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 other units than in the horizontal units, they must be converted to horizontal units using the parameter zscale. In GRASS GIS 7, vertical units are not assumed to be meters any more. For example, if both your vertical and horizontal units are feet, parameter zscale must not be used.
The aspect output raster map indicates the direction that slopes
are facing counterclockwise from East: 90 degrees is North, 180 is
West, 270 is South, 360 is East. Zero aspect indicates flat areas with
zero slope. Category and color table files are also generated for the
aspect raster map.
Note: These values can be transformed to azimuth values (90 is East, 180 is South, 270 is West, 360 is North) using r.mapcalc:
# convert angles from CCW from East to CW from North # modulus (%) can not be used with floating point aspect values r.mapcalc "azimuth_aspect = if(ccw_aspect == 0, 0, \ if(ccw_aspect < 90, 90 - ccw_aspect, \ 450 - ccw_aspect)))"
The aspect for slope equal to zero (flat areas) is set to zero (-9999 with -n flag). Thus, most cells with a very small slope end up having category 0, 45, ..., 360 in aspect output. It is possible to reduce the bias in these directions by filtering out the aspect in areas where the terrain is almost flat. A option min_slope can be used to specify the minimum slope for which aspect is computed. For all cells with slope < min_slope, both slope and aspect are set to zero.
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. Convex form values are positive and concave form values are negative.
Slope (degree) from example DEM
Aspect (degree) from example DEM
Tangential curvature (m-1) from example DEM
Profile curvature (m-1) from example DEM
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 raster elevation map. Thus, slope and aspect are not determineed for cells adjacent to the edges and NULL cells in the elevation map layer. These cells are by default set to nodata in output raster maps. With the -e flag, output values are estimated for these cells, avoiding cropping along the edges.
Horn's formula is used to find the first order 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.
g.region raster=elevation r.slope.aspect elevation=elevation slope=slope aspect=aspect pcurvature=pcurv tcurvature=tcurv # set nice color tables for output raster maps r.colors -n map=slope color=sepia r.colors map=aspect color=aspectcolr r.colors map=pcurv color=curvature r.colors map=tcurv color=curvature
Figure: Slope, aspect, profile and tangential curvature raster map (North Carolina dataset)
g.region raster=elevation -p # generate integer aspect map with degrees CCW from East r.slope.aspect elevation=elevation aspect=myaspect precision=CELL # generate compass orientation and classify four major directions (N, E, S, W) r.mapcalc "aspect_4_directions = eval( \\ compass=(450 - myaspect ) % 360, \\ if(compass >=0. && compass < 45., 1) \\ + if(compass >=45. && compass < 135., 2) \\ + if(compass >=135. && compass < 225., 3) \\ + if(compass >=225. && compass < 315., 4) \\ + if(compass >=315., 1) \\ )" # assign text labels r.category aspect_4_directions separator=comma rules=- << EOF 1,north 2,east 3,south 4,west EOF # assign color table r.colors aspect_4_directions rules=- << EOF 1 253,184,99 2 178,171,210 3 230,97,1 4 94,60,153 EOF
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