what information is decoded when the WRITE command is given.

Up to 7 observers may be specified in the list.

The SELECT ALL command should be used if no restrictions are to be placed on which observers are selected.

Current observer selection may be examined using the command SHOW OBSERVERS.

--------------------------------------------------------------------------------
Messages:

The following error messages are specific to the SELECT and SELECT OBSERVERS commands:


*** ERROR *** Specifying command SELECT
Command qualifiers are OBSERVER or ALL

*** ERROR *** Specifying command SELECT OBSERVER
Command requires at least 1 integer argument

*** ERROR *** Specifying command SELECT OBSERVER
Integer values should be in the range 1 to 8

--------------------------------------------------------------------------------
Examples:

This example assumes viewing parameters for 3 observers (observers 1 2 and 4) have been entered.

COVER>SELECT OBSERVERS 2 4
COVER>SHOW OBSERVERS


Current observer settings (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  2          130      140         70.6       30.0       60.0      120.5
  4          114      117         63.2      245.0       60.0      145.2 


COVER>SELECT ALL
COVER>SHOW OBSERVERS

Current observer settings (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  1          100      100         50.3        0.0      360.0        0.0
  2          130      140         70.6       30.0       60.0      120.5
  4          114      117         63.2      245.0       60.0      145.2 


COVER>

SET OFFSET COMMAND

--------------------------------------------------------------------------------
SET OFFSET

--------------------------------------------------------------------------------

FORMAT:		 SET OFFSET x_offset y_offset

--------------------------------------------------------------------------------
Command parameters:

--------------------------------------------------------------------------------
DESCRIPTION:

The SET OFFSET command is used to register a DTI file containing data on the height of surface features, to the input DTM.

By default the SW corner of the DTI file is registered to the SW corner of the DTM. The use of the command is therefore only necessary in situations were the SW corner of the two files do not represent the same geographical location.
The SET OFFSET command should only be given after the file containing the surface feature heights has been read in, either when opening the input DTM or by means of the ENABLE CULTURE command.

The SHOW SETTINGS command may be used to examine the current OFFSET values.

--------------------------------------------------------------------------------
Messages:

The following error messages are specific to the SET and SET OFFSET commands:


*** ERROR *** Specifying command SET
Command qualifiers are OFFSET or REFRACTION_COEFFICIENT

*** ERROR *** Specifying command SET OFFSET
Command requires 2 integer arguments

*** ERROR *** Specifying command SET OFFSET
Column and row values may not exceed DTM bounds

--------------------------------------------------------------------------------
Examples:

In this example, the SW corner of the DTI file containing surface feature height values is registered to column 50 and row 50 of the input DTM.

COVER>SET OFFSET 50 50
COVER>

SET REFRACTION_COEFFICIENT COMMAND

--------------------------------------------------------------------------------
SET REFRACTION_COEFFICIENT

--------------------------------------------------------------------------------

FORMAT:		 SET REFRACTION_COEFFICIENT coefficient

--------------------------------------------------------------------------------
Command parameters:

--------------------------------------------------------------------------------
DESCRIPTION:

This command allows the refraction coefficient value that is used when calculating the effect of terrestrial refraction on a line of sight, to be redefined. The supplied value is substituted in the equation:


		r = (d*d) * k/2R

where r is the effect of refraction (r), d is the distance between the observer and the target point in kilometres, R is the radius of the earth, and k is the coefficient of refraction.

By default COVER uses a coefficient value based on a 4/3 Earth of 0.125

The effect of refraction is only considered in intervisibility calculations if the ENABLE EARTH_CURVATURE command has been given.

The current value of the coefficient may be examined using the command SHOW SETTINGS.

