Provided by: grass-doc_6.4.3-3_all 
NAME
r.slope.aspect - Generates raster maps of slope, aspect, curvatures and partial
derivatives from a elevation raster map.
Aspect is calculated counterclockwise from east.
KEYWORDS
raster, terrain
SYNOPSIS
r.slope.aspect
r.slope.aspect help
r.slope.aspect [-qa] elevation=name [slope=name] [aspect=name] [format=string]
[prec=string] [pcurv=name] [tcurv=name] [dx=name] [dy=name] [dxx=name]
[dyy=name] [dxy=name] [zfactor=float] [min_slp_allowed=float] [--overwrite]
[--verbose] [--quiet]
Flags:
-q
Quiet
-a
Do not align the current region to the elevation layer
--overwrite
Allow output files to overwrite existing files
--verbose
Verbose module output
--quiet
Quiet module output
Parameters:
elevation=name
Name of elevation raster map
slope=name
Name for output slope raster map
aspect=name
Name for output aspect raster map
format=string
Format for reporting the slope
Options: degrees,percent
Default: degrees
prec=string
Type of output aspect and slope maps
Options: default,double,float,int
Default: float
pcurv=name
Name for output profile curvature raster map
tcurv=name
Name for output tangential curvature raster map
dx=name
Name for output first order partial derivative dx (E-W slope) raster map
dy=name
Name for output first order partial derivative dy (N-S slope) raster map
dxx=name
Name for output second order partial derivative dxx raster map
dyy=name
Name for output second order partial derivative dyy raster map
dxy=name
Name for output second order partial derivative dxy raster map
zfactor=float
Multiplicative factor to convert elevation units to meters
Default: 1.0
min_slp_allowed=float
Minimum slope val. (in percent) for which aspect is computed
Default: 0.0
DESCRIPTION
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. Convex
form values are positive and concave form values are negative.
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Example DEM
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Slope (degree) from example DEM
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Aspect (degree) from example DEM
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Tangential curvature (m-1) from example DEM
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Profile curvature (m-1) from example DEM
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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:
(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
NOTES
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 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.
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.
REFERENCE
Horn, B. K. P. (1981). Hill Shading and the Reflectance Map, Proceedings of
the IEEE, 69(1):14-47.
Mitasova, H. (1985). Cartographic aspects of computer surface modeling. PhD
thesis. Slovak Technical University , Bratislava
Hofierka, J., Mitasova, H., Neteler, M., 2009. Geomorphometry in GRASS GIS.
In: Hengl, T. and Reuter, H.I. (Eds), Geomorphometry: Concepts, Software,
Applications. Developments in Soil Science, vol. 33, Elsevier, 387-410 pp,
http://www.geomorphometry.org
SEE ALSO
r.mapcalc, r.neighbors, r.reclass, r.rescale
AUTHORS
Michael Shapiro, U.S.Army Construction Engineering Research Laboratory
Olga Waupotitsch, U.S.Army Construction Engineering Research Laboratory
Last changed: $Date: 2011-11-08 03:29:50 -0800 (Tue, 08 Nov 2011) $
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