1projectionintro(1)          GRASS GIS User's Manual         projectionintro(1)
2
3
4

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.
14
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.
20
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.
29
30       A graphical user interface is provided by wxGUI.
31
32       To simply translate a raster map (without stretching or rotation),  the
33       r.region command can be used.
34
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.
40
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.
50
51       A graphical user interface is provided by wxGUI.
52
53   References
54           ·   ASPRS Grids and Datum
55
56           ·   Projections Transform List (PROJ)
57
58           ·   Coordinate operations by PROJ (projections, conversions, trans‐
59               formations, pipeline operator)
60
61           ·   MapRef - The Collection of Map Projections and  Reference  Sys‐
62               tems for Europe
63
64           ·   Information  and  Service System for European Coordinate Refer‐
65               ence Systems - CRS
66
67   See also
68           ·   Introduction into raster data processing
69
70           ·   Introduction into 3D raster data (voxel) processing
71
72           ·   Introduction into vector data processing
73
74           ·   Introduction into image processing
75
76           ·   Introduction into temporal data processing
77
78           ·   Database management
79
80       Main index | Topics index | Keywords index |  Graphical  index  |  Full
81       index
82
83       © 2003-2020 GRASS Development Team, GRASS GIS 7.8.5 Reference Manual
84
85
86
87GRASS 7.8.5                                                 projectionintro(1)