| Abstract | The improvement in the speed and affordability of computers, the increased resolution of scanned airphoto and satellite images (5m and less), and the integration of cartographic features with GIS data
(raster and vector) have revolutionized the process of topographic mapping. However, a growing number of applications still require an orthorectification process so that images can be accurately corrected to a ground reference system. Because they
are planimetrically correct, the resulting orthoimages can be used as maps. Being able to correlate features on an orthoimage with what is observed on the ground is both time saving and cost effective. Highly accurate ground control points (GCPs)
are necessary to generate orthoimages. One method of collection is to use a GPS receiver. A high-quality GPS receiver with post-processing differential correction can yield GCPs with a horizontal and vertical accuracy of within 1m and 2m,
respectively. The problem with post-processing differential correction is that the user cannot accurately plot the GCPs on-site. For example, if the GPS antenna does not have a clear path to the satellites, the user is not immediately aware of the
error. A new service called real-time differential correction whereby the user can find out the coordinate of any point in real time is now available in many countries. This eliminates the post-processing differential correction step while yielding
GCPs of comparable horizontal and vertical accuracy. |