The orthophoto horizontal positional accuracy and the assurance of that accuracy depend, in part, on the accuracy of the data inputs to the rectification process. These inputs consist of the digital elevation model (DEM), aerotriangulation control and methods, the photograph source camera calibration, scanner calibration, and aerial photographs. The vertical accuracy of the verified USGS format DEM is equivalent to or better than a USGS level 1 or 2 DEM, with a root mean square error (RMSE) of no greater than 7.0 meters. Field control is acquired by sub-meter DGPS. Aerial cameras have current certification from the USGS, National Mapping Division, Optical Science Laboratory. Test calibration scans are performed on all source photograph scanners.
Erdas OrthoBASE is used to determine the horizontal positional accuracy for orthophoto data produced by Benchmark GIS. The program determines the accuracy by finding the line and sample coordinates of checkpoints in the orthophoto and fitting these to their ground coordinates to develop an RMSE. From 4 to 9 points are checked. Adjacent orthophoto's, when displayed together in a common planimetric coordinate system, may exhibit positional discrepancies across common orthophoto boundaries. Linear features, such as streets, may not be continuous. These edge mismatches, however, still conform to positional horizontal accuracy within the NMAS