mirror of
https://github.com/qgis/QGIS.git
synced 2025-02-27 00:33:48 -05:00
74 lines
4.3 KiB
HTML
74 lines
4.3 KiB
HTML
<html>
|
|
<head><link rel="stylesheet" type="text/css" href="help.css"/></head>
|
|
<body>
|
|
<h1 class='module'>D8 Contributing Area</h1>
|
|
<div class='author'>(c) 2010 by David G. Tarboton</div>
|
|
<div class='description'>Calculates a grid of contributing areas using the
|
|
single direction D8 flow model. The contribution of each grid cell is taken
|
|
as one (or when the optional weight grid is used, the value from the weight
|
|
grid). The contributing area for each grid cell is taken as its own contribution
|
|
plus the contribution from upslope neighbors that drain in to it according
|
|
to the D8 flow model.</div>
|
|
<div class='description'>If the optional outlet point shapefile is used,
|
|
only the outlet cells and the cells upslope (by the D8 flow model) of them
|
|
are in the domain to be evaluated.</div>
|
|
<div class='description'>By default, the tool checks for edge contamination.
|
|
This is defined as the possibility that a contributing area value may be
|
|
underestimated due to grid cells outside of the domain not being counted.
|
|
This occurs when drainage is inwards from the boundaries or areas with
|
|
"no data" values for elevation. The algorithm recognizes this
|
|
and reports "no data" for the contributing area. It is common
|
|
to see streaks of "no data" values extending inwards from boundaries
|
|
along flow paths that enter the domain at a boundary. This is the desired
|
|
effect and indicates that contributing area for these grid cells is unknown
|
|
due to it being dependent on terrain outside of the domain of data available.
|
|
Edge contamination checking may be turned off in cases where you know this
|
|
is not an issue or want to ignore these problems, if for example, the DEM
|
|
has been clipped along a watershed outline.</div>
|
|
|
|
<h2>Parameters</h2>
|
|
<dl class='parameters'>
|
|
<dt>Number of Processes <div class='type'>Integer</div></dt>
|
|
<dd>The number of stripes that the domain will be divided into and the
|
|
number of MPI parallel processes that will be spawned to evaluate each
|
|
of the stripes.</dd>
|
|
<dt>D8 Flow Direction Grid <div class='type'>Raster Grid</div></dt>
|
|
<dd>A grid of D8 flow directions which are defined, for each cell, as
|
|
the direction of the one of its eight adjacent or diagonal neighbors
|
|
with the steepest downward slope. This grid can be obtained as the
|
|
output of the "D8 Flow Directions" tool.</dd>
|
|
<dt>Outlets Shapefile <div class='type'>Point Shapefile (optional)</div></dt>
|
|
<dd>A point shapefile defining the outlets of interest. If this input
|
|
file is used, only the cells upslope of these outlet cells are considered
|
|
to be within the domain being evaluated.</dd>
|
|
<dt>Weight Grid <div class='type'>Raster Grid (optional)</div></dt>
|
|
<dd>A grid giving contribution to flow for each cell. These contributions
|
|
(also sometimes referred to as weights or loadings) are used in the
|
|
contributing area accumulation. If this input file is not used, the
|
|
contribution to flow will assumed to be one for each grid cell.</dd>
|
|
<dt>Check for edge contamination <div class='type'>Boolean</div></dt>
|
|
<dd>A flag that indicates whether the tool should check for edge
|
|
contamination. Edge contamination is defined as the possibility that
|
|
a contributing area value may be underestimated due to the fact that
|
|
grid cells outside of the domain have not been evaluated. This occurs
|
|
when drainage is inwards from the boundaries or areas with "no data"
|
|
values for elevation. The algorithm recognizes this and reports "no data"
|
|
for the impated cells. It is common to see streaks of "no data"
|
|
values extending inwards from boundaries along flow paths that enter
|
|
the domain at a boundary. This is the desired effect and indicates that
|
|
contributing area for these grid cells is unknown due to it being
|
|
dependent on terrain outside of the domain of available data. Edge
|
|
contamination checking may be turned off in cases where you know this
|
|
is not an issue, or want to ignore these problems, if for example, the
|
|
DEM has been clipped along a watershed outline.</dd>
|
|
</dl>
|
|
|
|
<h2>Outputs</h2>
|
|
<dl class='parameters'>
|
|
<dt>D8 Contributing Area Grid <div class='type'>Raster Grid</div></dt>
|
|
<dd>A grid of contributing area values calculated as the cells own
|
|
contribution plus the contribution from upslope neighbors that drain
|
|
in to it according to the D8 flow model.</dd>
|
|
</dl>
|
|
</body></html>
|