Name

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      AITOFF_GRID

Purpose

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      Produce an overlay of latitude and longitude lines over a plot or image

Explanation

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      The grid is plotted on the current graphics device. AITOFF_GRID
      assumes that the ouput plot coordinates span the x-range of
      -180 to 180 and the y-range goes from -90 to 90.

Calling Sequence

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      AITOFF_GRID [,DLONG,DLAT, LABEL=, /NEW, CHARTHICK=, CHARSIZE=,
                    FONT=, _EXTRA=]

Optional Inputs

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      DLONG = Optional input longitude line spacing in degrees. If left
                out, defaults to 30.
      DLAT = Optional input latitude line spacing in degrees. If left
                out, defaults to 30.

Optional Input Keywords

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      LABEL = Optional keyword specifying that the latitude and
                        longitude lines on the prime meridian and the
                        equator should be labeled in degrees. If LABELS is
                        given a value of 2, i.e. LABELS=2, then the longitude
                        labels will be in hours instead of degrees.
        CHARSIZE = If /LABEL is set, then CHARSIZE specifies the size
                        of the label characters (passed to XYOUTS)
        CHARTHICK = If /LABEL is set, then CHARTHICK specifies the
                        thickness of the label characters (passed to XYOUTS)
      FONT = scalar font graphics keyword (-1,0 or 1) for text
      /NEW = If this keyword is set, then AITOFF_GRID will create
                        a new plot grid, rather than overlay an existing plot.
      Any valid keyword to OPLOT such as COLOR, LINESTYLE, THICK can be
      passed to AITOFF_GRID (though the _EXTRA facility) to to specify the
      color, style, or thickness of the grid lines.

Outputs

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      Draws grid lines on current graphics device.

Example

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      Create a labeled Aitoff grid of the Galaxy, and overlay stars at
      specified Galactic longitudes, glong and latitudes, glat
      IDL> aitoff_grid,/label,/new ;Create labeled grid
      IDL> aitoff, glong, glat, x,y ;Convert to X,Y coordinates
      IDL> plots,x,y,psym=2 ;Overlay "star" positions

Procedures Used

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      AITOFF

Notes

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      If labeling in hours (LABEL=2) then the longitude spacing should be
      a multiple of 15 degrees

Author And Modifications

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      J. Bloch 1.2 6/2/91
      Converted to IDL V5.0 W. Landsman September 1997
      Create default plotting coords, if needed W. Landsman August 2000
      Added _EXTRA, CHARTHICK, CHARSIZE keywords W. Landsman March 2001
      Several tweaks, plot only hours not minutes W. Landsman January 2002
      Allow FONT keyword to be passed to XYOUTS. T. Robishaw Apr. 2006