This attribute is a boolean value which specifies how FITS "CAR"
(plate carree, or "Cartesian") projections should be treated when
reading a FrameSet from a foreign encoded FITS header. If zero (the
default), it is assumed that the CAR projection conforms to the
conventions described in the (draft) FITS world coordinate system
(FITS-WCS) paper by E.W. Greisen and M. Calabretta (A & A, in
preparation). If CarLin is non-zero, then these conventions are
ignored, and it is assumed that the mapping from pixel coordinates to
celestial coordinates is a simple linear transformation (hence the
attribute name "CarLin"). This is appropriate for some older FITS
data which claims to have a "CAR" projection, but which in fact do
not conform to the conventions of the FITS-WCS paper. Furthermore, if
CarLin is non-zero, it is assumed that CDELT and CD keywords are
in units of degrees rather than radians (as required by the
FITS-WCS papers).
The FITS-WCS paper specifies that headers which include a CAR projection
represent a linear mapping from pixel coordinates to "native spherical
coordinates", NOT celestial coordinates. An extra mapping is then
required from native spherical to celestial. This mapping is a 3D
rotation and so the overall Mapping from pixel to celestial coordinates
is NOT linear. See the FITS-WCS papers for further details.