Projections and spatial transformations in GRASS GIS

   Projection management in general
       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 coordinate system are defined.  To
       change the projection of maps, a new location 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 location 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 location (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.group, i.target, g.gui.gcp) and then
       transformed into the target location (i.rectify). Polynomial transformation of 1st, 2nd and 3rd order are
       supported.

       A graphical user interface is provided by wxGUI.

       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 location 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 coordinates, a forward transformation is
       performed. The unreferenced map is imported into the location with projection and  geocoded  within  this
       location  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).   Alternatively,
       v.rectify rectifies a vector by computing a coordinate transformation for each object in the vector based
       on the control points.

       A graphical user interface is provided by wxGUI.

   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 into raster data processing

           •   Introduction into 3D raster data (voxel) processing

           •   Introduction into vector data processing

           •   Introduction into image processing

           •   Introduction into temporal data processing

           •   Database management

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       © 2003-2019 GRASS Development Team, GRASS GIS 7.8.2 Reference Manual