TICPOS

       Specify distance between tic marks for astronomical coordinate overlays

       User inputs number an approximate distance
       between tic marks, and the axis length in degrees.  TICPOS will return
       a distance between tic marks such that the separation is a round
       multiple in arc seconds, arc minutes, or degrees

       TICPOS, deglen, pixlen, ticsize, incr, units

       deglen - length of axis in DEGREES
       pixlen - length of axis in plotting units (pixels)
       ticsize - distance between tic marks (pixels).  This value will be
               adjusted by TICPOS such that the distance corresponds to
               a round multiple in the astronomical coordinate.

       ticsize - distance between tic marks (pixels), positive scalar
       incr    - incremental value for tic marks in round units given
               by the UNITS parameter
       units - string giving units of ticsize, either 'ARC SECONDS',
               'ARC MINUTES', or 'DEGREES'

       Suppose a 512 x 512 image array corresponds to 0.2 x 0.2 degrees on
       the sky.   A tic mark is desired in round angular units, approximately
       every 75 pixels.
       IDL> ticsize = 75
       IDL> TICPOS,0.2,512,ticsize,incr,units
       ==> ticsize = 85.333, incr = 2. units = 'Arc Minutes'
       i.e. a good tic mark spacing is every 2 arc minutes, corresponding
       to 85.333 pixels.

       written by W. Landsman            November, 1988
       Converted to IDL V5.0   W. Landsman   September 1997
       Don't use all capital letters  W. Landsman May 2003
       Fix case where incr crosses degree/minute or minute/degree boundary
               A. Mortier/W.Landsman April 2005

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