Georeferencing raster image with QGIS

Georeferencing is the process of assigning geographic coordinates to an image or raster file from aerial or satellite. It is possible for several years, to obtain satellite images for free on the web from Google Earth.

In this exercise, an image capture on Google Earth somewhere over Morocco will be georeferenced. This is the same image used in the tutorial on georeferencing with MapInfo.

It is recommended to calibrate the image in a folder so that all generated files will be stored in that folder.
So to start georeferencing with Quantum GIS (1.8 Lisboa), click the Raster menu (1) > Georeferencer (2) > Georeferencer (3). A large dialog box opens.

Outil pour géoréferencer dans GIS

Click the Open Raster button (4). In the dialog box that appears, navigate to your folder and open the file or raster image in question.

 

Fenêtre Géoréférencer de QGIS

A new dialog box will open asking you to set the Coordinate Reference System (CRS). In our case, the geographic coordinate system is WGS 84 used by Google Earth. Then confirm with OK (5).

Choix du Système de Coordonnée de Référence (SCR)

The image loads into QGIS Georeferencer. Zoom in with the zoom tool of Georeferencer on the reference points marked on the image.

Click the Add Point button (6), then click on the reference point on the image and enter the X and Y coordinates in the dialog box that follows and press OK. A small red dot appears where you've clicked on the image.

 

Ajout des points de contrôle géographiques
Move the image with the Pan tool (hand) until the next reference point and repeat the addition of point coordinates.

Once all the points listed, click the Transformation Settings button (7).

Transformation Settings dialog box appears. Set the type of transformation, the method of resampling and the folder of the output raster file as displayed in the image below. Check Loads in QGIS when done below the dialog box to open the georeferenced image in QGIS and confirm these settings by hitting OK.

Paramètres de transformation pour le géoréférencement

Now, click Start Georeferencing button (picture below) to generate the new GeoTiff raster file.

Bouton commencer le géoréférencement

Once the new image generated, opening it in QGIS requires defining the Coordinate Reference System once again which is always WGS 84. After confirming with OK your new georeferenced image in GeoTIFF (. Tif) format is displayed in the QGIS canvas.

Enough chitchat, watch the video below.

 

 

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