--------------------------------------------------------------------------------
Messages:

The following error messages are specific to the SET and SET REFRACTION_COEFFICIENT commands:


*** ERROR *** Specifying command SET
Command qualifiers are OFFSET or REFRACTION_COEFFICIENT

*** ERROR *** Specifying command SET REFRACTION_COEFFICIENT
Command requires 1 real argument

--------------------------------------------------------------------------------
Examples:

COVER>SET REFRACTION_COEFFICIENT 0.3
COVER>

SETUP MAP COMMAND

--------------------------------------------------------------------------------
SETUP MAP

--------------------------------------------------------------------------------

FORMAT:		 SETUP MAP

--------------------------------------------------------------------------------
DESCRIPTION:

The SETUP MAP command allows a source document to be registered to the input DTM.

In order to register the map and a DTM, the DTM should have first been specified, and a source document should have been securely attached to the surface of a digitising table.

On giving the command you will be asked to digitise 4 rectangular registration points. The registration points represent 4 points on the map that correspond to the 4 corners of the DTM. The points are digitised in the order top left (NW), bottom left (SW), bottom right (SE) and top right (NE) using any button on the table puck. A point should be digitised in response to a prompt on the terminal.
An error message is generated if any of the angles of the digitised rectangle are less than 88 degrees, or greater than 92 degrees (ie. if the corner points of the rectangle are more than 2 degrees off rectangular). In this case you will be asked to redigitise the 4 registration points.
Setup of the map may be aborted using (pressing the Ctrl and Z keys together), or by pressing Button F on the table puck.

Following the registration of a map to the DTM, coordinate values required by the WINDOW, POSITION, TARGET and POINT commands, may be input using an appropriate button on the table puck.

--------------------------------------------------------------------------------
Messages:

The following error messages are specific to the SETUP and SETUP MAP commands:


*** ERROR *** Specifying command SETUP
Command qualifier is MAP

*** ERROR *** Specifying command SETUP MAP
Command is invalid if the table has not been initialised

*** ERROR *** Specifying command SETUP MAP
The input DTM must be specified before SETUP MAP

*** ERROR *** Setting up MAP
Badly digitised corner points; try again

--------------------------------------------------------------------------------
Examples:

COVER>SETUP MAP


Digitise map NW corner>

Digitise map SW corner>

Digitise map SE corner>

Digitise map NE corner>


COVER>

SHOW ENABLE COMMAND

--------------------------------------------------------------------------------
SHOW ENABLE

--------------------------------------------------------------------------------

FORMAT:		 SHOW ENABLE

--------------------------------------------------------------------------------
Command parameters: None.

--------------------------------------------------------------------------------
DESCRIPTION:

Displays the current status of all the COVER options that may be enabled or disabled using the ENABLE and DISABLE commands.
The name of the option is shown, followed by either the word ON or OFF to indicate its current status.
If the command SHOW ENABLE is used before any ENABLE or DISABLE commands have been given, the default status of the options is displayed.

--------------------------------------------------------------------------------
Messages:

The following message is specific to the SHOW command:


*** ERROR *** Specifying command SHOW
Command qualifiers are ENABLE, FILES, OBSERVERS, SETTINGS or TARGETS

--------------------------------------------------------------------------------
Examples:

COVER>SHOW ENABLE


Current status:

ABSOLUTE         On        ADD_TARGET       Off       CULTURE          Off

DIAGNOSTICS      On        EARTH_CURVATURE  Off       TABLE            Off


COVER>

SHOW FILES COMMAND

--------------------------------------------------------------------------------
SHOW FILES

--------------------------------------------------------------------------------

FORMAT:		 SHOW FILES

--------------------------------------------------------------------------------
Command parameters: None.

--------------------------------------------------------------------------------
DESCRIPTION:

Details extracted from the header of the DTM are displayed on the terminal, along with details of the current window or area of interest.

The header values are shown in the current units of measurement. This is dependent on the header type of the input file, or may be set explicitly using the UNITS command.

Information on the output visibility file selected with the FILEOUT command and the culture file selected with the ENABLE CULTURE command is also output.

