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Harris Geospatial / Docs Center / Libraries / Beaumont / DENSITYMAP






  This procedure creates a smoothed map of the suface density of
  objects on the sky or in a plane.


  image processing, catalog processing

Calling Sequence

  DENSITYMAP, x, y, num, map, errmap, head, [cdelt = cdelt, out =


    x: Vector of x-coordinate positions for a group of objects, in degrees
    y: Corresponding y-coordinate positions for these
      objects, in degrees.
  num: The number of objects to use when smoothing. See PROCEDURE
      section for details.
  map: A named variable to hold the density map
  errmap: A named variable to hold the error map. This is just map /
        sqrt(num - 1)
  head: A named variable to hold the fits header describing the map


  map, errmap, and head are updated with the correct information.

Keyword Parameters

  cdelt: The pixel size of the smoothed map, in degrees /
        pixel. Either a scalar or two element array. The default
        pixel size is roughly 1/2 the mean input point spacings.
  crval: The output map center. The default is the middle of the
        input data. A two element array.
  naxis: The output map size, in pixels. The default is the smallest
        necessary to overlap all input data, given the values for
        cdelt and crval. A scalar or two element array.
    out: A name used when writing out files. The map and error map
        will be written to the file out_map.fits and out_err.fits
  galactic: If set, then the input is assumed to be in galactic
  verbose: If set, this procedure will print information as it
          works. Set to a value 1-4 to control the amount of output
  robust: If set, the distances between objects will be computed
          using rigorous spherical trigonometry. This will be MUCH slower, as
          this appoach is not vectorized well. The default behavior is to
          treat the coordinates as cartesian, which is adequate for small
  debug: Keyword which runs both robust and non-robust methods, and
          reports for how many pixels the non-robust method gets the
          wrong result.
  cartesian: By default, the procedure assumes that the input
          coordinates are on the sky. Set the CARTESIAN keyword to treat the
          coordinates as on a flat plane.


  The method is taken from section 3.5 of Gutermuth et al
  2005ApJ...632..397G. For each pixel, the procedure finds
  the distance r to the (num)'th closest object. The surface
  density of that pixel is num / (pi r^2).


  x = randomn(seed, 500)
  y = randomn(seed, 500)
  num = 10
  DENSITYMAP, x, y, num, map, errmap, head, /verbose, /cartesian
  tvscl, congrid(map, 500, 500)

Modification History

  Written by: Chris Beaumont, December 2008
  May 11 2009: Added CARTESIAN keyword. cnb.
  June 15 2009: Updated things so that VERBIAGE handles text
                output. cnb.
  June 15 2009: Fixed some bugs related to the CARTESIAN
                keyword. cnb.
  June 15 2009: Enhancements to cdelt. Added crval and naxis
                keywords. cnb.
  Oct 8 2009: Fixed a bug which led to integer overflow when
                manipulating naxis1, naxis2 keywords. cnb

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