trusty (1) projectionintro.1grass.gz

NAME

       projectionintro - Projections and spatial transformations

       Projections and spatial transformations

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.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
                      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