--------------------------------------------------------------------------------
Messages:

The following messages are specific to the commands SHOW and SHOW FILES:


*** ERROR *** Specifying command SHOW
Command qualifiers are ENABLE, FILES, OBSERVERS, SETTINGS or TARGETS

*** WARNING *** The input DTM is undefined

--------------------------------------------------------------------------------
Examples:

COVER>SHOW FILES


LSL$DTI:TEST.DTI	
Header: LSLA	 Data: WORD

Units are DTI Matrix Values

Matrix Coverage    SW:    1     1    NE:   301   251
Matrix Window      SW:    1     1    NE:   200   200
Matrix Interval	    E:	  1	      N:     1
Value Range	     :      0   to    851

File LSL$DTI:COVER_TEST.VIS selected for output.

COVER>

SHOW OBSERVERS COMMAND

--------------------------------------------------------------------------------
SHOW OBSERVERS

--------------------------------------------------------------------------------

FORMAT:		 SHOW OBSERVERS

--------------------------------------------------------------------------------
Command parameters: None.

--------------------------------------------------------------------------------
DESCRIPTION:

The SHOW OBSERVERS command is used to display information about currently selected observers at the terminal.

Details of the coordinate position, height and field of view of each observer, are displayed.
The information is shown in the current units of measurement. This is dependent on the header type of the input DTM, or may be set explicitly using the UNITS command.

--------------------------------------------------------------------------------
Messages:

The following messages are specific to the commands SHOW and SHOW OBSERVERS


*** ERROR *** Specifying command SHOW
Command qualifiers are ENABLE, FILES, OBSERVERS, SETTINGS or TARGETS

*** WARNING *** No observers are currently selected

--------------------------------------------------------------------------------
Examples:

COVER>SHOW OBSERVERS


Current observer settings (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  1          100      100         50.3        0.0      360.0        0.0
  2          130      140         70.6       30.0       60.0      120.5
  4          114      117         63.2      245.0       60.0      145.2 


COVER>

SHOW SETTINGS COMMAND

--------------------------------------------------------------------------------
SHOW SETTINGS

--------------------------------------------------------------------------------

FORMAT:		 SHOW SETTINGS

--------------------------------------------------------------------------------
Command parameters: None.

--------------------------------------------------------------------------------
DESCRIPTION:

Displays the name of all COVER parameters that may be set using the SET command, and their current values.
If the SHOW SETTINGS command is given before using a SET command, the values shown are the default values that are allocated to the parameters by COVER.

--------------------------------------------------------------------------------
Messages:

The following message is specific to the command SHOW:


*** ERROR *** Specifying command SHOW
Command qualifiers are ENABLE, FILES, OBSERVERS, SETTINGS or TARGETS

--------------------------------------------------------------------------------
Examples:

COVER>SHOW SETTINGS


Current values:

OFFSET     1    1       REFRACTION_COEFFICIENT  0.125


COVER>

SHOW TARGETS COMMAND

--------------------------------------------------------------------------------
SHOW TARGETS

--------------------------------------------------------------------------------

FORMAT:		 SHOW TARGETS

--------------------------------------------------------------------------------
Command parameters: None.

--------------------------------------------------------------------------------
DESCRIPTION:

This command is used to display at the terminal, the target view point associated with each currently selected observer.

Details are shown in the current units of measurement. This is dependent on the header type of the input DTM, or may be set explicitly using the UNITS command.

--------------------------------------------------------------------------------
Messages:

The following messages are specific to the commands SHOW and SHOW TARGETS


*** ERROR *** Specifying command SHOW
Command qualifiers are ENABLE, FILES, OBSERVERS, SETTINGS or TARGETS

*** WARNING *** No observers are currently selected

--------------------------------------------------------------------------------
Examples:

COVER>SHOW TARGETS


Current Target Points (Units are DTI Matrix Values)

Observer      Target View Point

  1             130      160
  2              70      325


COVER>

SPAWN COMMAND

--------------------------------------------------------------------------------
SPAWN

--------------------------------------------------------------------------------
FORMAT: SPAWN command-line

--------------------------------------------------------------------------------
Command parameters:

--------------------------------------------------------------------------------
DESCRIPTION:

The SPAWN command enables you to create a subprocess while within COVER. When the subprocess terminates, control is returned to COVER.

