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70 lines
3.6 KiB
HTML
70 lines
3.6 KiB
HTML
<html>
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<head><link rel="stylesheet" type="text/css" href="help.css"/></head>
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<body>
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<h1 class='module'>D8 Flow Directions</h1>
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<div class='author'>(c) 2010 by David G. Tarboton</div>
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<div class='description'>Creates 2 grids. The first contains the flow
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direction from each grid cell to one of its adjacent or diagonal neighbors,
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calculated using the direction of steepest descent. The second contain the
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slope, as evaluated in the direction of steepest descent, and is reported
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as drop/distance, i.e. tan of the angle. Flow direction is reported as
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"no data" for any grid cell adjacent to the edge of the DEM domain,
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or adjacent to a "no data" value in the DEM. In flat areas, flow
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directions are assigned away from higher ground and towards lower ground
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using the method of Garbrecht and Martz (1997). The D8 flow direction
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algorithm may be applied to a DEM that has not had its pits filled, but
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it will then result in "no data" values for flow direction
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and slope at the lowest point of each pit.</div>
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<div class='description'>D8 Flow Direction Coding:</div>
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<ul>
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<li>1 — East</li>
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<li>2 — Northeast</li>
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<li>3 — North</li>
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<li>4 — Northwest</li>
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<li>5 — West</li>
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<li>6 — Southwest</li>
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<li>7 — South</li>
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<li>8 — Southeast</li>
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</ul>
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<p align="center"><img src="img/d8index.gif"></img></p>
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<div class='description'>The flow direction routing across flat areas is
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performed according to the method described by Garbrecht, J. and L. W.
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Martz, (1997), "The Assignment of Drainage Direction Over Flat Surfaces
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in Raster Digital Elevation Models", Journal of Hydrology, 193: 204-213.</div>
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<h2>Parameters</h2>
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<dl class='parameters'>
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<dt>Number of Processes <div class='type'>Integer</div></dt>
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<dd>The number of stripes that the domain will be divided into and the
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number of MPI parallel processes that will be spawned to evaluate each
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of the stripes.</dd>
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<dt>Pit Filled Elevation Grid <div class='type'>Raster Grid</div></dt>
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<dd>A grid of elevation values. This is usually the output of the "Pit
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Remove" tool, in which case it is elevations with pits removed.
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Pits are low elevation areas in digital elevation models (DEMs) that
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are completely surrounded by higher terrain. They are generally taken
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to be artifacts of the digitation process that interfere with the
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processing of flow across DEMs. So they are removed by raising their
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elevation to the point where they just drain off the domain. This step
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is not essential if you have reason to believe that the pits in your
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DEM are real. If a few pits actually exist and so should not be removed,
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while at the same time others are believed to be artifacts that need
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to be removed, the actual pits should have "no data" elevation
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values inserted at their lowest point. "No data" values serve
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to define edges of the domain in the flow field, and elevations are
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only raised to where flow is off an edge, so an internal "no data"
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value will stop a pit from being removed, if necessary.</dd>
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</dl>
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<h2>Outputs</h2>
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<dl class='parameters'>
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<dt>D8 Flow Direction Grid <div class='type'>Raster Grid</div></dt>
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<dd>A grid of D8 flow directions which are defined, for each cell, as
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the direction of the one of its eight adjacent or diagonal neighbors
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with the steepest downward slope.</dd>
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<dt>D8 Slope Grid <div class='type'>Raster Grid</div></dt>
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<dd>A grid giving slope in the D8 flow direction. This is measured as
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drop/distance.</dd>
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</dl>
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</body></html>
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