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  1. <!-- meta page description: Raster data processing in GRASS GIS -->
  2. <!-- meta page index: raster -->
  3. <h3>Raster maps in general</h3>
  4. A "raster map" is a data layer consisting of a gridded array of cells.
  5. It has a certain number of rows and columns, with a data point (or null
  6. value indicator) in each cell. These may exist as a 2D grid or as a 3D
  7. cube made up of many smaller cubes, i.e. a stack of 2D grids.
  8. <p>
  9. The geographic boundaries of the raster map are described by the north,
  10. south, east, and west fields. These values describe the lines which bound
  11. the map at its edges. These lines do NOT pass through the center of the
  12. grid cells at the edge of the map, but along the edge of the map itself.
  13. i.e. the geographic extent of the map is described by the outer bounds of
  14. all cells within the map.
  15. <p>
  16. As a general rule in GRASS:
  17. <ol>
  18. <li> Raster output maps have their bounds and resolution equal to those
  19. of the current computational region.
  20. <li> Raster input maps are automatically cropped/padded and rescaled
  21. (using nearest-neighbour resampling) to match the current region.
  22. <li> Raster input maps are automatically masked if a raster map named
  23. MASK exists. The MASK is only applied when <i>reading</i> maps
  24. from the disk.
  25. </ol>
  26. There are a few exceptions to this:
  27. <tt>r.in.*</tt> programs read the data cell-for-cell, with no resampling. When
  28. reading non-georeferenced data, the imported map will usually have its
  29. lower-left corner at (0,0) in the location's coordinate system; the user
  30. needs to use <a href="r.region.html">r.region</a> to "place" the imported map.
  31. <p>
  32. Some programs which need to perform specific types of resampling (e.g.
  33. <a href="r.resamp.rst.html">r.resamp.rst</a>) read the input maps at
  34. their original resolution then do the resampling themselves.
  35. <p>
  36. <a href="r.proj.html">r.proj</a> has to deal with two regions (source
  37. and destination) simultaneously; both will have an impact upon the
  38. final result.
  39. <h3>Raster import and export</h3>
  40. The module <a href="r.in.gdal.html">r.in.gdal</a> offers a common
  41. interface for many different raster formats. Additionally, it also
  42. offers options such as on-the-fly location creation or extension of
  43. the default region to match the extent of the imported raster map.
  44. For special cases, other import modules are available. The full map
  45. is always imported.
  46. <p>
  47. For importing scanned maps, the user will need to create a
  48. x,y-location, scan the map in the desired resolution and save it into
  49. an appropriate raster format (e.g. tiff, jpeg, png, pbm) and then use
  50. <a href="r.in.gdal.html">r.in.gdal</a> to import it. Based on
  51. reference points the scanned map can be recified to obtain geocoded
  52. data.
  53. <p>
  54. Raster maps are exported with <a href="r.out.gdal.html">r.out.gdal</a>
  55. into common formats. Also <a href="r.out.bin.html">r.out.bin</a>,
  56. <a href="r.out.vtk.html">r.out.vtk</a>, <a href="r.out.ascii.html">r.out.ascii</a>
  57. and other export modules are available. They export the data according
  58. to the current region settings. If those differ from the original map,
  59. the map is resampled on the fly (nearest neighbor algorithm). In other
  60. words, the output will have as many rows and columns as the current region.
  61. To export maps with various grid spacings (e.g, 500x500 or 200x500), you
  62. can just change the region resolution with <a href="g.region.html">g.region</a>
  63. and then export the map. The resampling is done with nearest neighbor
  64. algorithm in this case. If you want some other form of resampling,
  65. first change the region, then explicitly resample the map with e.g.
  66. <a href="r.resamp.interp.html">r.resamp.interp</a> or
  67. <a href="r.resamp.stats.html">r.resamp.stats</a>, then export the
  68. resampled map.
  69. <h3>Metadata</h3>
  70. The <a href="r.info.html">r.info</a> module displays general information
  71. about a map such as region extent, data range, data type, creation history,
  72. and other metadata.
  73. Metadata such as map title, units, vertical datum etc. can be updated
  74. with <a href="r.support.html">r.support</a>. Timestamps are managed
  75. with <a href="r.timestamp.html">r.timestamp</a>. Region extent and
  76. resolution are mangaged with <a href="r.region.html">r.region</a>.