--------------------------------------------------------------------------------
Messages:

The following error messages are specific to the SPAWN command:


*** ERROR *** Specifying command SPAWN 
Command requires a valid DCL command line

*** ERROR *** Unable to spawn command, returning to COVER

--------------------------------------------------------------------------------
Examples:

COVER> SPAWN DIRECTORY LSL$DTI:*_._DTI

Directory DUA3:[LSL.DTI]
TEST1_.DTI;1              8/8       18-AUG-1987 07:56  [LSL,DAVEC]
TEST2_.DTI;1              7/8       18-AUG-1987 17:17  [LSL,DAVEC]
TEST2_.DTI;2              7/8       18-AUG-1987 17:34  [LSL,DAVEC]
Total of 3 files, 22/24 blocks.

COVER>

TARGET COMMAND

--------------------------------------------------------------------------------
TARGET

--------------------------------------------------------------------------------

FORMAT:		 TARGET x_coordinate y_coordinate

--------------------------------------------------------------------------------
Command parameters:

The format of the command parameters is controlled by the UNITS command.

Note that in all cases, the input values are adjusted to the nearest column and row values.

--------------------------------------------------------------------------------
DESCRIPTION:

Defines the position of the target view point of the observer currently selected by means of the DEFINE OBSERVER command.

The TARGET command provides an alternative means to the DIRECTION and DISTANCE commands, of defining an observer's direction and distance of view.

If a cone of vision for the selected observer has not been previously defined when the TARGET command is given, or the current cone value is 360 degrees, a cone of vision of 90 degrees is applied to the observer.

The target point must be inside the geographical bounds of the DTM, but may lie outside the current area of interest defined by means of the WINDOW command.

Note that once defined, the target position is applied by default to subsequent observers when a further DEFINE OBSERVER command is given.

If a digitising table is available, and a map has been registered to the DTM using the SETUP MAP command, puck button 9 may be used to digitise the position of the target point.

The use of the commands SHOW TARGETS and SHOW OBSERVERS is recommended to confirm the correct definition of target position, and observer field of view parameters.

--------------------------------------------------------------------------------
Messages:

The following error messages are specific to TARGET command:


*** ERROR *** Specifying command TARGET using puck button
No map set up has been performed

*** ERROR *** Specifying command TARGET using puck button
Digitised point must lie within DTM bounds

*** ERROR *** Specifying command TARGET
The input DTM is undefined

*** ERROR *** Specifying command TARGET
No observer has been selected using the DEFINE OBSERVER command

*** ERROR *** Specifying command TARGET
Command requires 2 real arguments

*** ERROR *** Specifying command TARGET
Latitude and longitude values supplied in wrong format

--------------------------------------------------------------------------------
Examples:

COVER>UNITS MATRIX
COVER>DEFINE OBSERVER 1
COVER>POSITION 10 10
COVER>TARGET 400 300
COVER>SHOW OBSERVERS


Current observer settings (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  1          10        10         50.3        56.4      90.0      486.0


COVER>SHOW TARGETS

Current Target Points (Units are DTI Matrix Values)

Observer      Target View Point

  1             400      300


COVER>

UNITS COMMAND

--------------------------------------------------------------------------------
UNITS

--------------------------------------------------------------------------------
FORMAT: UNITS units

--------------------------------------------------------------------------------
Command parameters:

--------------------------------------------------------------------------------
DESCRIPTION:

The UNITS command defines in which units of measurement arguments to the DISTANCE, POINT, POSITION and TARGET commands are specified, and the units of measurement used when defining an area of interest in the input DTM by means of the WINDOW command.
The command also controls in what format details from the header of the DTM, and observer and target details are displayed, when the SHOW FILES, SHOW OBSERVERS and SHOW TARGET commands are given.

The command should be given after defining the input DTM since an appropriate default units of measurement is set up when the file is opened. If the DTM file is of header type TED4 or UHL1 (ie DTED files), then the default is latitude and longitude specified in degrees, minutes and seconds; for all other header types, matrix units (ie. columns and rows) are assumed.

