Geografic-Information-System/raster/r.slope.aspect/r.slope.aspect.html

<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.

<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 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 &lt;
<i>min_slp_allowed</i> is set to <b>null</b>.


<h2>REFERENCE</h2>

Horn, B. K. P. (1981). Hill Shading and the Reflectance Map, Proceedings
of the IEEE, 69(1):14-47.

<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>