Name

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      TICPOS

Purpose

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      Specify distance between tic marks for astronomical coordinate overlays

Explanation

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      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

Calling Sequence

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      TICPOS, deglen, pixlen, ticsize, incr, units

Inputs

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      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.

Outputs

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      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'

Example

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      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.

Revison History

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      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