1projectionintro(1)            Grass User's Manual           projectionintro(1)
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Projections and spatial transformations in GRASS GIS

6   Introduction
7       A  GRASS location is referenced with a single projection and coordinate
8       system (or unreferenced as XY location). When creating a  new  location
9       from  an  existing  raster or vector map using the tools available from
10       the startup screen or the map import commands, projection  and  coordi‐
11       nate system are defined.  To change the projection of maps, a new loca‐
12       tion has to be created and the desired maps have to be reprojected into
13       it from the source location as explained below.
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15   Raster map projections
16       Raster  maps  are backward projected in GRASS. This means that the user
17       has to run the raster projection command (r.proj) in the  target  loca‐
18       tion  and "pull" the map from the source location.  Both locations need
19       to have a projection defined.
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21   Raster map transformation
22       To transform an unprojected map from a XY  location  into  a  projected
23       location  (or  another  XY  location), a forward transformation is per‐
24       formed.  The unreferenced map is geocoded within  the  XY  location  by
25       defining four corner points or by seeking several ground control points
26       (i.target, i.points) and then  transformed  into  the  target  location
27       (i.rectify).  Polynomial  transformation  of 1st, 2nd and 3rd order are
28       supported.
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30       A graphical user interface is provided by gis.m.
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32       To simply translate a raster map (without stretching or rotation),  the
33       r.region command can be used.
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35   Vector map projections
36       Vector  maps  are backward projected in GRASS. This means that the user
37       has to run the vector projection command (v.proj) in the  target  loca‐
38       tion  and "pull" the map from the source location.  Both locations need
39       to have a projection defined.
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41   Vector map transformation
42       To transform an unprojected map (e.g. CAD map) into  projected  coordi‐
43       nates,  a  forward transformation is performed. The unreferenced map is
44       imported into the location with projection  and  geocoded  within  this
45       location  by  defining  four corner points or by seeking several ground
46       control points.  These points are stored into an ASCII  file  and  then
47       transformed within the same location (v.transform).
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49       A graphical user interface is provided by gis.m.
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51   References
52                      ASPRS Grids and Datum
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54                      Projections Transform List (PROJ4)
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56                      MapRef - The Collection of Map Projections and Reference
57                     Systems for Europe
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59                      Information and Service System for  European  Coordinate
60                     Reference Systems - CRS
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62   See also
63                     Introduction to GRASS raster map processing
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65                     Introduction to GRASS image processing
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67                     Introduction to GRASS vector map processing
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69       full index
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73GRASS 6.3.0                                                 projectionintro(1)