  77. <h3>Raster map operations</h3>
  78. <h4>Resampling methods and interpolation methods</h4>
  79. GRASS raster map processing is always performed in the current region
  80. settings (see <a href="g.region.html">g.region</a>), i.e. the current
  81. region extent and current raster resolution is used. If the resolution
  82. differs from that of the input raster map(s), on-the-fly resampling is
  83. performed (nearest neighbor resampling). If this is not desired, the
  84. input map(s) has/have to be resampled beforehand with one of the dedicated
  85. modules.
  86. <p>
  87. The built-in nearest-neighbour resampling of raster data calculates
  88. the centre of each region cell, and takes the value of the raster cell
  89. in which that point falls.
  90. <p>
  91. If the point falls exactly upon a grid line, the exact result will be
  92. determined by the direction of any rounding error. One consequence of
  93. this is that downsampling by a factor which is an even integer will
  94. always sample exactly on the boundary between cells, meaning that the
  95. result is ill-defined.
  96. <p>
  97. The following modules are available for reinterpolation of "filled"
  98. raster maps (continuous data) to a different resolution:
  99. <ul>
  100. <li><a href="r.resample.html">r.resample</a> uses the built-in resampling,
  101. so it should produce identical results as the on-the-fly resampling done
  102. via the raster import modules.</li>
  103. <li><a href="r.resamp.interp.html">r.resamp.interp</a> Resampling with
  104. nearest neighbor, bilinear, and bicubic method: <b>method=nearest</b> uses the
  105. same algorithm as <a href="r.resample.html">r.resample</a>, but not the same
  106. code, so it may not produce identical results in cases which are decided
  107. by the rounding of floating-point numbers.
  108. <br>
  109. For <a href="r.resamp.interp.html">r.resamp.interp</a> <b>method=bilinear</b>
  110. and <b>method=bicubic</b>, the raster values are treated as samples at each
  111. raster cell's centre, defining a piecewise-continuous surface. The resulting
  112. raster values are obtained by sampling the surface at each region cell's centre.
  113. As the algorithm only interpolates, and doesn't extrapolate, a margin of 0.5
  114. (for bilinear) or 1.5 (for bicubic) cells is lost from the extent of the original
  115. raster. Any samples taken within this margin will be null.</li>
  116. <li><a href="r.resamp.rst.html">r.resamp.rst</a> Regularized Spline with Tension
  117. (RST) interpolation 2D: Behaves similarly, i.e. it computes a surface assuming
  118. that the values are samples at each raster cell's centre, and samples the surface
  119. at each region cell's centre.</li>
  120. <li><a href="r.resamp.bspline.html">r.resamp.bspline</a> Bicubic or bilinear
  121. spline interpolation with Tykhonov regularization.</li>
  122. <li>For <a href="r.resamp.stats.html">r.resamp.stats</a> without <b>-w</b>, the value of
  123. each region cell is the chosen aggregate of the values from all of the raster
  124. cells whose centres fall within the bounds of the region cell.
  125. <br>
  126. With <b>-w</b>, the samples are weighted according to the proportion of the
  127. raster cell which falls within the bounds of the region cell, so the
  128. result is normally unaffected by rounding error (a miniscule difference
  129. in the position of the boundary results in the addition or subtraction of
  130. a sample weighted by a miniscule factor; also, The min and max aggregates
  131. can't use weights, so <b>-w</b> has no effect for those).</li>
  132. <li> <a href="r.fillnulls.html">r.fillnulls</a> for Regularized Spline with Tension (RST)
  133. interpolation 2D for hole filling (e.g., SRTM DEM)</li>
  134. </ul>
  135. <p>
  136. Furthermore, there are modules available for reinterpolation of "sparse"
  137. (scattered points or lines) maps:
  138. <ul>
  139. <li> Inverse distance weighted average (IDW) interpolation
  140. (<a href="r.surf.idw.html">r.surf.idw</a> and <a href="r.surf.idw2.html">r.surf.idw2</a>)</li>
  141. <li> Interpolating from contour lines (<a href="r.contour.html">r.contour</a>)</li>
  142. </ul>
  143. For Lidar and similar data, <a href="r.in.lidar.html">r.in.lidar</a> and <a href="r.in.xyz.html">r.in.xyz</a>
  144. support loading and binning of ungridded x,y,z ASCII data into a new raster map.
  145. The user may choose from a variety of statistical methods in creating the new raster map.
  146. <p>
  147. Otherwise, for interpolation of scattered data, use the <em>v.surf.*</em> set of
  148. modules.
  149. <h4>Raster MASKs</h4>
  150. If a raster map named "MASK" exists, most GRASS raster modules will operate
  151. only on data falling inside the masked area, and treat any data falling
  152. outside of the mask as if its value were NULL. The mask is only applied
  153. when <em>reading</em> an existing GRASS raster map, for example when used
  154. in a module as an input map.
