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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