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r.slope.aspect

Generates raster maps of slope, aspect, curvatures and partial derivatives from an elevation raster map.

Aspect is calculated counterclockwise from east.

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] [--verbose] [--quiet] [--qq] [--ui]

Example:

r.slope.aspect elevation=name slope=name

grass.script.run_command("r.slope.aspect", elevation, slope=None, aspect=None, format="degrees", precision="FCELL", pcurvature=None, tcurvature=None, dx=None, dy=None, dxx=None, dyy=None, dxy=None, zscale=1.0, min_slope=0.0, nprocs=1, memory=300, flags=None, overwrite=False, verbose=False, quiet=False, superquiet=False)

Example:

gs.run_command("r.slope.aspect", elevation="name", slope="name")

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
    Allowed values: degrees, percent
    Default: degrees
precision=string
    Type of output aspect and slope maps
    Storage type for resultant raster map
    Allowed values: 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
-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
--qq
    Very quiet module output
--ui
    Force launching GUI dialog

elevation : str, required
    Name of input elevation raster map
    Used as: input, raster, name
slope : str, optional
    Name for output slope raster map
    Used as: output, raster, name
aspect : str, optional
    Name for output aspect raster map
    Used as: output, raster, name
format : str, optional
    Format for reporting the slope
    Allowed values: degrees, percent
    Default: degrees
precision : str, optional
    Type of output aspect and slope maps
    Storage type for resultant raster map
    Allowed values: CELL, FCELL, DCELL
    CELL: Integer
    FCELL: Single precision floating point
    DCELL: Double precision floating point
    Default: FCELL
pcurvature : str, optional
    Name for output profile curvature raster map
    Used as: output, raster, name
tcurvature : str, optional
    Name for output tangential curvature raster map
    Used as: output, raster, name
dx : str, optional
    Name for output first order partial derivative dx (E-W slope) raster map
    Used as: output, raster, name
dy : str, optional
    Name for output first order partial derivative dy (N-S slope) raster map
    Used as: output, raster, name
dxx : str, optional
    Name for output second order partial derivative dxx raster map
    Used as: output, raster, name
dyy : str, optional
    Name for output second order partial derivative dyy raster map
    Used as: output, raster, name
dxy : str, optional
    Name for output second order partial derivative dxy raster map
    Used as: output, raster, name
zscale : float, optional
    Multiplicative factor to convert elevation units to horizontal units
    Default: 1.0
min_slope : float, optional
    Minimum slope value (in percent) for which aspect is computed
    Default: 0.0
nprocs : int, optional
    Number of threads for parallel computing
    Default: 1
memory : int, optional
    Maximum memory to be used (in MB)
    Cache size for raster rows
    Used as: memory in MB
    Default: 300
flags : str, optional
    Allowed values: a, e, n
    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: bool, optional
    Allow output files to overwrite existing files
    Default: False
verbose: bool, optional
    Verbose module output
    Default: False
quiet: bool, optional
    Quiet module output
    Default: False
superquiet: bool, optional
    Very quiet module output
    Default: False

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.

aspect_diagram

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.

dem
Example DEM

slope
Slope (degree) from example DEM

aspect
Aspect (degree) from example DEM

tcurv
Tangential curvature (1/m) from example DEM

pcurv
Profile curvature (1/m) 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.

benchmark for number of cells benchmark for memory size
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

r_slope_aspect_slope r_slope_aspect_aspect r_slope_aspect_pcurv r_slope_aspect_tcurv

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
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)
Latest change: Sunday Apr 06 10:10:58 2025 in commit 68cf12d