User Coordinates

The positions and displacements within a Version 1 ARD description are interpreted as application coordinates by default. To supply positions and displacements in some other linearly-related coordinate system, the ARD description would contain statement fields defining the mapping from user coordinates to application coordinates. Such mappings were set up using the statements COEFFS, OFFSET, SCALE, TWIST and STRETCH.

For instance, if the application expects coordinates to be given in arc-seconds but the user wishes to give them in arc-minutes, then the statement SCALE( 60 ) should be included in the ARD description before the first keyword. This mapping is then concatenated with the mapping supplied by the application to get the mapping from user coordinates to pixel coordinates.

The user coordinate system may be changed at any point within an ARD description using suitable statement (COEFFS, etc.), and the new mapping will be used to interpret all further positions until the user coordinate system is modified again.

Arguments which specify distances (such as the radius of a circle for instance) are subject to the current mapping. Thus, for instance, a circle may be transformed into an ellipse if the coordinate axes have different scales.