oracular (1) projectionintro.1grass.gz

Projections and spatial transformations in GRASS GIS

   Projection management in general
       A  GRASS  project  is  referenced  with  a single projection and coordinate system (or unreferenced as XY
       project). When creating a new project 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 project  has  to  be  created  and  the  desired  maps  have  to  be
       reprojected into it from the source project as explained below.

   Reprojecting raster maps
       Rasters  are reprojected using the raster projection tool r.proj.  The tool is used in the target project
       to "pull" a map from its source project.  Both projects need to have a  projection  defined,  i.e.,  they
       cannot be XY (unprojected).

   Raster map transformation
       To  transform  an  unprojected  map from a XY project into a projected project (or another XY project), a
       forward transformation is performed.  The unreferenced map is geocoded within the XY project 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 project (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
       Vectors  are reprojected using the vector projection tool v.proj.  The tool is used in the target project
       to "pull" a map from its source project.  Both projects need to have a  projection  defined,  i.e.,  they
       cannot be XY (unprojected).

   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 project with projection  and  geocoded  within  this
       project  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  project  (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

SOURCE CODE

       Available at: Projections and spatial transformations in GRASS GIS source code (history)

       Accessed: Saturday Jul 27 11:27:22 2024

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       © 2003-2024 GRASS Development Team, GRASS GIS 8.4.0 Reference Manual