Subject: New release of WCS draft

From: egreisen at nrao.edu (Eric Greisen)
Date: Mon, 9 Sep 1996 10:42:46 -0400

This broadcast is to announce a new version of the paper
describing the coordinates proposal by Mark Calabretta and myself.
The paper is available as wcs.all.ps, wcs.all.ps.Z and wcs.none.ps.Z
(only) from the fits ftp/WWW site (fits.cv.nrao.edu) in the
fits/documents/wcs directory.

There have been a lot of changes in the paper since the last broadcast
version of about two (!) years ago.  Some intermediate versions of the
paper were placed in the fits area, but not broadcast widely.  I have
attempted to run 'diff' to find out what the changes are, but expect
that I have missed some.  What I found is listed below:

1.  The paper has been converted to A & A format with suitable style
    changes, re-arranging of long equations, dropping excess figures,
    etc.

2,  The subject of default coordinate types (there is none now), and
    general longitude and latitude types is addressed.

3.  The finding of the native North pole from the reference point
    (when it is not the North pole itself) required a lengthy
    generalization and discussion and the introduction of the LATPOLE
    keyword.

4.  AIR projection mathematics corrected.

5.  CYL now called CYP.

6.  CEA definition of lambda corrected.

7.  Conical projections reference pixels now at the central Native
    latitude and PROJP1 gives the central latitude and PROJP2 gives
    1/2 of the difference of the 2 latitudes of the projection.  All
    conical projections required a little change, mostly in Y_0, in
    the math.

8.  COP - add an alternate form of Rtheta.

9.  COO - change symbols.

10. BON - simpler formulation.

11. GLS - correct formula for phi.

12. PAR - correct units.

13. QSC - better symbols, corrected formulae

14. TSC - extra form for x_f, y_f

15. Added appendix defining Pixel regularization image.

16. Added references to Calabretta's software package known as wcslib
    (version 2.0 now in preparation).

17.  Correct accuracy of numbers in numeric example.

18.  Generalize conics slightly to allow ref latitude < 0.

19.  Figures 30 and 31 had the computation of the difference between
     ZEA and SIN corrected.  ZEA is very similar to ARC and STG now.
     (Thanks to Steve Allen.)

20.  The translation between the PC matrix and the old CDELT/CROTA
     system was corrected.  The order of the matrix multiplies matters
     and was done wrongly in developing those formulae.

21.  References to the PROJPn, particularly in Table 5, were
     incomplete and out of date. 

22.  A strong recommendation to make the PC matrix positive as well as
     diagonal was added.  The sentences on CDmmmnnn and CDn_m were
     made into a separate (short) paragraph.

23.  The reference to wcslib was brought up to rev 2.3 and the
     misstatement about public domain was corrected.

24.  The increments in the pixel regularization image do not have to
     integer image pixels, but can be any appropriate value.  A
     statement to clarify this was added.

25.  The wording in the abstract re other kinds of coordinates was
     modified.

26.  An appendix by Bill Pence, Arnold Rots, and Lorella Angelini
     suggesting extensions to the main conventions to cover
     coordinates in tables has been included. 

27.  Casual wording regarding "reference pixel" was improved and
     generally changed to "reference point".

This seems like a lot - I guess #15 (and maybe #26) are the biggest
changes which should get wide discussion.  #2, 3, 7 are significant as
well, while #1 was surprisingly easy and mostly cosmetic.

Please let us know what you think.

Eric Greisen (egreisen at nrao.edu)
Mark Calabretta (mcalabre at atnf.csiro.au)

---

From: Peter Bunclark <psb at ast.cam.ac.uk>
Date: Fri, 13 Sep 1996 12:24:46 +0100

> fits/documents/wcs directory.

I just pulled and printed it - it looks exactly the same as the document that has been there
many months - dated 30.10.1995...

Pete.

---

From: Jeff Bloch <jbloch at alexis.cv.nrao.edu>
Date: Sat, 14 Sep 1996 10:45:55 -0600

The version I pulled is dated 9.9.1996 and appears to have differences 
from previous versions.

However, there is something that I would like to see expanded. I restate a
plea that I made many months ago. I would like to see more EXPLICIT language
in the description of the CSC, QSC, and TSC on how to treat the existence or
non-existence of the CUBEFACE axis type. 

As we understand and use Eric's wcslib software package, it is possible to
lay out the whole sky in the "lazy T" configuration in a single plane of the
FITS image, not use the CUBEFACE axis type keyword, and the reference pixel
will be located at the center of face "1". However, at any valid location on
the map, a value for "CUBEFACE" is implied. It is also possible, for
compatibility with the COBE convention, to store the six planes of the
projection as a data cube with the third axis having the type CUBEFACE. 

This arrangement is sort of implied in the document, but I am afraid that 
authors of new software implementing these conventions may not fully 
appreciate the two ways of dealing with these projections from the 
description.

Here is an in-use example header for a map that does not use the CUBEFACE 
axis type:

SIMPLE  =                    T / Written by IDL:  11-Sep-1996 07:27:32.00
BITPIX  =                   32 /
NAXIS   =                    2 /
NAXIS1  =                 1440 /
NAXIS2  =                 1080 /
DATE    = '11/09/96'           / Creation date (DD/MM/YY) of FITS header
CTYPE1  = 'RA---QSC'           /COORDINATE TYPE
CTYPE2  = 'DEC--QSC'           /COORDINATE TYPE
EQUINOX =            1996.6948 /ALEXIS MAPS ARE IN CURRENT EPOCH
CD001001=            1.0000000 /DEGREES/PIXEL
CD002001=            0.0000000 /DEGREES/PIXEL
CD001002=            0.0000000 /DEGREES/PIXEL
CD002002=            1.0000000 /DEGREES/PIXEL
CDELT1  =          -0.25000000 /
CDELT2  =           0.25000000 /
CRPIX1  =              1260.50 /REFERENCE PIXEL IN X
CRPIX2  =              540.500 /REFERENCE PIXEL IN Y
CRVAL1  =            0.0000000 /R.A. (DEGREES)
CRVAL2  =            0.0000000 /DEC  (DEGREES)
LONGPOLE=              180.000 /LONGITUDE OF POLE (DEGREES)

I have placed the complete map as a gziped file called 
"sample_wcs_qsc.fits.gz" at our anonymous ftp site:

nis-ftp.lanl.gov

in directory "incoming".

						Jeff

> many months - dated 30.10.1995...

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