Raster data processing in GRASS GIS
Raster maps in general
The geographic boundaries of the raster file 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.
As a general rule in GRASS:
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.
- Raster output maps have their bounds and resolution equal to those
of the current region.
- Raster input maps are automatically cropped/padded and rescaled
(using nearest-neighbour resampling) to match the current region.
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
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. Always the full
map is 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
Raster map operations
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
Raster map statistics
A couple of commands are available to calculate local statistics
(r.neighbors), and global statistics
r.sum). Profiles and transects can be generated
r.transect) as well as histograms
(d.histogram) and polar diagrams
Univariate statistics (r.univar) and
reports are also available (r.report,r.stats, r.volume).
Raster map algebra and aggregation
The r.mapcalc command provides raster map
The r.average command aggregates one map based on a second map.
Raster map resampling and interpolation methods
GRASS offers various raster resampling and interpolation methods:
Hydrologic modeling toolbox
Watershed modeling related modules are
Water flow related modules are
Flooding can be simulated with r.lake.
Hydrologic simulation model are available as
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