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projectionintro(1grass)     GRASS GIS User's Manual    projectionintro(1grass)

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  coordi-
       nate system are defined.  To change the projection of maps, a new loca-
       tion 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  loca-
       tion  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 lo-
       cation (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 lo-
       cation (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  loca-
       tion  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  coordi-
       nates,  a  forward transformation is performed. The unreferenced map is
       imported into the location with projection and geocoded within this lo-
       cation  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, trans-
               formations, pipeline operator)

           •   MapRef - The Collection of Map Projections and  Reference  Sys-
               tems for Europe

           •   Information  and  Service System for European Coordinate Refer-
               ence 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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       © 2003-2022 GRASS Development Team, GRASS GIS 7.8.7 Reference Manual

GRASS 7.8.7                                            projectionintro(1grass)

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