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- <h2>DESCRIPTION</h2>
- <b>r.slope.aspect </b>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 <i>elevation</i> file name
- and at least one output file name. The user can also specify the
- <i>format</i> for slope (degrees, percent; default=degrees), and the
- <i>zfactor</i>: multiplicative factor to convert elevation units to meters;
- (default 1.0).
- <p>
- The <i>elevation</i> input raster map specified by the user must contain true
- elevation values, <b>not</b> rescaled or categorized data. If the elevation
- values are in feet or other units than meters (with a conversion factor
- <i>meters:</i>, defined in PROJ_UNITS), they must be converted to meters using
- the parameter <i>zfactor</i>.
- <p>
- The <i>aspect</i> 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.
- <p>
- The <i>slope</i> output raster map contains slope values, stated in degrees of
- inclination from the horizontal if <i>format</i>=degrees option (the default)
- is chosen, and in percent rise if <i>format</i>=percent option is chosen.
- Category and color table files are generated.
- <p>
- 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.
- <p><table width="100%" border="0">
- <tr valign="baseline">
- <td>
- <center>
- <img src="dem.png" border="1">
- <p> Example DEM
- <br><br>
- </center>
- </td>
- <td>
- </td>
- </tr>
- <tr valign="baseline">
- <td>
- <center>
- <img src="slope.png" border="1">
- <p> Slope (degree) from example DEM
- <br><br>
- </center>
- </td>
- <td>
- <center>
- <img src="aspect.png" border="1">
- <p> Aspect (degree) from example DEM
- <br><br>
- </center>
- </td>
- </tr>
- <tr valign="baseline">
- <td>
- <center>
- <img src="tcurv.png" border="1">
- <p> Tangential curvature (m<sup>-1</sup>) from example DEM
- <br><br>
- </center>
- </td>
- <td>
- <center>
- <img src="pcurv.png" border="1">
- <p> Profile curvature (m<sup>-1</sup>) from example DEM
- <br><br>
- </center>
- </td>
- <td>
- </td>
- </tr>
- </table>
- <p>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
- <i><a href="r.reclass.html">r.reclass</a></i>. An example of a useful
- reclassification is given below:
- <div class="code"><pre> 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</pre></div>
- <h2>NOTES</h2>
- To ensure that the raster elevation map layer 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 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 elevation map
- resampled to the current region resolution, the -a flag should be specified.
- <p>
- The current mask is ignored.
- <p>
- 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.
- <p>
- Horn's formula is used to find the first order derivatives in x and y directions.
- <p>
- 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.
- <p>
- 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 <i>min_slp_allowed</i> was added to specify the minimum slope for which
- aspect is computed. The aspect for all cells with slope <
- <i>min_slp_allowed</i> is set to <b>null</b>.
- <h2>REFERENCE</h2>
- <ul>
- <li> Horn, B. K. P. (1981). <i>Hill Shading and the Reflectance Map</i>, Proceedings
- of the IEEE, 69(1):14-47.
- <li> Mitasova, H. (1985). <i>Cartographic aspects of computer surface modeling. PhD thesis.</i>
- Slovak Technical University , Bratislava
- <li> Hofierka, J., Mitasova, H., Neteler, M., 2009. <i>Geomorphometry in GRASS GIS.</i>
- In: Hengl, T. and Reuter, H.I. (Eds), <i>Geomorphometry: Concepts, Software, Applications. </i>
- Developments in Soil Science, vol. 33, Elsevier, 387-410 pp,
- <a href="http://www.geomorphometry.org">http://www.geomorphometry.org</a>
- </ul>
- <h2>SEE ALSO</h2>
- <em><a href="r.mapcalc.html">r.mapcalc</a></em>,
- <em><a href="r.neighbors.html">r.neighbors</a></em>,
- <em><a href="r.reclass.html">r.reclass</a></em>,
- <em><a href="r.rescale.html">r.rescale</a></em>
- <h2>AUTHORS</h2>
- Michael Shapiro, U.S.Army Construction Engineering Research Laboratory<br>
- Olga Waupotitsch, U.S.Army Construction Engineering Research Laboratory
- <p>
- <i>Last changed: $Date$</i>
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