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78 lines
4.5 KiB
HTML
78 lines
4.5 KiB
HTML
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<head><link rel="stylesheet" type="text/css" href="help.css"/></head>
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<h1 class='module'>D-Infinity Flow Directions</h1>
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<div class='author'>(c) 2010 by David G. Tarboton</div>
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<div class='description'>Assigns a flow direction based on the D-infinity
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flow method using the steepest slope of a triangular facet (Tarboton, 1997,
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"A New Method for the Determination of Flow Directions and Contributing
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Areas in Grid Digital Elevation Models", Water Resources Research,
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33(2): 309-319). Flow direction is defined as steepest downward slope on
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planar triangular facets on a block centered grid. Flow direction is encoded
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as an angle in radians counter-clockwise from east as a continuous (floating
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point) quantity between 0 and 2π. The flow direction angle is determined
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as the direction of the steepest downward slope on the eight triangular
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facets formed in a 3 x 3 grid cell window centered on the grid cell of
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interest. The resulting flow in a grid is then usually interpreted as being
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proportioned between the two neighboring cells that define the triangular
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facet with the steepest downward slope.</div>
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<p align="center"><img src="img/tardemfig.gif"></img></p>
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<div class='description'>A block-centered representation is used with each
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elevation value taken to represent the elevation of the center of the
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corresponding grid cell. Eight planar triangular facets are formed between
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each grid cell and its eight neighbors. Each of these has a downslope vector
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which when drawn outwards from the center may be at an angle that lies within
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or outside the 45 degree (π/4 radian) angle range of the facet at the center
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point. If the slope vector angle is within the facet angle, it represents
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the steepest flow direction on that facet. If the slope vector angle is
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outside a facet, the steepest flow direction associated with that facet
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is taken along the steepest edge. The slope and flow direction associated
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with the grid cell is taken as the magnitude and direction of the steepest
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downslope vector from all eight facets. Slope is measured as drop/distance,
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i.e. tan of the slope angle.</div>
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<div class='description'>In the case where no slope vectors are positive
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(downslope), the flow direction is set using the method of Garbrecht and
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Martz (1997) for the determination of flow across flat areas. This makes
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flat areas drain away from high ground and towards low ground. The flow
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path grid to enforce drainage along existing streams is an optional input,
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and if used, takes precedence over elevations for the setting of flow
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directions.</div>
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<div class='description'>The D-infinity flow direction algorithm may be
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applied to a DEM that has not had its pits filled, but it will then result
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in "no data" values for the D-infinity flow direction and slope
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associated with the lowest point of the pit.</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
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"Pit Remove" tool, in which case it is elevations with pits
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removed.</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>D-Infinity Flow Direction Grid <div class='type'>Raster Grid</div></dt>
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<dd>A grid of flow directions based on the D-infinity flow method using
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the steepest slope of a triangular facet. Flow direction is determined
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as the direction of the steepest downward slope on the 8 triangular
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facets of a 3 x 3 block centered grid. Flow direction is encoded as
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an angle in radians, counter-clockwise from east as a continuous
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(floating point) quantity between 0 and 2π. The resulting flow in
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a grid is then usually interpreted as being proportioned between the
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two neighboring cells that define the triangular facet with the steepest
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downward slope.</dd>
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<dt>D-Infinity Slope Grid <div class='type'>Raster Grid</div></dt>
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<dd>A grid of slope evaluated using the D-infinity method described in
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Tarboton, D. G., (1997), "A New Method for the Determination of
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Flow Directions and Contributing Areas in Grid Digital Elevation Models",
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Water Resources Research, 33(2): 309-319. This is the steepest outwards
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slope on one of eight triangular facets centered at each grid cell,
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measured as drop/distance, i.e. tan of the slope angle.</dd>
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</dl>
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</body></html>
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