r.slope.aspect - Generates raster map layers of slope, aspect, curvatures and partial derivatives from a raster map layer of true elevation values. Aspect is calculated counterclockwise from east.
r.slope.aspect [-aq] elevation=string [slope=string] [format=string] [prec=string] [aspect=string] [pcurv=string] [tcurv=string] [dx=string] [dy=string] [dxx=string] [dyy=string] [dxy=string] [zfactor=float] [min_slp_allowed=float] [--overwrite]
- Do not align the current region to the elevation layer
- Force overwrite of output files
- Raster elevation file name
- Output slope filename
- Format for reporting the slope
- Options: degrees,percent
- Default: degrees
- Type of output aspect and slope maps
- Options: default,double,float,int
- Default: float
- Output aspect filename
- Output profile curvature filename
- Output tangential curvature filename
- Output first order partial derivative dx (E-W slope) filename
- Output first order partial derivative dy (N-S slope) filename
- Output second order partial derivative dxx filename
- Output second order partial derivative dyy filename
- Output second order partial derivative dxy filename
- Multiplicative factor to convert elevation units to meters
- Default: 1.0
- Minimum slope val. (in percent) for which aspect is computed
- Default: 0.0
r.slope.aspect generates raster maps of slope, aspect, curvatures and
first and second order partial derivatives from a raster map layer 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
zfactor: multiplicative factor to convert elevation units to meters;
The raster elevation map layer 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 defined in PROJ_UNITS), they must be converted to
meters using the parameter zfactor.
Do not align the settings of the current geographic region (to which the
output slope and aspect map layers will be set) to those of the elevation
layer. See NOTES.
The raster aspect map layer which is created 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, and the aspect value 0 is used to indicate undefined aspect in
flat areas with slope=0.
The resulting raster slope map layer will contain slope values,
stated in degrees of inclination from the horizontal if format=degrees
option (which is also default ) is chosen, and in percent rise if format=percent
option is chosen. The category file, but not the color table, is generated
by r.slope.aspect for the raster slope map layer.Profile and tangential
curvatures are the curvatures in the direction of steepest slope and in
the direction of the contour tangent respectively.
For some applications, the user will wish to use a reclassified map
layer 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
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
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, if set, 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.
Because most cells with a very small slope end up having category 0,
45,..., 360 it is sometimes possible to reduce bias in these directions
by filtering out computation of aspect in areas where terrain is almost
flat. The new option
was added (minimum slope for which aspect is computed). The aspect for
all cells with slope < min_slp is set to 0 (no value).
When working with floating point elevation models, no such aspect bias
Horn, B. K. P. (1981). Hill Shading and the Reflectance Map, Proceedings
of the IEEE, 69(1):14-47.
Michael Shapiro, U.S.Army Construction Engineering Research Laboratory
Olga Waupotitsch, U.S.Army Construction Engineering Research Laboratory
Last changed: $Date: 2006/04/25 18:37:47 $
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