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diff --git a/pkg/utilities/nttools/stxtools/doc/wcs.doc b/pkg/utilities/nttools/stxtools/doc/wcs.doc new file mode 100644 index 00000000..8154a944 --- /dev/null +++ b/pkg/utilities/nttools/stxtools/doc/wcs.doc @@ -0,0 +1,177 @@ +Routines for conversion between pixel coordinates and world coordinates. + + This package contains the following high-level routines for converting +between world coordinates and pixel coordinates: + +xt_wcs_init initialize structure for world coordinate system +xt_wcs_init_c initialize from input cdelt, crota, etc +xt_wcs_init_cd initialize from input CD matrix, etc +xt_wc_pix convert from world coordinates to pixel coordinates +xt_pix_wc convert from pixel coordinates to world coordinates +xt_wcs_free deallocate wcs struct + +After calling any one of the three initialization routines, either or both +of the conversion routines (xt_wc_pix, xt_pix_wc) may be called any number +of times. When finished, xt_wcs_free should be called to deallocate memory +that was allocated by the initialization routine. + + Eight different projection geometries are currently implemented for +spherical coordinates. The projection type is obtained from the CTYPE +parameter, or the type defaults to gnomonic if nothing is specified in +CTYPE. Here is a list of CTYPE values for the right ascension axis with +the various projection types. For Aitoff and Mercator projections the +reference pixel is assumed to be on the equator. In addition, for Aitoff +projection the difference between right ascension and RA at the reference +pixel is limited to 180 degrees. + + CTYPE projection type + ----- --------------- + RA---TAN gnomonic (tangent) + RA---SIN radial distance proportional to sine of angle + RA---ARC radial distance proportional to angle + RA---NCP north celestial pole + RA---GBS global sine (equal area) + RA---STG stereographic + RA---AIT Aitoff equal area + RA---MER Mercator + + + To use the following sample program, extract into a file "ttt.x" +and compile and link with: + + xc -p tables ttt.x -lstxtools + +task ttt + +include <imhdr.h> + +procedure ttt() + +pointer im, wcs +double phys[IM_MAXDIM] +real pix[IM_MAXDIM], opix[IM_MAXDIM] +int naxis, k +char input[SZ_FNAME] +pointer immap() +int scan() + +begin + call clgstr ("input", input, SZ_FNAME) + im = immap (input, READ_ONLY, NULL) + naxis = IM_NDIM(im) + call xt_wcs_init (im, wcs) # initialize + call imunmap (im) + + call printf ("naxis = %d\n") + call pargi (naxis) + call printf ("enter pixel coordinates\n") + + while (scan() != EOF) { + do k = 1, naxis + call gargr (pix[k]) + call xt_pix_wc (wcs, pix, phys, naxis) # to world coords + call xt_wc_pix (wcs, phys, opix, naxis) # to pixel coords + # Print the input pixel coordinates, the world coordinates, + # and the output pixel coordinates (which should be the same + # as the input). + do k = 1, naxis { + call printf ("%.3f %18.10g %.3f\n") + call pargr (pix[k]) + call pargd (phys[k]) + call pargr (opix[k]) + } + } + call xt_wcs_free (wcs) # free memory +end + + + +# xt_wcs_init -- initialize wcs struct +# This routine allocates space for a structure describing the world +# coordinate system for an image, fills in the values or defaults, and +# returns a pointer to that structure. + +call xt_wcs_init (im, wcs) + +pointer im # i: pointer to image descriptor +pointer wcs # o: pointer to world coord system struct + + +# xt_wcs_init_c -- initialize wcs struct +# xt_wcs_init_c and xt_wcs_init_cd allocate space for a structure +# describing the world coordinate system for an image, fill in the values +# or defaults, and return a pointer to that structure. They differ from +# xt_wcs_init in that these take the coordinate parameters as arguments +# rather than getting them from the image. +# xt_wcs_init_c takes cdelt & crota, and xt_wcs_init_cd takes the CD matrix. + +call xt_wcs_init_c (crval, crpix, cdelt, crota, ctype, naxis, wcs) + +double crval[naxis] # i: coordinate values at reference pixel +real crpix[naxis] # i: reference pixel +real cdelt[naxis] # i: pixel spacing +real crota # i: rotation angle (if 2-D) +char ctype[SZ_CTYPE,naxis] # i: e.g. "RA---TAN" +int naxis # i: size of arrays +pointer wcs # o: pointer to world coord system struct + + +call xt_wcs_init_cd (crval, crpix, cd, ctype, naxis, wcs) + +double crval[naxis] # i: coordinate values at reference pixel +real crpix[naxis] # i: reference pixel +real cd[naxis,naxis] # i: CD matrix +char ctype[SZ_CTYPE,naxis] # i: e.g. "RA---TAN" +int naxis # i: size of arrays +pointer wcs # o: pointer to world coord system struct + + +# xt_wcs_free -- deallocate wcs struct +# This routine deallocates space for a wcs structure. + +call xt_wcs_free (wcs) + +pointer wcs # io: pointer to world coord system struct + + + +# xt_wc_pix -- wcs to pixels +# This routine converts world coordinates to pixel coordinates. +# +# In the 1-D case, CRVAL is subtracted from the coordinate, the +# result is divided by CDELT (same as CD1_1), and CRPIX is added. +# +# For 2-D or higher dimension, if two of the axes are like RA and Dec, +# the input coordinates are converted to standard coordinates Xi +# and Eta. The (Xi, Eta) vector is then multiplied on the left by +# the inverse of the CD matrix, and CRPIX is added. +# The units for axes like Ra & Dec are degrees, not hours or radians. +# For linear axes the conversion is the same as for 1-D. + +call xt_wc_pix (wcs, phys, pix, naxis) + +pointer wcs # i: pointer to world coord system struct +double phys[naxis] # i: physical (world) coordinates (e.g. degrees) +real pix[naxis] # o: pixel coordinates +int naxis # i: size of arrays + + + +# xt_pix_wc -- pixels to wcs +# This routine converts pixel coordinates to world coordinates. +# +# In the 1-D case, CRPIX is subtracted from the pixel coordinate, +# the result is multiplied by CDELT (same as CD1_1), and CRVAL is added. +# +# For 2-D or higher dimension, CRPIX is subtracted, and the result is +# multiplied on the left by the CD matrix. If two of the axes are like +# RA and Dec, the pixel coordinates are converted to standard coordinates +# Xi and Eta. The (Xi, Eta) vector is then converted to differences +# between RA and Dec and CRVAL, and then CRVAL is added to each coordinate. + +call xt_pix_wc (wcs, pix, phys, naxis) + +pointer wcs # i: pointer to world coord system struct +real pix[naxis] # i: pixel coordinates +double phys[naxis] # o: physical (world) coordinates (e.g. degrees) +int naxis # i: size of arrays |