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

<h2>DESCRIPTION</h2>

<em>r.slope.aspect</em> 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 <b>elevation</b> raster map
and at least one output raster maps. The user can also specify the
<b>format</b> for slope (degrees, percent; default=degrees), and the 
<b>zscale</b>: multiplicative factor to convert elevation units to horizontal units;
(default 1.0).

<p>
The <b>elevation</b> input raster map specified by the user must contain true
elevation values, <em>not</em> 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 <b>zscale</b>.
<em>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 <b>zscale</b> must not be used</em>.

<p>
The <b>aspect</b> output raster map indicates the direction that slopes 
are facing counterclockwise from East: 90 degrees is North, 180 is 
West, 270 is South, 360 is East. Zero aspect indicates flat areas with 
zero slope. Category and color table files are also generated for the 
aspect raster map. <br> Note: These values can be transformed to 
azimuth values (90 is East, 180 is South, 270 is West, 360 is North) 
using <a href="r.mapcalc.html">r.mapcalc</a>:

<div class="code"><pre>
# convert angles from CCW from East to CW from North
# modulus (%) can not be used with floating point aspect values
r.mapcalc "azimuth_aspect = if(ccw_aspect == 0, 0, \
                            if(ccw_aspect < 90, 90 - ccw_aspect, \
                            450 - ccw_aspect)))"
</pre></div>

Alternatively, the <b>-n</b> flag can be used to produce aspect as 
degrees CW from North. Aspect for flat areas is then set to -9999 
(default: 0). Note: The reason for using -9999 is to be compliant with
<b>gdaldem</b> which uses -9999 by default as the nodata value.

<p>
The aspect for slope equal to zero (flat areas) is set to zero (-9999 
with <b>-n</b> flag). Thus, most cells with a very small slope end up 
having category 0, 45, ..., 360 in <b>aspect</b> 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 <b>min_slope</b> can 
be used to specify the minimum slope for which aspect is computed. For 
all cells with slope&nbsp;&lt;&nbsp;<b>min_slope</b>, both slope and 
aspect are set to zero.

<center>
  <img src="aspect_diagram.png" border="0">
</center>

<p>
The <b>slope</b> output raster map contains slope values, stated in degrees of
inclination from the horizontal if <b>format</b>=degrees option (the default)
is chosen, and in percent rise if <b>format</b>=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
<em><a href="r.reclass.html">r.reclass</a></em>. 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 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 <b>-a</b> 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 raster elevation map. Thus, 
slope and aspect are not determineed for cells adjacent to 
the edges and NULL cells in the elevation map layer. These cells are 
by default set to nodata in output raster maps. With the <b>-e</b> flag, 
output values are estimated for these cells, avoiding cropping along 
the edges.

<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.

<h3>PERFORMANCE</h3>
To enable parallel processing, the user can specify the number of threads to be
used with the <b>nprocs</b> parameter (default 1). The <b>memory</b> parameter
(default 300) can also be provided to determine the size of the buffer for 
computation.

<div align="center" style="margin: 10px">
     <img src="r_slope_aspect_benchmark_size.png" alt="benchmark for number of cells" border="0">
     <img src="r_slope_aspect_benchmark_memory.png" alt="benchmark for memory size" border="0">
     <br>
     <i>Figure: Benchmark on the left shows execution time for different
     number of cells, benchmark on the right shows execution time
     for different memory size for 5000x5000 raster. See benchmark scripts in source code.
     (Intel Core i9-10940X CPU @ 3.30GHz x 28) </i>
     </div>

<p>To reduce the memory requirements to minimum, set option <b>memory</b> to zero.
To take advantage of the parallelization, GRASS GIS
needs to compiled with OpenMP enabled.

<h2>EXAMPLES</h2>

<h3>Calculation of slope, aspect, profile and tangential curvature</h3>

In this example a slope, aspect, profile and tangential curvature map
are computed from an elevation raster map (North Carolina sample
dataset):

<div class="code"><pre>
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
</pre></div>

<center>
  <img src="r_slope_aspect_slope.png" border="0">
  <img src="r_slope_aspect_aspect.png" border="0">
  <img src="r_slope_aspect_pcurv.png" border="0">
  <img src="r_slope_aspect_tcurv.png" border="0">
  <p>
Figure: Slope, aspect, profile and tangential curvature raster map (North Carolina dataset)
</center>

<h3>Classification of major aspect directions in compass orientation</h3>

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):
 
<div class="code"><pre>
g.region raster=elevation -p

# generate integer aspect map with degrees CCW from East
r.slope.aspect elevation=elevation aspect=myaspect precision=CELL

# 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
</pre></div>

<center>
<img src="r_slope_aspect_4_directions.png" alt="Aspect map classified to four major compass directions"><br>
Aspect map classified to four major compass directions (zoomed subset shown)
</center>

<h2>REFERENCES</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>,
  <a href="r.neighbors.html">r.neighbors</a>,
  <a href="r.reclass.html">r.reclass</a>,
  <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