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 (PROJ)

           •   Coordinate operations by PROJ (projections, conversions, transformations, pipeline
               operator)

           •   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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       © 2003-2023 GRASS Development Team, GRASS GIS 8.3.0 Reference Manual