rasterintro(1)

Raster data processing in GRASS GIS

  Raster maps in general
A "raster map" is a data layer consisting of a gridded array of cells. It has a certain number of rows and columns, with a data point (or null value indicator) in each cell. These may exist as a 2D grid or as a 3D cube made up of many smaller cubes, i.e. a stack of 2D grids.

The geographic boundaries of the raster map are described by the north, south, east, and west fields. These values describe the lines which bound the map at its edges. These lines do NOT pass through the center of the grid cells at the edge of the map, but along the edge of the map itself. i.e. the geographic extent of the map is described by the outer bounds of all cells within the map.

As a general rule in GRASS:

1
     Raster output maps have their bounds and resolution equal to those of the current computational region.

2
     Raster input maps are automatically cropped/padded and rescaled (using nearest-neighbour resampling) to match the current region.

3
     Raster input maps are automatically masked if a raster map named MASK exists. The MASK is only applied when reading maps from the disk.

There are a few exceptions to this: r.in.* programs read the data cell-for-cell, with no resampling. When reading non-georeferenced data, the imported map will usually have its lower-left corner at (0,0) in the location's coordinate system; the user needs to use r.region to "place" the imported map.

Some programs which need to perform specific types of resampling (e.g. r.resamp.rst) read the input maps at their original resolution then do the resampling themselves.

r.proj has to deal with two regions (source and destination) simultaneously; both will have an impact upon the final result.

  Raster import and export
The module r.in.gdal offers a common interface for many different raster formats. Additionally, it also offers options such as on-the-fly location creation or extension of the default region to match the extent of the imported raster map. For special cases, other import modules are available. The full map is always imported.

For importing scanned maps, the user will need to create a x,y-location, scan the map in the desired resolution and save it into an appropriate raster format (e.g. tiff, jpeg, png, pbm) and then use r.in.gdal to import it. Based on reference points the scanned map can be recified to obtain geocoded data.

Raster maps are exported with r.out.gdal into common formats. Also r.out.bin, r.out.ascii and other export modules are available. They export the data according to the current region settings. If those differ from the original map, the map is resampled on the fly (nearest neighbor algorithm). In other words, the output will have as many rows and columns as the current region. To export maps with various grid spacings (e.g, 500x500 or 200x500), you can just change the region resolution with g.region and then export the map. The resampling is done with nearest neighbor algorithm in this case. If you want some other form of resampling, first change the region, then explicitly resample the map with e.g. r.resamp.interp or r.resamp.stats, then export the resampled map.

  Metadata
The r.info module displays general information about a map such as region extent, data range, data type, creation history, and other metadata. Metadata such as map title, units, vertical datum etc. can be updated with r.support. Timestamps are managed with r.timestamp. Region extent and resolution are mangaged with r.region.

  Raster map operations

  Resampling methods and interpolation methods
GRASS raster map processing is always performed in the current region settings (see g.region), i.e. the current region extent and current raster resolution is used. If the resolution differs from that of the input raster map(s), on-the-fly resampling is performed (nearest neighbor resampling). If this is not desired, the input map(s) has/have to be resampled beforehand with one of the dedicated modules.

The built-in nearest-neighbour resampling of raster data calculates the centre of each region cell, and takes the value of the raster cell in which that point falls.

If the point falls exactly upon a grid line, the exact result will be determined by the direction of any rounding error. One consequence of this is that downsampling by a factor which is an even integer will always sample exactly on the boundary between cells, meaning that the result is ill-defined.

