r.slope.aspect.html 6.5 KB

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  1. <h2>DESCRIPTION</h2>
  2. <b>r.slope.aspect </b>generates raster maps of slope, aspect, curvatures and
  3. first and second order partial derivatives from a raster map of true
  4. elevation values. The user must specify the input <i>elevation</i> file name
  5. and at least one output file name. The user can also specify the
  6. <i>format</i> for slope (degrees, percent; default=degrees), and the
  7. <i>zfactor</i>: multiplicative factor to convert elevation units to meters;
  8. (default 1.0).
  9. <p>
  10. The <i>elevation</i> input raster map specified by the user must contain true
  11. elevation values, <b>not</b> rescaled or categorized data. If the elevation
  12. values are in feet or other units than meters (with a conversion factor
  13. <i>meters:</i>, defined in PROJ_UNITS), they must be converted to meters using
  14. the parameter <i>zfactor</i>.
  15. <p>
  16. The <i>aspect</i> output raster map indicates the direction that slopes are
  17. facing. The aspect categories represent the number degrees of east. Category
  18. and color table files are also generated for the aspect map layer. The aspect
  19. categories represent the number degrees of east and they increase
  20. counterclockwise: 90deg is North, 180 is West, 270 is South 360 is East. The
  21. aspect value 0 is used to indicate undefined aspect in flat areas with slope=0.
  22. <p>
  23. The <i>slope</i> output raster map contains slope values, stated in degrees of
  24. inclination from the horizontal if <i>format</i>=degrees option (the default)
  25. is chosen, and in percent rise if <i>format</i>=percent option is chosen.
  26. Category and color table files are generated.
  27. <p>
  28. Profile and tangential curvatures are the curvatures in the direction of
  29. steepest slope and in the direction of the contour tangent respectively. The
  30. curvatures are expressed as 1/metres, e.g. a curvature of 0.05 corresponds to a
  31. radius of curvature of 20m. Convex form values are positive and concave form values
  32. are negative.
  33. <p><table width="100%" border="0">
  34. <tr valign="baseline">
  35. <td>
  36. <center>
  37. <img src="dem.png" border="1">
  38. <p> Example DEM
  39. <br><br>
  40. </center>
  41. </td>
  42. <td>
  43. </td>
  44. </tr>
  45. <tr valign="baseline">
  46. <td>
  47. <center>
  48. <img src="slope.png" border="1">
  49. <p> Slope (degree) from example DEM
  50. <br><br>
  51. </center>
  52. </td>
  53. <td>
  54. <center>
  55. <img src="aspect.png" border="1">
  56. <p> Aspect (degree) from example DEM
  57. <br><br>
  58. </center>
  59. </td>
  60. </tr>
  61. <tr valign="baseline">
  62. <td>
  63. <center>
  64. <img src="tcurv.png" border="1">
  65. <p> Tangential curvature (m<sup>-1</sup>) from example DEM
  66. <br><br>
  67. </center>
  68. </td>
  69. <td>
  70. <center>
  71. <img src="pcurv.png" border="1">
  72. <p> Profile curvature (m<sup>-1</sup>) from example DEM
  73. <br><br>
  74. </center>
  75. </td>
  76. <td>
  77. </td>
  78. </tr>
  79. </table>
  80. <p>For some applications, the user will wish to use a reclassified raster map
  81. of slope that groups slope values into ranges of slope. This can be done using
  82. <i><a href="r.reclass.html">r.reclass</a></i>. An example of a useful
  83. reclassification is given below:
  84. <div class="code"><pre> category range category labels
  85. (in degrees) (in percent)
  86. 1 0- 1 0- 2%
  87. 2 2- 3 3- 5%
  88. 3 4- 5 6- 10%
  89. 4 6- 8 11- 15%
  90. 5 9- 11 16- 20%
  91. 6 12- 14 21- 25%
  92. 7 15- 90 26% and higher
  93. The following color table works well with the above
  94. reclassification.
  95. category red green blue
  96. 0 179 179 179
  97. 1 0 102 0
  98. 2 0 153 0
  99. 3 128 153 0
  100. 4 204 179 0
  101. 5 128 51 51
  102. 6 255 0 0
  103. 7 0 0 0</pre></div>
  104. <h2>NOTES</h2>
  105. To ensure that the raster elevation map layer is not inappropriately resampled,
  106. the settings for the current region are modified slightly (for the execution
  107. of the program only): the resolution is set to match the resolution of
  108. the elevation map and the edges of the region (i.e. the north, south, east
  109. and west) are shifted, if necessary, to line up along edges of the nearest
  110. cells in the elevation map. If the user really wants the elevation map
  111. resampled to the current region resolution, the -a flag should be specified.
  112. <p>
  113. The current mask is ignored.
  114. <p>
  115. The algorithm used to determine slope and aspect uses a 3x3 neighborhood
  116. around each cell in the elevation file. Thus, it is not possible to determine
  117. slope and aspect for the cells adjacent to the edges in the elevation map
  118. layer. These cells are assigned a "zero slope" value (category 0) in both
  119. the slope and aspect raster map layers.
  120. <p>
  121. Horn's formula is used to find the first order derivatives in x and y directions.
  122. <p>
  123. Only when using integer elevation models, the aspect is biased in 0,
  124. 45, 90, 180, 225, 270, 315, and 360 directions; i.e., the distribution
  125. of aspect categories is very uneven, with peaks at 0, 45,..., 360 categories.
  126. When working with floating point elevation models, no such aspect bias occurs.
  127. <p>
  128. Because most cells with a very small slope end up having category 0,
  129. 45, ..., 360, it is sometimes possible to reduce the bias in these directions
  130. by filtering out the aspect in areas where the terrain is almost flat. A new
  131. option <i>min_slp_allowed</i> was added to specify the minimum slope for which
  132. aspect is computed. The aspect for all cells with slope &lt;
  133. <i>min_slp_allowed</i> is set to <b>null</b>.
  134. <h2>REFERENCE</h2>
  135. <ul>
  136. <li> Horn, B. K. P. (1981). <i>Hill Shading and the Reflectance Map</i>, Proceedings
  137. of the IEEE, 69(1):14-47.
  138. <li> Mitasova, H. (1985). <i>Cartographic aspects of computer surface modeling. PhD thesis.</i>
  139. Slovak Technical University , Bratislava
  140. <li> Hofierka, J., Mitasova, H., Neteler, M., 2009. <i>Geomorphometry in GRASS GIS.</i>
  141. In: Hengl, T. and Reuter, H.I. (Eds), <i>Geomorphometry: Concepts, Software, Applications. </i>
  142. Developments in Soil Science, vol. 33, Elsevier, 387-410 pp,
  143. <a href="http://www.geomorphometry.org">http://www.geomorphometry.org</a>
  144. </ul>
  145. <h2>SEE ALSO</h2>
  146. <em><a href="r.mapcalc.html">r.mapcalc</a></em>,
  147. <em><a href="r.neighbors.html">r.neighbors</a></em>,
  148. <em><a href="r.reclass.html">r.reclass</a></em>,
  149. <em><a href="r.rescale.html">r.rescale</a></em>
  150. <h2>AUTHORS</h2>
  151. Michael Shapiro, U.S.Army Construction Engineering Research Laboratory<br>
  152. Olga Waupotitsch, U.S.Army Construction Engineering Research Laboratory
  153. <p>
  154. <i>Last changed: $Date$</i>