1projectionintro(1)          GRASS GIS 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 lo‐
23       cation (or another XY location), a forward transformation is performed.
24       The unreferenced map is geocoded within the  XY  location  by  defining
25       four  corner  points  or  by  seeking  several  ground  control  points
26       (i.group, i.target, g.gui.gcp) and then transformed into the target lo‐
27       cation (i.rectify). Polynomial transformation of 1st, 2nd and 3rd order
28       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 lo‐
45       cation  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 (PROJ)
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58           •   Coordinate operations by PROJ (projections, conversions, trans‐
59               formations, pipeline operator)
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61           •   MapRef - The Collection of Map Projections and  Reference  Sys‐
62               tems for Europe
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64           •   Information  and  Service System for European Coordinate Refer‐
65               ence Systems - CRS
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67   See also
68           •   Introduction into raster data processing
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70           •   Introduction into 3D raster data (voxel) processing
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72           •   Introduction into vector data processing
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74           •   Introduction into image processing
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76           •   Introduction into temporal data processing
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78           •   Database management
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80       Main index | Topics index | Keywords index | Graphical index | Full in‐
81       dex
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83       © 2003-2023 GRASS Development Team, GRASS GIS 8.2.1 Reference Manual
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87GRASS 8.2.1                                                 projectionintro(1)