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

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  coordi-
       nate system are defined.	 To change the projection of maps, a new loca-
       tion 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	 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 lo-
       cation (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.tar-
       get, i.points) and then transformed into	the  target  location  (i.rec-
       tify).  Polynomial  transformation  of  1st, 2nd	and 3rd	order are sup-
       ported.

       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	 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  coordi-
       nates,  a  forward transformation is performed. The unreferenced	map is
       imported	into the location with projection and geocoded within this lo-
       cation  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

       (C) 2008-2012 GRASS Development Team

GRASS 6.4.6						    projectionintro(1)

NAME | Projections and spatial transformations in GRASS GIS

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