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Projections and spatial transformations in GRASS GIS
Introduction
A GRASS location is referenced with a single projection and coordinate system
(or unreferenced as XY location). When creating a new location from an existing
raster or vector map using the tools available from the startup screen or
the map import commands, projection and coordinate system are defined.
To change the projection of maps, a new location has to be created and
the desired maps have to be reprojected into it from the source location
as explained below.
Raster map projections
Raster maps are backward projected in GRASS. This means that the
user has to run the raster projection command (r.proj)
in the target location and "pull" the map from the source location.
Both locations need to have a projection defined.
Raster map transformation
To transform an unprojected map from a XY location into a projected
location (or another XY location), a forward transformation is performed.
The unreferenced map is geocoded within the XY location by defining
four corner points or by seeking several ground control points
(i.group, i.target,
i.points) and then transformed into the
target location (i.rectify). Polynomial
transformation of 1st, 2nd and 3rd order are supported.
A graphical user interface is provided by wxGUI
(old interface: gis.m).
To simply translate a raster map (without stretching or rotation), the
r.region command can be used.
Vector map projections
Vector maps are backward projected in GRASS. This means that the
user has to run the vector projection command (v.proj)
in the target location and "pull" the map from the source location.
Both locations need to have a projection defined.
Vector map transformation
To transform an unprojected map (e.g. CAD map) into projected coordinates,
a forward transformation is performed. The unreferenced map is imported
into the location with projection and geocoded within this location by
defining four corner points or by seeking several ground control points.
These points are stored into an ASCII file and then transformed within
the same location (v.transform).
A graphical user interface is provided by wxGUI
(old interface: gis.m).
References
See also
Main index - full index
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