In the next step we georeference the image using some source of location control (for example, topographic maps, digital roads, or ground control points measured using GPS receivers). In this example, location control is provided using roads depicted on 1:100,000 scale Digital Line Graph (DLG) data downloaded from the US Geological Survey. Prominent road junctions are located throughout the image, and their map locations measured from the DLG file. Once a suitable set of location control points is defined in this way, the image data are then digitally “resampled” to fit a desired map projection.