NAME¶
projectionintro - Projections and spatial transformations
Projections and spatial transformations
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.
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.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.
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¶
-
ASPRS Grids and Datum
-
Projections Transform List (PROJ4)
-
MapRef - The Collection of Map Projections and Reference Systems for
Europe
-
Information and Service System for European Coordinate Reference Systems -
CRS
See also¶
- Introduction to GRASS raster map processing
- Introduction to GRASS image processing
- Introduction to GRASS vector map processing
full index
© 2008-2012 GRASS Development Team