This process is to establish the relationship between page coordinates (i.e. x,y) of a planar map of image with known real-world coordinates (i.e. longitude/latitude, UTM, etc). Typically, ground control points (GCPs) can be collected with GPS technique. The collected GCPs can be used to geo-reference the images. A digital orthophoto quadrangle (DOQ) is a computer-generated image of an aerial photograph in which image displacement caused by terrain relief and camera tilts has been removed. It combines the image characteristics of a photograph with the geometric qualities of a map. All DOQ’s are referenced to the North American Datum of 1983 (NAD83) and cast on the Universal Transverse Mercator (UTM) projection. Primary (NAD83) and secondary (NAD27) datum coordinates for the upper left pixel are included in the header to allow users to spatially reference other digital data with the DOQ. So, Digital Ortho Quarter Quads (DOQQ's) can be also used to geo-reference the images.