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NAME

KEYWORDS

SYNOPSIS

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 value (in percent) for which aspect is computed

Default: 0.0

DESCRIPTION

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.

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

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

AUTHORS

Last changed: $Date: 2011-11-08 03:29:50 -0800 (Tue, 08 Nov 2011) $

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