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r.slope.aspect generates raster maps of slope, aspect, curvatures and
first and second order partial derivatives from a raster map of true
elevation values. The user must specify the input elevation file name
and at least one output file name. The user can also specify the format
for slope (degrees, percent; default=degrees), and the zfactor:
multiplicative factor to convert elevation units to meters; (default
1.0).
The elevation input raster map specified by the user must contain true
elevation values, not rescaled or categorized data. If the elevation
values are in feet or other units than meters (with a conversion factor
meters:, defined in PROJ_UNITS), they must be converted to meters using
the parameter zfactor.
The aspect output raster map indicates the direction that slopes are
facing. The aspect categories represent the number degrees of east.
Category and color table files are also generated for the aspect map
layer. The aspect categories represent the number degrees of east and
they increase counterclockwise: 90deg is North, 180 is West, 270 is
South 360 is East. The aspect value 0 is used to indicate undefined
aspect in flat areas with slope=0.
The slope output raster map contains slope values, stated in degrees of
inclination from the horizontal if format=degrees option (the default)
is chosen, and in percent rise if format=percent option is chosen.
Category and color table files are generated.
Profile and tangential curvatures are the curvatures in the direction
of steepest slope and in the direction of the contour tangent
respectively. The curvatures are expressed as 1/metres, e.g. a
curvature of 0.05 corresponds to a radius of curvature of 20m.
For some applications, the user will wish to use a reclassified raster
map of slope that groups slope values into ranges of slope. This can be
done using [1]r.reclass. An example of a useful reclassification is
given below:
category range category labels
(in degrees) (in percent)
1 0- 1 0- 2%
2 2- 3 3- 5%
3 4- 5 6- 10%
4 6- 8 11- 15%
5 9- 11 16- 20%
6 12- 14 21- 25%
7 15- 90 26% and higher
The following color table works well with the above
reclassification.
category red green blue
0 179 179 179
1 0 102 0
2 0 153 0
3 128 153 0
4 204 179 0
5 128 51 51
6 255 0 0
7 0 0 0
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To ensure that the raster elevation map layer is not inappropriately
resampled, the settings for the current region are modified slightly
(for the execution of the program only): the resolution is set to match
the resolution of the elevation map and the edges of the region (i.e.
the north, south, east and west) are shifted, if necessary, to line up
along edges of the nearest cells in the elevation map. If the user
really wants the elevation map resampled to the current region
resolution, the -a flag should be specified.
The current mask is ignored.
The algorithm used to determine slope and aspect uses a 3x3
neighborhood around each cell in the elevation file. Thus, it is not
possible to determine slope and aspect for the cells adjacent to the
edges in the elevation map layer. These cells are assigned a "zero
slope" value (category 0) in both the slope and aspect raster map
layers.
Horn's formula is used to find the derivatives in x and y directions.
Only when using integer elevation models, the aspect is biased in 0,
45, 90, 180, 225, 270, 315, and 360 directions; i.e., the distribution
of aspect categories is very uneven, with peaks at 0, 45,..., 360
categories. When working with floating point elevation models, no such
aspect bias occurs.
Because most cells with a very small slope end up having category 0,
45, ..., 360, it is sometimes possible to reduce the bias in these
directions by filtering out the aspect in areas where the terrain is
almost flat. A new option min_slp_allowed was added to specify the
minimum slope for which aspect is computed. The aspect for all cells
with slope < min_slp_allowed is set to null.
ССЫÐÐÐ
Horn, B. K. P. (1981). Hill Shading and the Reflectance Map,
Proceedings of the IEEE, 69(1):14-47.
СÐ. ТÐÐÐÐ
[2]r.mapcalc, [3]r.neighbors, [4]r.reclass, [5]r.rescale
ÐÐТÐРЫ
Michael Shapiro, U.S.Army Construction Engineering Research Laboratory
Olga Waupotitsch, U.S.Army Construction Engineering Research Laboratory
Last changed: $Date: 2006-11-19 02:50:25 -0600 (ÐÑ, 19 Ð½Ð¾Ñ 2006) $
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References
1. file://localhost/root/tmp/2/fin/r.reclass.html
2. file://localhost/root/tmp/2/fin/r.mapcalc.html
3. file://localhost/root/tmp/2/fin/r.neighbors.html
4. file://localhost/root/tmp/2/fin/r.reclass.html
5. file://localhost/root/tmp/2/fin/r.rescale.html