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<b>r.slope.aspect </b>generates raster maps of slope, aspect, curvatures and
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first and second order partial derivatives from a raster map of true
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elevation values. The user must specify the input <i>elevation</i> file name
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and at least one output file name. The user can also specify the
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<i>format</i> for slope (degrees, percent; default=degrees), and the
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<i>zfactor</i>: multiplicative factor to convert elevation units to meters;
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The <i>elevation</i> input raster map specified by the user must contain true
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elevation values, <b>not</b> rescaled or categorized data. If the elevation
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values are in feet or other units than meters (with a conversion factor
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<i>meters:</i>, defined in PROJ_UNITS), they must be converted to meters using
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the parameter <i>zfactor</i>.
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The <i>aspect</i> output raster map indicates the direction that slopes are
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facing. The aspect categories represent the number degrees of east. Category
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and color table files are also generated for the aspect map layer. The aspect
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categories represent the number degrees of east and they increase
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counterclockwise: 90deg is North, 180 is West, 270 is South 360 is East. The
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aspect value 0 is used to indicate undefined aspect in flat areas with slope=0.
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The <i>slope</i> output raster map contains slope values, stated in degrees of
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inclination from the horizontal if <i>format</i>=degrees option (the default)
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is chosen, and in percent rise if <i>format</i>=percent option is chosen.
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Category and color table files are generated.
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Profile and tangential curvatures are the curvatures in the direction of
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steepest slope and in the direction of the contour tangent respectively. The
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curvatures are expressed as 1/metres, e.g. a curvature of 0.05 corresponds to a
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radius of curvature of 20m. Convex form values are positive and concave form values
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<table width="100%" border="0">
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<tr valign="baseline">
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<img src="dem.png" border="1">
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<tr valign="baseline">
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<img src="slope.png" border="1">
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Slope (degree) from example DEM
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<img src="aspect.png" border="1">
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Aspect (degree) from example DEM
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<tr valign="baseline">
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<img src="tcurv.png" border="1">
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Tangential curvature (m<sup>-1</sup>) from example DEM
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<img src="pcurv.png" border="1">
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Profile curvature (m<sup>-1</sup>) from example DEM
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For some applications, the user will wish to use a reclassified raster map
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of slope that groups slope values into ranges of slope. This can be done using
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<i><a href="r.reclass.html">r.reclass</a></i>. An example of a useful
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reclassification is given below:
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<div class="code"><pre> category range category labels
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(in degrees) (in percent)
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7 15- 90 26% and higher
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The following color table works well with the above
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category red green blue
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To ensure that the raster elevation map layer is not inappropriately resampled,
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the settings for the current region are modified slightly (for the execution
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of the program only): the resolution is set to match the resolution of
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the elevation map and the edges of the region (i.e. the north, south, east
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and west) are shifted, if necessary, to line up along edges of the nearest
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cells in the elevation map. If the user really wants the elevation map
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resampled to the current region resolution, the -a flag should be specified.
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The current mask is ignored.
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The algorithm used to determine slope and aspect uses a 3x3 neighborhood
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around each cell in the elevation file. Thus, it is not possible to determine
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slope and aspect for the cells adjacent to the edges in the elevation map
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layer. These cells are assigned a "zero slope" value (category 0) in both
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the slope and aspect raster map layers.
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Horn's formula is used to find the first order derivatives in x and y directions.
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Only when using integer elevation models, the aspect is biased in 0,
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45, 90, 180, 225, 270, 315, and 360 directions; i.e., the distribution
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of aspect categories is very uneven, with peaks at 0, 45,..., 360 categories.
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When working with floating point elevation models, no such aspect bias occurs.
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Because most cells with a very small slope end up having category 0,
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45, ..., 360, it is sometimes possible to reduce the bias in these directions
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by filtering out the aspect in areas where the terrain is almost flat. A new
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option <i>min_slp_allowed</i> was added to specify the minimum slope for which
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aspect is computed. The aspect for all cells with slope <
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<i>min_slp_allowed</i> is set to <b>null</b>.
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<li> Horn, B. K. P. (1981). <i>Hill Shading and the Reflectance Map</i>, Proceedings
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of the IEEE, 69(1):14-47.
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<li> Mitasova, H. (1985). <i>Cartographic aspects of computer surface modeling. PhD thesis.</i>
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Slovak Technical University , Bratislava
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<li> Hofierka, J., Mitasova, H., Neteler, M., 2009. <i>Geomorphometry in GRASS GIS.</i>
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In: Hengl, T. and Reuter, H.I. (Eds), <i>Geomorphometry: Concepts, Software, Applications. </i>
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Developments in Soil Science, vol. 33, Elsevier, 387-410 pp,
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<a href="http://www.geomorphometry.org">http://www.geomorphometry.org</a>
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<em><a href="r.mapcalc.html">r.mapcalc</a></em>,
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<em><a href="r.neighbors.html">r.neighbors</a></em>,
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<em><a href="r.reclass.html">r.reclass</a></em>,
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<em><a href="r.rescale.html">r.rescale</a></em>
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Michael Shapiro, U.S.Army Construction Engineering Research Laboratory<br>
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Olga Waupotitsch, U.S.Army Construction Engineering Research Laboratory
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<i>Last changed: $Date: 2011-11-08 12:29:50 +0100 (Tue, 08 Nov 2011) $</i>