View source for Wikipedia:Graphics Lab/Resources/QGIS/Reprojection

{{stub}}
{{in progress|—Contribute to this tutorial by completing one section welcome !}}

== Projection  (0%) ==
:{{waiting}} → waiting a volunteer
[[File:Equirectangular projection SW.jpg|300px|thumb|Equirectangular projection of the world; the standard parallel is the equator (plate carrée projection).]]
[[Image:Tissot indicatrix world map equirectangular proj.svg|thumb|right|300px|The equirectangular projection with [[Tissot's indicatrix]] of deformation]]
[[File:Winkel triple projection SW.jpg|thumb|right|300px|[[Winkel tripel projection]], lowest balance of distortion, used by the [[National Geographic]].]]
[[File:Tissot indicatrix world map Winkel Tripel proj.svg|thumb|right|300px|The Winkel tripel projection with [[Tissot's indicatrix]] of deformation.]]
;Background
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: {{green|'''WGS84 lat/lon (EPSG:4326)'''}}. 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.

;Source projection
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:
: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)

;{{in progress}} Which new projection for my map ?
Secondly, you have to choice your target projection.
{{stub}}

;{{in progress}}Target projection
Third, you have to find the ''definition'' of this 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
{{stub}}
;Reproject a vector layer
{{stub}}
;Frequent projections
{{main|List of National Coordinate Reference Systems}}

== Links ==
* [[Liste of National Coordinate Reference Systems]]
== References ==
<references />
{{Digital cartography tutorials}}

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

Travaux impressionnant:
<gallery widths="600px" heights="300px">
file:Porquerolles_topographic_map-fr.svg|
file:Rivers_of_New_Brunswick_map-fr.svg | Ombre: SRTM3 v.2 + [[:en:3DEM]] 
file:Rivers_of_New_Brunswick_topographic_map-fr.svg |
File:China edcp location map.svg | '''{{green|Geocoded}}''' File:China edcp location map.svg, part of the +500  [[:de:Wikipedia:Kartenwerkstatt/Positionskarten|Location maps]].
File:China edcp relief location map.jpg|'''{{green|Geocoded}}''' File:China edcp relief location map.jpg (with paralleles and lattitudes)
</gallery>
}}