RELATIVE ACCURACY OF RECTIFICATIONS USING COORDINATES DETERMINED FROMMAPS AND THE GLOBAL POSITIONING SYSTEM

Global positioning systems (GPS) that use code phase receivers have th e potential for providing more accurate coordinates for ground control points (GCPs), than do 7.5-minute quadrangle maps. To evaluate the ef fect of the greater accuracy of GPS in determining coordinates for sat ellite images, GCP coordinates were measured on maps and by GPS techni ques and were then used to rectify Landsat Thematic Mapper, SPOT multi spectral, and SPOT panchromatic images from the same area. There were major differences between rectifications using map and CPS coordinates . For Thematic !Mapper data, 24 percent of the values assigned to pixe ls in the rectification performed using map coordinates differed from the values assigned to the same pixels in the rectification performed using the GPS coordinates. For SPOT multispectral and SPOT panchromati c images, the percentage of pixels with different assignments were gre ater than 40 percent. The improvements in accuracy when using the GPS data were substantial for even the relatively large Thematic Mapper pi xels and warrant the use of GPS where position accuracy is essential.