Ubuntu Manpage: r.slope.aspect - Generates raster maps of slope, aspect, curvatures and partial derivatives from an

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Provided by: grass-doc_7.0.3-1build1_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

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

AUTHORS

       Michael Shapiro, U.S.Army Construction Engineering Research Laboratory
       Olga Waupotitsch, U.S.Army Construction Engineering Research Laboratory

       Last changed: $Date: 2015-08-12 11:48:51 +0200 (Wed, 12 Aug 2015) $

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