3
<em>r.surf.area</em> calculates area of regular 3D triangulated points
4
(centers of cells) in current region by adding areas of triangles.
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Therefore, area of a flat surface will be reported as (<tt>rows + cols
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-1) * (area of cell)</tt> less than area of flat region due to a half
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row and half column missing around the perimeter.
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This calculation is heavily dependent on data resolution (think of it
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as a fractal shoreline problem, the more resolution the more detail,
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the more area, etc). This module uses the <b>current region
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settings</b>, not the resolution of the raster map. This is especially
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important for surfaces with <tt>NULL</tt> values and highly irregular
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edges. The module does not [currently] attempt to correct for the
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error introduced by this <em>edge effect</em>.
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<p>This version actually calculates area twice for each triangle pair,
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keeping a running minimum and maximum area depending on the direction
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<p>Reported totals are:
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<li>"Plan" area of <tt>NULL</tt> values within the current GRASS
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<li>"Plan" area within calculation region (<tt>rows-1 * cols-1 *
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<li>Average of the minimum and maximum calculated 3d triangle area
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within this region</li>
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<li>"Plan" area within current computational region (<tt>rows * cols *
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<li>Scaling of calculated area to current region</li>
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<p><em>r.surf.area</em> works best when the surface being evaluated
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extends to the edges of the current region and the cell resolution is
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small. Surfaces which are especially long and thin and have highly
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irregular boudaries will tend to have underestimated surface areas.
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Setting a high cell resolution (small area) will greatly reduce this
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impact, but will cause longer processing times.
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<div class="code"><pre>
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g.region -p raster=elevation
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r.surf.area map=elevation units=hectares
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Null value area ignored in calculation: 0.000000
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Plan area used in calculation: 20221.510000
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Surface area calculation(low, high, avg):
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20294.310421 20320.936368 20307.623395
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Current region plan area: 20250.000000
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Estimated region Surface Area: 20336.234719
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<a href="g.region.html">g.region</a>,
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<a href="r.surf.idw.html">r.surf.idw</a>,
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<a href="r.surf.fractal.html">r.surf.fractal</a>,
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<a href="r.surf.gauss.html">r.surf.gauss</a>,
64
<a href="r.volume.html">r.volume</a>,
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<a href="r.slope.aspect.html">r.slope.aspect</a>,
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<a href="v.to.rast.html">v.to.rast</a>
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Bill Brown, USACERL December 21, 1994
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Modified for floating point rasters and <tt>NULL</tt> values by Eric
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G. Miller (October 17, 2000)
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Updated for GRASS 7, and units option by Martin Landa, Czech Technical
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University in Prague, Czech Republic (October 2011)
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<p><i>Last changed: $Date: 2014-12-27 23:51:00 +0100 (Sat, 27 Dec 2014) $</i>