r.slope.aspectgrass - Online in the Cloud

PROGRAM:

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

SYNOPSIS

r.slope.aspect
r.slope.aspect --help
r.slope.aspect [-a] 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] [--overwrite]
[--help] [--verbose] [--quiet] [--ui]

Flags:
-a
Do not align the current region to the raster elevation map

--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
Options: CELL, FCELL, DCELL
Default: FCELL

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

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. The aspect
categories represent the number degrees of east. Category and color table files are also
generated for the aspect raster map. The aspect categories represent the number degrees of
east and they increase counterclockwise: 90 degrees is North, 180 is West, 270 is South
360 is East.
Note: These values can be transformed to azimuth (0 is North, 90 is East, etc) values
using r.mapcalc:
# convert angles from CCW to north up
r.mapcalc "azimuth_aspect = (450 - ccw_aspect) % 360"

The aspect is not defined for slope equal to zero. 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. The aspect for all cells with slope < min_slope is set to null (no-data).

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, 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 maps.

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.

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 aspect map with CCW orientation
r.slope.aspect elevation=elevation aspect=myaspect
# 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,
http://www.geomorphometry.org

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