Wikipedia:Graphics Lab/Resources/QGIS/Reprojection

Projection (0%)

 * → waiting a volunteer

QGIS display your data using the projection you tell it to use. By default, the lat/long projection (= Equirectangular projection, example aside) is used, code:. This projection is the most convenient for georeferencing, but imply strong distortions at polar latitudes. You may be interested to set a projection when you start a project, or to reproject (warp) an existing lat/long project to a specific projection just before to export your final image or svg. Indeed, each country often have a specific conventional projection to minimized distortions of its geographic space.
 * Background

First, you have to find the definition of your source projection/file. It's a kind of long equation which definite the properties of each projection. So:
 * Source projection
 * QGis (1.7) > (double click on the layer to reproject) > Layer's properties (window pop up) > tab 'Coordinates Reference System (CRS)' > Layer Spatial Reference System : [projection's definition] > (copy it)

Secondly, you have to choice your target projection.
 * Which new projection for my map ?

Third, you have to find the definition of this target projection.
 * Target projection
 * QGis (1.7) > File > Project's properties (a window pop up) > tab 'Coordinates Reference System (CRS)'
 * >> Select your target projection* > copy the definition (the long equation) >
 * >> ?create a personalized projection.?
 * QGis (1.7) > Raster > Warp (reproject) > (a window pop up)
 * > fill the fields witht he source SRS, and the target SRS > open in canvas > run > get the result.


 * Reproject a raster layer


 * Reproject a vector layer


 * Frequent projections

Links

 * Liste of National Coordinate Reference Systems

(I should read before writing)
Read: fr:Wikipédia:Atelier graphique/Didacticiels cartographiques/Didacticiels pour la cartographie numérique géoréférencée
 * 1) /Didacticiel d'installation de winGRASS GIS
 * 2) http://www.qgis.org/wiki/Users_Corner
 * En:User:Captain_Blood/GMT_Example

Travaux impressionnant:

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