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 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
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.
Olga Waupotitsch, U.S.Army Construction Engineering Research Laboratory
Last changed: $Date: 2006-11-19 00:50:25 -0800 (Sun, 19 Nov 2006) $
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