Georectification of the aerial photographs involves the establishment of ground control points that link each image to its corresponding aerial coverage on a 1995 USGS digital orthophoto quarter quad (DOQQ). Points are chosen on the image that can be matched to points on the DOQQ. Road intersections and other cultural features are preferred as reference points rather than natural features. However, in many cases cultural features are lacking and features such as trees, shrubs, and the edges of water bodies are used. Where possible, points are evenly spaced across the image surface, with special emphasis on the edges of the image, and on areas near to the shoreline. The number of ground control points used for each image varies depending on how distorted the image is, and on the availability of suitable reference features. The average range is approximately 30 to 60 points per image. Once all the ground control points have been established, the image is rectified. The images underwent a delaunay triangulation algorithm, in which each ground control point became a vertex in a mesh of triangle. A single polynomial warp was applied to each triangle in order to correct for local distortion. Once the rectification was complete, the image was made semi-transparent and overlain on the DOQQ. In some areas a "double image" would occur, indicating that either more ground control points were needed, or that one or more existing ground control points had been placed incorrectly. In these cases, images were reprocessed to correct any remaining distortion.