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          An attempt to explain the contents of the position
          parameter common /LOCATI/.


1.  The common is allocated by:
         INCLUDE DLOC.INC
         INCLUDE CLOC.INC
    And initialized by calling
         CALL SETLOC (IDEPTH)
    where IDEPTH is I*2(5) and is the location of the plane on axes
    3 - 7.

2.  Contents: 

       RPVAL   R*8(3)    Reference pixel value axes x, y, "z"
       RPLOC   R*4(3)    Reference pixel position 
       AXINC   R*4(3)    Axis increment
       CTYP    R*4(2,3)  Axis type: 4 characters/f.p.
       CPREF   R*4(2)    Units prefix: x, y axes only
       ROT     R*4       Rotation in degrees of the latitude-like axis
                         (+) into the + values of the longitude-like
                         axis
       ZDEPTH  I*2       IDEPTH(n) where n is the axis called Z
       AXTYP   I*2       Code: are x-y-z related? (see below)
       CORTYP  I*2       Code: non-linear corrections needed
       LABTYP  I*2       Code: how to label axes
       SGNROT  I*2       Extra sign to apply to rotation 

3.  Explanations:

    (a).  The "Z" axis is used only if one of the x and y axes is
          a position axis (i.e. ra, dec, ll, elon, ...) and the other
          is not a corresponding position axis (i.e. it is  freq,
          vel, ...).  If one of the other 5 axes is the corresponding
          position axis, then it is called the "Z" axis.  This is 
          particularly important in the case where, for example,
          the maps are initially constructed with a rotation of MM
          into LL and then transposed to VEL - LL - MM order.  To
          get a correct "position" of a point in a plane requires
          using all 3 axes.

   (b).   AXTYP code = 0 : x, y not related, no z, no rotation
                       1 : use x with y
                       2 : use x with z
                       3 : use y with z

   (c).   CORTYP code 0 : effectively x, y, z linear coords
                      1 : x is LL    y is MM
                      2 : y is LL    x is MM
                      3 : x is LL    z is MM
                      4 : z is LL    x is MM
                      5 : y is LL    z is MM
                      6 : z is LL    y is MM

   (d).   LABTYP code is sum of 2 codes:
                      0 x axis: use CPREF, CTYP
                      1       : use RIGHT ASCENSION
                      2       : use DECLINATION
                     00 y axis: use CPREF, CTYP
                     10       : use RIGHT ASCENSION
                     20       : use DECLINATION

   (e).   SGNROT +1 : latitude-like axis is Y
                 -1 : latitude-like axis is X
          This is a bit confused and will have to be watched like a
          hawk while doing the matrix transpose task.
