FITS-WCS projection parameters
"
PV2_1=45.0"
would specify a value for projection parameter 1
of axis 2 in a WcsMap.
These projection parameters correspond exactly to the values stored using the FITS-WCS keywords "
PV1_1"
, "
PV1_2"
, etc. This means that projection parameters which correspond to angles must be
given in degrees (despite the fact that the angular coordinates and other attributes used by a WcsMap
are in radians).
The set of projection parameters used by a WcsMap depends on the type of projection, which is determined by its WcsType parameter. Most projections either do not require projection parameters, or use parameters 1 and 2 associated with the latitude axis. You should consult the FITS-WCS paper for details.
Some projection parameters have default values (as defined in the FITS-WCS paper) which apply if no
explicit value is given. You may omit setting a value for these "
optional"
parameters and the default
will apply. Some projection parameters, however, have no default and a value must be explicitly
supplied. This is most conveniently done using the OPTIONS argument of AST_WCSMAP (q.v.)
when a WcsMap is first created. An error will result when a WcsMap is used to transform
coordinates if any of its required projection parameters has not been set and lacks a default
value.
A "
get"
operation for a parameter which has not been assigned a value will return the default value
defined in the FITS-WCS paper, or AST__BAD if the paper indicates that the parameter has no
default. A default value of zero is returned for parameters which are not accessed by the
projection.
Note, the FITS-WCS paper reserves parameters 1 and 2 on the longitude axis to hold the native longitude and latitude of the fiducial point of the projection, in degrees. The default values for these parameters are determined by the projection type. The AST-specific TPN projection does not use this convention - all projection parameters for both axes are used to represent polynomical correction terms, and the native longitude and latitude at the fiducial point may not be changed from the default values of zero and 90 degrees.
The value of this attribute may changed only if the WcsMap has no more than one reference. That is, an error is reported if the WcsMap has been cloned, either by including it within another object such as a CmpMap or FrameSet or by calling the AST_CLONE function.
If the projection parameter values given for a WcsMap do not satisfy all the required constraints (as defined in the FITS-WCS paper), then an error will result when the WcsMap is used to transform coordinates.