A WcsMap is used to represent sky coordinate projections as described in the (draft) FITS world coordinate system (FITS-WCS) paper by E.W. Griesen and M. Calabretta (A & A, in preparation). This paper defines a set of functions, or sky projections, which transform longitude-latitude pairs representing spherical celestial coordinates into corresponding pairs of Cartesian coordinates (and vice versa).
A WcsMap is a specialised form of Mapping which implements these sky projections and applies them to a specified pair of coordinates. All the projections in the FITS-WCS paper are supported, plus the now deprecated "TAN with polynomial correction terms" projection which is refered to here by the code "TPN". Using the FITS-WCS terminology, the transformation is between "native spherical" and "projection plane" coordinates. These coordinates may, optionally, be embedded in a space with more than two dimensions, the remaining coordinates being copied unchanged. Note, however, that for consistency with other AST facilities, a WcsMap handles coordinates that represent angles in radians (rather than the degrees used by FITS-WCS).
The type of FITS-WCS projection to be used and the coordinates (axes) to which it applies are specified when a WcsMap is first created. The projection type may subsequently be determined using the WcsType attribute and the coordinates on which it acts may be determined using the WcsAxis(lonlat) attribute.
Each WcsMap also allows up to 100 "projection parameters" to be associated with each axis. These specify the precise form of the projection, and are accessed using PVi_m attribute, where "i" is the integer axis index (starting at 1), and m is an integer "parameter index" in the range 0 to 99. The number of projection parameters required by each projection, and their meanings, are dependent upon the projection type (most projections either do not use any projection parameters, or use parameters 1 and 2 associated with the latitude axis). Before creating a WcsMap you should consult the FITS-WCS paper for details of which projection parameters are required, and which have defaults. When creating the WcsMap, you must explicitly set values for all those required projection parameters which do not have defaults defined in this paper.
If the sky projection to be implemented requires projection parameter values to be set, then this should normally be done here via the PVi_m attribute (see the "Examples" section). Setting values for these parameters is mandatory if they do not have default values (as defined in the FITS-WCS paper).
AST A Library for Handling World Coordinate Systems in Astronomy