The source map or orthophoto is registered geographically, and the data points can then be defined relative to this system. Manual digitization is slow and is being widely supplanted by autodigitization and vectorization techniques. A range of representative systems is given in Table 1. It operates on raster images.
All processing is done on a desktop computer.
They are processed to reduce the amount of dithering and pixilation of colors. The user then selects a color to follow and clicks two points on a line of that color. This can be quite useful if the line being digitized crosses other lines of the same color or has many tight bends. In order to tile the images together we needed to convert the state plane coordinates to latitudes and longitudes. The original topographic map data upon which the digital data are based were derived from 1978 overflights.