--------------------------------------------------------------------------------
Messages:

The following error messages are specific to the UNITS command:


*** ERROR *** Specifying command UNITS
Command qualifiers are MATRIX, METRES, PROJECTION, SECONDS or LATLONG

*** ERROR *** Specifying command UNITS
Command qualifier is invalid for the input file

--------------------------------------------------------------------------------
Examples:


COVER> UNITS MATRIX<CR>
COVER>

WAIT COMMAND

--------------------------------------------------------------------------------
WAIT

--------------------------------------------------------------------------------
FORMAT: WAIT seconds

--------------------------------------------------------------------------------
Command parameters:

--------------------------------------------------------------------------------
DESCRIPTION:

The WAIT command causes processing to be suspended for a specified number of seconds. It is designed for use in software demonstration situations and is of no value in a production flowline.

--------------------------------------------------------------------------------
Messages:

The following error message is specific to the WAIT command:


*** ERROR *** Specifying command WAIT
Command requires a real argument

--------------------------------------------------------------------------------
Examples:


COVER> WAIT 4.0<CR>
COVER>

WINDOW COMMAND

--------------------------------------------------------------------------------
WINDOW

--------------------------------------------------------------------------------

FORMAT:		WINDOW  xmin ymin xmax ymax

--------------------------------------------------------------------------------
Command parameters:

--------------------------------------------------------------------------------
DESCRIPTION:

The WINDOW command is used limit intervisibility calculations to a particular rectangular geographical area. Only nodes in the DTM that lie within this area are used in line of sight calculations.
The area of interest should lie within the geographical bounds of the DTM.

The command also determines the number of columns and number of rows that will be present in the output cover map.

If the logical name LSL$MATRIX_DEFAULT_WINDOW is defined with a value of "1", then a default area of interest defined in matrix units of bottom left hand corner 1,1 and top right hand corner 200,200 is set when the file is opened. If the logical name is absent or has any other value, or if the input DTI file has fewer than 200 columns or 200 rows, then a default window equivalent to the total matrix is set.

If a digitising table is available, puck Button B may be used to give the command WINDOW. When the button is pressed, you will be asked to digitise 2 further points inside the map area, defining the SW (bottom lefthand) and NE (top righthand) corners of the rectangle. Definition of the area of interest may be abandoned by pressing puck Button F.

--------------------------------------------------------------------------------
Messages:

The following messages are specific to the WINDOW command:


*** ERROR *** Specifying command WINDOW
The input DTM must be specified before WINDOW

*** ERROR *** Specifying command WINDOW using puck button
No map set up has been performed

*** ERROR *** Specifying command WINDOW
Command requires 4 arguments

*** ERROR *** Specifying command WINDOW
NE corner values must exceed SW corner values

*** ERROR *** Specifying command WINDOW
Supplied values exceed matrix extents

*** ERROR *** Specifying command WINDOW
Latitude and longitude values supplied in wrong format

--------------------------------------------------------------------------------
Examples:

COVER>WINDOW 1 1 200 200
COVER>UNITS LATLONG
COVER>WINDOW 42 00 00N 3 00 00E 42 30 00N 2 58 40E
COVER>

WRITE COMMAND

--------------------------------------------------------------------------------
WRITE

--------------------------------------------------------------------------------

FORMAT:		WRITE infile-spec outfile-spec

--------------------------------------------------------------------------------
Command parameters:

--------------------------------------------------------------------------------
DESCRIPTION:

The WRITE command creates an output DTI file containing visibility information decoded from a cover map.

Visibility information in a cover map is held in a bitwise manner. Each bit of a data value is related to a single observer; a bit is set or unset depending on whether a node is visible or hidden from view of an observer. The header of a cover map holds position and field of view parameters for each observer.
These data and header values are decoded when the DISPLAY or REDISPLAY commands are given in order to produce a colour coded image showing which parts of the terrain are visible to an observer or observers, and which parts of the terrain lie outside their field of view. The information is read only by the module COVER; other modules within the TVES package are unable to correctly interpret the data values in a cover map.

