Georeferencing a raster from control points

Sometimes, a raster that represents features on the earth is just an image with no georeferencing information. That is certainly the case with historical scanned maps. However, you can use a referenced data set of the same area to create tie points, or ground control points, and then use an algorithm to warp the image to fit the model of the earth. It is common for data collection systems to just store the ground control points (GCP) along with the raster to keep the image in as raw a format as possible. Each change to an image holds the possibility of losing data, so georeferencing an image on demand is often the best approach.

In this recipe, we'll georeference a historical survey map of the Louisiana and Mississippi Gulf Coast from 1853. The control points were manually created with the QGIS Georeferencer plugin and saved to a standardized control point file.

Getting ready

Download the following ZIP file, unzip the contents, and put the georef...