The following modules are available for reinterpolation of "filled" raster maps (continuous data) to a different resolution:

        r.resample uses the built-in resampling, so it should produce identical results as the on-the-fly resampling done via the raster import modules.

        r.resamp.interp Resampling with nearest neighbor, bilinear, and bicubic method: method=nearest uses the same algorithm as r.resample, but not the same code, so it may not produce identical results in cases which are decided by the rounding of floating-point numbers.
        For r.resamp.interp method=bilinear and method=bicubic, the raster values are treated as samples at each raster cell's centre, defining a piecewise-continuous surface. The resulting raster values are obtained by sampling the surface at each region cell's centre. As the algorithm only interpolates, and doesn't extrapolate, a margin of 0.5 (for bilinear) or 1.5 (for bicubic) cells is lost from the extent of the original raster. Any samples taken within this margin will be null.

        r.resamp.rst Regularized Spline with Tension (RST) interpolation 2D: Behaves similarly, i.e. it computes a surface assuming that the values are samples at each raster cell's centre, and samples the surface at each region cell's centre.

        For r.resamp.stats without -w, the value of each region cell is the chosen aggregate of the values from all of the raster cells whose centres fall within the bounds of the region cell.
        With -w, the samples are weighted according to the proportion of the raster cell which falls within the bounds of the region cell, so the result is normally unaffected by rounding error (a miniscule difference in the position of the boundary results in the addition or subtraction of a sample weighted by a miniscule factor; also, The min and max aggregates can't use weights, so -w has no effect for those).

         r.fillnulls for Regularized Spline with Tension (RST) interpolation 2D for hole filling (e.g., SRTM DEM)

Furthermore, there are modules available for reinterpolation of "sparse" (scattered points or lines) maps:

         Inverse distance weighted average (IDW) interpolation (r.surf.idw2)

         Interpolating from contour lines (r.contour)
For Lidar and similar data, r.in.xyz supports loading and binning of ungridded x,y,z ASCII data into a new raster map. The user may choose from a variety of statistical methods in creating the new raster.

Otherwise, for interpolation of scattered data, use the v.surf.* set of modules.

  Raster MASKs
If a raster map named "MASK" exists, most GRASS raster modules will operate only on data falling inside the masked area, and treat any data falling outside of the mask as if its value were NULL. The mask is only applied when reading an existing GRASS raster map, for example when used in a module as an input map.

The mask is read as an integer map. If MASK is actually a floating-point map, the values will be converted to integers using the map's quantisation rules (this defaults to round-to-nearest, but can be changed with r.quant).

(see r.mask)

  Raster map statistics
A couple of commands are available to calculate local statistics (r.neighbors), and global statistics (r.surf.area, r.sum). Profiles and transects can be generated (r.profile, r.transect) as well as histograms (d.histogram) and polar diagrams (d.polar). Univariate statistics (r.univar) and reports are also available (r.report,<a href="r.stats.html">r.stats, r.volume).

  Raster map algebra and aggregation
The r.mapcalc command provides raster map algebra methods. The r.resamp.stats command resamples raster map layers using various aggregation methods, the r.average command aggregates one map based on a second map. r.resamp.interp resamples raster map layers using interpolation.

  Hydrologic modeling toolbox
Watershed modeling related modules are r.basins.fill, r.water.outlet, r.watershed, and r.terraflow. Water flow related modules are r.carve, r.drain, r.fill.dir, r.fillnulls, r.flow, and r.topidx. Flooding can be simulated with r.lake. Hydrologic simulation model are available as r.sim.sediment, r.sim.water, and r.topmodel.

  Raster format
Raster data can be stored in GRASS as 2D or 3D grids. 2D rasters support 3 data types: 32bit signed integer, single- and double-precision floating-point. 3D rasters support only single- and double-precision floating-point. In most GRASS resources, 2D raster maps are usually called "raster", their integer data type "CELL", single-precision floating-point data type "FCELL" and double-precision floating-point "DCELL". The 3D raster map type is usually called "3D raster" but other names like "G3D", "voxel", "volume", "GRID3D" or "3d cell" are common. 3D raster's single-precision data type is most often called "float", and the double-precision one "double".

GRASS raster format is architecture independent and portable between 32bit and 64bit machines.

GRASS distinguishes NULL and zero. When working with NULL data, it is important to know that operations on NULL cells lead to NULL cells.

  See also

        Introduction to GRASS vector map processing

        Introduction to GRASS 3D raster map (voxel) processing

raster index - full index

© 2008 GRASS Development Team