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

6   Projection management in general
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.group, i.target, g.gui.gcp) and then  transformed  into  the  target
27       location  (i.rectify).  Polynomial  transformation  of 1st, 2nd and 3rd
28       order are supported.
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30       A graphical user interface is provided by wxGUI.
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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).  Alternatively,
48       v.rectify rectifies a vector by computing a  coordinate  transformation
49       for each object in the vector based on the control points.
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51       A graphical user interface is provided by wxGUI.
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53   References
54           ·   ASPRS Grids and Datum
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56           ·   Projections Transform List (PROJ4)
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58           ·   MapRef  -  The Collection of Map Projections and Reference Sys‐
59               tems for Europe
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61           ·   Information and Service System for European  Coordinate  Refer‐
62               ence Systems - CRS
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64   See also
65           ·   Introduction into raster data processing
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67           ·   Introduction into 3D raster data (voxel) processing
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69           ·   Introduction into vector data processing
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71           ·   Introduction into image processing
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73           ·   Introduction into temporal data processing
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75           ·   Database management
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77       Main  index  |  Topics  index | Keywords index | Graphical index | Full
78       index
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80       © 2003-2019 GRASS Development Team, GRASS GIS 7.4.4 Reference Manual
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84GRASS 7.4.4                                                 projectionintro(1)