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.
" CTYPEi"
keyword. You should consult the FITS-WCS paper for a list of the available projections. The
additional code of AST__TPN can be supplied which represents a TAN projection with
polynomial correction terms as defined in an early draft of the FITS-WCS 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).
’ ’ , STATUS ) ’ PV3_1=40.0’ , STATUS ) " PV3_1" ) is required and has no
default, so is set explicitly to 40.0 degrees. Projection parameter 2 (corresponding to FITS
keyword " PV3_2" ) is required but has a default of zero, so need not be specified. The forward transformation of a WcsMap converts between FITS-WCS " native spherical" and "
relative physical" coordinates, while the inverse transformation converts in the opposite direction.
This arrangement may be reversed, if required, by using AST_INVERT or by setting the Invert
attribute to a non-zero value.
If any set of coordinates cannot be transformed (for example, many projections do not cover the entire celestial sphere), then a WcsMap will yield coordinate values of AST__BAD.
The validity of any projection parameters given via the PVi_m parameter in the OPTIONS string is not checked by this function. However, their validity is checked when the resulting WcsMap is used to transform coordinates, and an error will result if the projection parameters do not satisfy all the required constraints (as defined in the FITS-WCS paper).
A null Object pointer (AST__NULL) will be returned if this function is invoked with STATUS set to an error value, or if it should fail for any reason.
" int
status"
.