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NAME  - Surface area estimation for rasters.


       raster, statistics

SYNOPSIS help input=string  [vscale=float]   [--verbose]  [--quiet]

           Raster file for surface

           Vertical scale

DESCRIPTION  Calculates  area  of  regular  3D  triangulated  points (centers of cells) in
       current region by adding areas of triangles.  Therefore, area of a flat  surface  will  be
       reported  as  (rows  + cols -1)*(area of cell) less than area of flat region due to a half
       row and half column missing around the perimeter.


       This calculation is heavily dependent on  data  resolution  (think  of  it  as  a  fractal
       shoreline problem, the more resolution the more detail, the more area, etc).  This program
       uses the CURRENT GRASS REGION,  not  the  resolution  of  the  map.   This  is  especially
       important  for surfaces with NULL values and highly irregular edges.  The program does not
       [currently] attempt to correct for the error introduced by this edge effect.

       This version actually calculates area twice for each  triangle  pair,  keeping  a  running
       minimum and maximum area depending on the direction of the diagonal used.

       Reported totals are:

       1      "Plan" area of NULL values within the current GRASS region

       2      "Plan" area within calculation region (rows-1 * cols-1 * cellarea)

       3      Average of the minimum and maximum calculated 3d triangle area within this region

       4      "Plan" area within current GRASS region (rows * cols * cellarea)

       5      Scaling of calculated area to current GRASS region (see NOTE)

NOTES  works  best  when  the  surface  being  evaluated extends to the edges of the
       current GRASS region and the cell resolution is small.  Surfaces which are especially long
       and  thin  and  have  highly  irregular boudaries will tend to have underestimated surface
       areas.  Setting a high cell resolution (small area) will greatly reduce this  impact,  but
       will cause longer processing times.


       r.sum,,,,,  r.volume,,
       r.slope.aspect, g.region


       Bill Brown, USACERL December 21, 1994

       Modified for floating point rasters and NULL values by Eric G. Miller (October 17, 2000)

       Last changed: $Date: 2008-05-16 12:09:06 -0700 (Fri, 16 May 2008) $

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       © 2003-2013 GRASS Development Team