projectionintro(1grass) 					Grass User's Manual					   projectionintro(1grass)

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 loca-
       tion 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 repro-
       jected 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 loca-
       tion 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 per-
       formed.	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 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 loca-
       tion 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 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

       (C) 2008-2011 GRASS Development Team

GRASS 6.4.2														   projectionintro(1grass)