PLON/PLAT

[Mark Calabretta]

Recent discussion on sci.astro.fits has indicated that it would be useful to
formally recognize a PLON/PLAT coordinate system in WCS.

While WCS was developed primarily for mapping the celestial sphere it is also
well suited for other forms of cartography.  The particular requirement which
PLON/PLAT would address is that of describing off-limb phenomena in planetary,
solar, lunar, etc. mapping.

An image using PLON/PLAT coordinates for longitude and latitude on the sphere
would have an implied Cartesian coordinate system which would apply to all
pixels in the image, including those beyond the limb of the sphere.  Secondary
axis descriptors could optionally be used to define a scale and coordinate
type for this Cartesian system.

Note that PLON/PLAT is simply an application of the methods already defined in
the WCS draft using the xLON/xLAT provision.  The proposal here is effectively
to register the name of this application; it does not constitute a fundamental
change to WCS (for example, it won't require a change to WCSLIB).

Examples of usage:

   a) Solar prominences.
   b) Volcanic plumes on Io.
   c) Terrestrial aurorae.
   d) Mapping of the positions of a planet's moons, rings, impacting comets,
      magnetic fields, radiation belts, spacecraft trajectories, etc. in
      relation to its disk.

PLON/PLAT would have the following defining characteristics:
 
   1) Used mainly in conjunction with the azimuthal perspective projection,
      AZP (with mu << -1), for planetary/solar mapping.
 
   2a) The PC matrix is chosen so that the perimeter of the sphere has unit
      radius.

   2b) The map is oriented so that the north pole is at x' = 0, y' >= 0, where
      x' and y' are the coordinate elements corresponding to the PLON and PLAT
      axes obtained after applying the PC matrix to a pixel coordinate.

   2c) The CDELTn header cards scale (x',y') to (x,y) in the "degree" units
      required by WCS.  Their signs are set to invert the native longitude as
      appropriate for a sphere observed from the outside as opposed to the
      usual case of the celestial sphere observed from the inside.
 
   3) Secondary axis descriptors can be used to assign physical units in a
      complementary Cartesian coordinate system for all points in the image,
      including those beyond the perimeter of the sphere.


Mark Calabretta
ATNF