The Georeferencer Plugin is a tool for generating world files for rasters. It allows you to reference rasters to geographic or projected coordinate systems by creating a new GeoTiff or by adding a world file to the existing image. The basic approach to georeferencing a raster is to locate points on the raster for which you can accurately determine their coordinates.
|Open raster||Start georeferencing|
|Generate GDAL Script||Load GCP Points|
|Save GCP Points As||Transformation settings|
|Add Point||Delete Point|
|Move GCP Point||Pan|
|Zoom In||Zoom Out|
|Zoom To Layer||Zoom Last|
|Zoom Next||Link Georeferencer to QGIS|
|Link QGIS to Georeferencer|
Table Georeferencer 1: Georeferencer Tools
As X and Y coordinates (DMS (dd mm ss.ss), DD (dd.dd) or projected coordinates (mmmm.mm) which correspond with the selected point on the image, two alternative procedures can be used:
The usual procedure for georeferencing an image involves selecting multiple points on the raster, specifying their coordinates, and choosing a relevant transformation type. Based on the input parameters and data, the plugin will compute the world file parameters. The more coordinates you provide, the better the result will be.
The first step is to start QGIS, load the Georeferencer Plugin (see Section Loading a QGIS Core Plugin) and click on the Georeferencer icon which appears in the QGIS toolbar menu. The Georeferencer Plugin dialog appears as shown in figure_georeferencer_1.
For this example, we are using a topo sheet of South Dakota from SDGS. It can later be visualized together with the data from the GRASS spearfish60 location. You can download the topo sheet here: http://grass.osgeo.org/sampledata/spearfish_toposheet.tar.gz.
Figure Georeferencer 1:
Figure Georeferencer 2:
The points that are added to the map will be stored in a separate text file ([filename].points) usually together with the raster image. This allows us to reopen the Georeferencer plugin at a later date and add new points or delete existing ones to optimize the result. The points file contains values of the form: mapX, mapY, pixelX, pixelY. You can use the Load GCP Points and Save GCP Points buttons to manage the files.
Within the GCP table you can click on a column header and therewith enable e.g. numerical sorting. The GCP list is automatically updated.
After you have added your GCPs to the raster image, you need to define the transformation settings for the georeferencing process.
Figure Georeferencer 3:
Depending on how many ground control point you have captured, you may want to use different transformation algorithms. Choice of transformation algorithm is also dependent on the type and quality of input data and the amount of geometric distortion that you are willing to introduce to final result.
Currently, following algorithms are available:
The type of resampling you choose will likely depending on your input data and the ultimate objective of the exercise. If you don’t want to change statistics of the image, you might want to choose ‘Nearest neighbour’, whereas a ‘Cubic resampling’ will likely provide a more smoothed result.
It is prossible to choose between five different resampling methods.
There are several options that need to be defined for the georeferenced output raster.
Clicking on the Raster properties dialog in the Settings menu opens the raster properties of the layer that you want to georeference.