Wikipedia:Graphics Lab/Resources/QGIS/Reprojection

 —Contribute to this tutorial by completing one section welcome !

Projection (0%)

C → waiting a volunteer

Equirectangular projection of the world; the standard parallel is the equator (plate carrée projection).

The equirectangular projection with Tissot's indicatrix of deformation

Winkel tripel projection, lowest balance of distortion, used by the National Geographic.

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: 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.

 In progressTarget 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

Reproject a vector layer

Frequent projections

Main article: List of National Coordinate Reference Systems

Links

• Liste of National Coordinate Reference Systems

References

The table above has links to various tutorials and resources which can help in the creation of Wikipedia maps from digital georeferenced data (GIS).

== (I should read before writing) == Read: /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: Ombre: SRTM3 v.2 + en:3DEM Geocoded File:China edcp location map.svg, part of the +500 Location maps. Geocoded File:China edcp relief location map.jpg (with paralleles and lattitudes)