Provided by: grass-doc_8.4.1-1_all 
NAME
r.slope.aspect - Generates raster maps of slope, aspect, curvatures and partial derivatives from an
elevation raster map.
Aspect is calculated counterclockwise from east.
KEYWORDS
raster, terrain, aspect, slope, curvature, parallel
SYNOPSIS
r.slope.aspect
r.slope.aspect --help
r.slope.aspect [-aen] elevation=name [slope=name] [aspect=name] [format=string] [precision=string]
[pcurvature=name] [tcurvature=name] [dx=name] [dy=name] [dxx=name] [dyy=name] [dxy=name]
[zscale=float] [min_slope=float] [nprocs=integer] [memory=memory in MB] [--overwrite] [--help]
[--verbose] [--quiet] [--ui]
Flags:
-a
Do not align the current region to the raster elevation map
-e
Compute output at edges and near NULL values
-n
Create aspect as degrees clockwise from North (azimuth), with flat = -9999
Default: degrees counter-clockwise from East, with flat = 0
--overwrite
Allow output files to overwrite existing files
--help
Print usage summary
--verbose
Verbose module output
--quiet
Quiet module output
--ui
Force launching GUI dialog
Parameters:
elevation=name [required]
Name of input 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
precision=string
Type of output aspect and slope maps
Storage type for resultant raster map
Options: CELL, FCELL, DCELL
Default: FCELL
CELL: Integer
FCELL: Single precision floating point
DCELL: Double precision floating point
pcurvature=name
Name for output profile curvature raster map
tcurvature=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
zscale=float
Multiplicative factor to convert elevation units to horizontal units
Default: 1.0
min_slope=float
Minimum slope value (in percent) for which aspect is computed
Default: 0.0
nprocs=integer
Number of threads for parallel computing
Default: 1
memory=memory in MB
Maximum memory to be used (in MB)
Cache size for raster rows
Default: 300
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 raster
map and at least one output raster maps. The user can also specify the format for slope (degrees,
percent; default=degrees), and the zscale: multiplicative factor to convert elevation units to horizontal
units; (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 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)))"
Alternatively, the -n flag can be used to produce aspect as degrees CW from North. Aspect for flat areas
is then set to -9999 (default: 0). Note: The reason for using -9999 is to be compliant with gdaldem which
uses -9999 by default as the nodata value.
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.
Example DEM
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
NOTES
To ensure that the raster elevation map 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 raster 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 raster 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 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.
PERFORMANCE
To enable parallel processing, the user can specify the number of threads to be used with the nprocs
parameter (default 1). The memory parameter (default 300) can also be provided to determine the size of
the buffer for computation.
Figure: Benchmark on the left shows execution time for different number of cells, benchmark on the right
shows execution time for different memory size for 5000x5000 raster. See benchmark scripts in source
code. (Intel Core i9-10940X CPU @ 3.30GHz x 28)
To reduce the memory requirements to minimum, set option memory to zero. To take advantage of the
parallelization, GRASS GIS needs to compiled with OpenMP enabled.
EXAMPLES
Calculation of slope, aspect, profile and tangential curvature
In this example a slope, aspect, profile and tangential curvature map are computed from an elevation
raster map (North Carolina sample dataset):
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)
Classification of major aspect directions in compass orientation
In the following example (based on the North Carolina sample dataset) we first generate the standard
aspect map (oriented CCW from East), then convert it to compass orientation, and finally classify four
major aspect directions (N, E, S, W):
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
Aspect map classified to four major compass directions (zoomed subset shown)
REFERENCES
• 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, doi:10.1016/S0166-2481(08)00017-2,
https://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
SOURCE CODE
Available at: r.slope.aspect source code (history)
Accessed: Friday Apr 04 01:19:33 2025
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GRASS 8.4.1 r.slope.aspect(1grass)