The WRITE command is provided to allow the visibility data to be stored in a DTI file in a decoded form. In this form, these data can be processed through other modules such as VECTORISE and ROVER.
The data values written to the output DTI file indicate as on the graphics screen, how many observers can see a particular area of terrain. A value of 0 indicates the node is visible to no observers; a value of 1 that it is visible to 1 observer and so on. A value of -1 indicates that the node is outside the cone of vision of all observers.

The SELECT OBSERVERS command is used to control what information is decoded from the cover map. Only information related to the selected observers is used.

--------------------------------------------------------------------------------
Messages: _None

--------------------------------------------------------------------------------
Examples:

COVER>WRITE TEST_COVER TEST_OUTPUT
COVER>

EXAMPLE COVER SESSIONS

--------------------------------------------------------------------------------

EXAMPLE COVER SESSION

EXAMPLE COVER SESSIONS

$ COVER

COVER> filein test

LSL$DTI:TEST.DTI	
Header: LSLA	 Data: WORD

Units are Metres 

Matrix Coverage    SW:   440000    80000    NE:   460000   100000
Matrix Window      SW:   440000    80000    NE:   460000   100000
Matrix Interval	    E:	 50	      N:    50
Value Range	     :      0   to    190

COVER> units matrix
COVER> window 50 50 150 100
COVER> define observer 1
COVER> position 50 50
COVER> show observers

Current observer settings (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  1          50        50         50.3        0.0       360.0      0.0

COVER> define observer 2
COVER> pos -10 -10
COVER> height 32.5
COVER> cone 60
COVER> target 130 130
COVER> show observers

Current observer settings (Units are DTI Matrix Values)

Observer       Position          Height    Direction    Cone    Distance
  1          50        50         50.3        0.0      360.0       0.0
  2         -10       -10         32.5	     45.0       60.0     197.9

COVER> fileout test
File LSL$DTI:test.vis  selected for output.
COVER> go

+-----------------------------------------------------------+
|                                                           |
|               Determining areas visible to                |
|                       Observer  1                         |
|                                                           |
+-----------------------------------------------------------+

Observer 1   Visible  33%   Not Visible  67%   Outside   0%

+-----------------------------------------------------------+
|                                                           |
|               Determining areas visible to                |
|                       Observer  2                         |
|                                                           |
+-----------------------------------------------------------+

Observer 2   Visible  14%   Not Visible  22%   Outside  64%

COVER> exit

In this example, file LSL$DTI:TEST.DTI is selected as the input DTM. The UNITS MATRIX and WINDOW commands have been used to define an area of interest of 100 columns and 50 rows. Only the visibility of DTM nodes inside the specified rectangle will be computed.
The position and field of view of 2 observers is defined.
In the case of observer 1, only a position command has been given. The default cone of vision (360 degrees) and distance (infinity) values will therefore be used. The observer has been positioned inside the DTM, so that the height of the surface at the observation point is automatically derived from the DTM.
Observer 2 has been placed outside the DTM. The height command is used to set the height at the observation point. The cone command is used in conjunction with the target command to restrict the observer's field of view. Only the visibility of DTM nodes that lie within this field of view will be determined for observer 2.
The FILEOUT command is used to specify the filename of the output cover map, and the GO command is used to initiate intervisibility calculation.

MESSAGES (OTHER)

--------------------------------------------------------------------------------

MESSAGES (OTHER)

In addition to messages which are generated by the program itself, other messages may be produced by Laser-Scan libraries. In particular, messages may be generated the DTI library, and by the Laser-Scan I/O library, LSLLIB.

DTI library messages are introduced by '%DTI', and are documented in the DTILIB Reference Manual. In all cases the messages indicate a fatal error, that will cause processing to halt.

LSLLIB messages are introduced by '%LSLLIB' and are generally self-explanatory. Such messages rarely indicate a fatal error, and are generated most frequently by entering a command in an invalid format in response to the COVER prompt.