The SQUARE command allows the geometry of features to be constrained so
as to produce a "neat" appearance. LITES2 implements two different
algorithms which are known as "angle squaring" and "OS squaring"
This section of the user manual explains in detail how squaring is achieved and how to use the commands.
The angle squaring command has only one tolerance (an angle), and is totally independent of the units or size of either the map (IFF data), or the sheet. It can however only sensibly cope with features having a single orientation of corners to be squared. As it has no distance tolerance, corners can be moved by significant amounts if the feature is off-square.
The OS squaring algorithm was a direct implementation of the D14 algorithm as used in the ICL mainframe DMB system at OS Southampton. It has been enhanced to include facilities from the algorithm used by OS in their later programs. These additional facilities are selected by the ENABLE FIXED command (on by default) and include:
The angle squaring algorithm is invoked by the SQUARE ANGLE command. It requires that a linear feature has already been found at a corner vertex that is to become a right angle. This nominated corner defines the orientations for parallel and perpendicular for the rest of the feature. The actions of the algorithm are as follows:
The OS D14 squaring algorithm is invoked by the SQUARE WHOLE or SQUARE
PART commands. Both these commands require that a linear feature has
already been found. The position on the feature by which it was found is
immaterial. The only difference between SQUARE WHOLE and SQUARE PART is
that for SQUARE WHOLE the value of the tolerance parameter SQMT is
ignored and assumed to be infinite.
The actions of the basic algorithm are as follows:
A warning is given if any vertex is moved by more than the warning tolerance (SQWT).
More significant movements of points can occur when the opening and closing sides of a feature are parallel. In this case the last point (and perhaps others) will be removed, and the first point may be moved a long way in the direction of the closing side, to close the feature.
Default settings of things that control squaring may need to be altered. These are:-
TOLERANCE DEGREES real - Angle squaring tolerance in degrees TOLERANCE RADIANS real - Angle squaring tolerance in radians TOLERANCE SQDEF - restore to default setting all OS SQxx parameters TOLERANCE SBMT real - as SQMT but for based squaring TOLERANCE SBLT real - as SQLT but for based squaring TOLERANCE SQBT real - length of base must be longer than this distance (mm) TOLERANCE SQCT real - distance (mm) to be used when comparing the end points of a feature for closure TOLERANCE SQMT real - distance (mm) to be used when deciding if a side is to be included in this squaring pass. It is the maximum lateral distance a point might be expected to move TOLERANCE SQLT real - minimum length of line to be moved (mm) TOLERANCE SQPL real - angle (degrees) to be used in OS squaring to test if two sides are parallel TOLERANCE SQWT real - warning issued when point moved more than this (mm)