  155. <p>
  156. The mask is read as an integer map. If MASK is actually a
  157. floating-point map, the values will be converted to integers using the
  158. map's quantisation rules (this defaults to round-to-nearest, but can
  159. be changed with r.quant).
  160. <p>
  161. (see <a href="r.mask.html">r.mask</a>)
  162. <h3>Raster map statistics</h3>
  163. A couple of commands are available to calculate local statistics
  164. (<a href="r.neighbors.html">r.neighbors</a>), and global statistics
  165. (<a href="r.statistics.html">r.statistics</a>, <a href="r.surf.area.html">r.surf.area</a>).
  166. Profiles and transects can be generated
  167. (<a href="d.profile.html">d.profile</a>, <a href="r.profile.html">r.profile</a>,
  168. <a href="r.transect.html">r.transect</a>) as well as histograms
  169. (<a href="d.histogram.html">d.histogram</a>) and polar diagrams
  170. (<a href="d.polar.html">d.polar</a>).
  171. Univariate statistics (<a href="r.univar.html">r.univar</a>) and
  172. reports are also available (<a href="r.report.html">r.report</a>,<a
  173. href="r.stats.html">r.stats</a>, <a href="r.volume.html">r.volume</a>).
  174. <h3>Raster map algebra and aggregation</h3>
  175. The <a href="r.mapcalc.html">r.mapcalc</a> command provides raster map
  176. algebra methods.
  177. The <a href="r.resamp.stats.html">r.resamp.stats</a> command resamples raster
  178. map layers using various aggregation methods, the <a href="r.statistics.html">r.statistics</a>
  179. command aggregates one map based on a second map.
  180. <a href="r.resamp.interp.html">r.resamp.interp</a> resamples raster map layers using interpolation.
  181. <h3>Hydrologic modeling toolbox</h3>
  182. Watershed modeling related modules are
  183. <a href="r.basins.fill.html">r.basins.fill</a>,
  184. <a href="r.water.outlet.html">r.water.outlet</a>,
  185. <a href="r.watershed.html">r.watershed</a>, and
  186. <a href="r.terraflow.html">r.terraflow</a>.
  187. Water flow related modules are
  188. <a href="r.carve.html">r.carve</a>,
  189. <a href="r.drain.html">r.drain</a>,
  190. <a href="r.fill.dir.html">r.fill.dir</a>,
  191. <a href="r.fillnulls.html">r.fillnulls</a>,
  192. <a href="r.flow.html">r.flow</a>, and
  193. <a href="r.topidx.html">r.topidx</a>.
  194. Flooding can be simulated with <a href="r.lake.html">r.lake</a>.
  195. Hydrologic simulation model are available as
  196. <a href="r.sim.sediment.html">r.sim.sediment</a>,
  197. <a href="r.sim.water.html">r.sim.water</a>, and
  198. <a href="r.topmodel.html">r.topmodel</a>.
  199. <h3>Raster format</h3>
  200. Raster data can be stored in GRASS as 2D or 3D grids. 2D rasters
  201. support 3 data types: 32bit signed integer, single- and double-precision
  202. floating-point. 3D rasters support only single- and double-precision
  203. floating-point. In most GRASS resources, 2D raster maps are usually
  204. called "raster", their integer data type "CELL", single-precision
  205. floating-point data type "FCELL" and double-precision floating-point
  206. "DCELL". The 3D raster map type is usually called "3D raster" but other
  207. names like "RASTER3D", "voxel", "volume", "GRID3D" or "3d cell" are common.
  208. 3D raster's single-precision data type is most often called "float", and the
  209. double-precision one "double".
  210. <p>
  211. GRASS raster format is architecture independent and portable between
  212. 32bit and 64bit machines.
  213. <p>
  214. GRASS distinguishes NULL and zero. When working with NULL data, it is
  215. important to know that operations on NULL cells lead to NULL cells.
  216. <h3>See also</h3>
  217. <ul>
  218. <li><a href="raster3dintro.html">Introduction into 3D raster data (voxel) processing</a></li>
  219. <li><a href="vectorintro.html">Introduction into vector data processing</a></li>
  220. <li><a href="imageryintro.html">Introduction into image processing</a></li>
  221. <li><a href="databaseintro.html">Database management</a></li>
  222. <li><a href="projectionintro.html">Projections and spatial transformations</a></li>
  223. </ul>