Geografic-Information-System/doc/projectionintro.html

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<h3>Projection management in general</h3>

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.

<h3>Raster map projections</h3>

Raster maps are backward projected in GRASS. This means that the
user has to run the raster projection command (<a href=r.proj.html>r.proj</a>)
in the target location and "pull" the map from the source location.
Both locations need to have a projection defined.

<h3>Raster map transformation</h3>

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
(<a href="i.group.html">i.group</a>, <a href="i.target.html">i.target</a>,
 <a href="g.gui.gcp.html">g.gui.gcp</a>) and then transformed into the
target location (<a href="i.rectify.html">i.rectify</a>). Polynomial
transformation of 1st, 2nd and 3rd order are supported.
<p>
A graphical user interface is provided by <a href="wxGUI.html">wxGUI</a>.
<p>
To simply translate a raster map (without stretching or rotation), the
<a href="r.region.html">r.region</a> command can be used.


<h3>Vector map projections</h3>

Vector maps are backward projected in GRASS. This means that the
user has to run the vector projection command (<a href=v.proj.html>v.proj</a>)
in the target location and "pull" the map from the source location.
Both locations need to have a projection defined.

<h3>Vector map transformation</h3>

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 (<a href="v.transform.html">v.transform</a>).

Alternatively, <a href="v.rectify.html">v.rectify</a> rectifies a vector
by computing a coordinate transformation for each object in the vector
based on the control points. 
<p>
A graphical user interface is provided by <a href="wxGUI.html">wxGUI</a>.

<h3>References</h3>

<ul>
<li> <a href="https://www.asprs.org/asprs-publications/grids-and-datums">ASPRS Grids and Datum</a>
<li> <a href="http://geotiff.maptools.org/proj_list/">Projections Transform List</a> (PROJ)
<li> <a href="https://proj.org/operations/index.html">Coordinate operations</a> by PROJ (projections, conversions, transformations, pipeline operator) 
<li> <a href="http://www.mapref.org">MapRef - The Collection of Map Projections and Reference Systems for Europe</a>
<li> <a href="http://crs.bkg.bund.de/crs-eu/">Information and Service System for European Coordinate Reference Systems - CRS</a>
</ul>

<h3>See also</h3>

<ul>
  <li><a href="rasterintro.html">Introduction into raster data processing</a></li>
  <li><a href="raster3dintro.html">Introduction into 3D raster data (voxel) processing</a></li>
  <li><a href="vectorintro.html">Introduction into vector data processing</a></li>
  <li><a href="imageryintro.html">Introduction into image processing</a></li>
  <li><a href="temporalintro.html">Introduction into temporal data processing</a></li>
  <li><a href="databaseintro.html">Database management</a></li>
</ul>