However, using water features was necessary in this case, due to the fact that human-built features were generally absent in all but the most recent images, and then only in a limited southern area of the NDA. To help correct for this weakness, each image was assigned a relatively high number of GCPs; the mathematical minimum required per image is three, although each LPS block project in this case used anywhere from forty (one image, 1977) to 164 GCPs (10 overlapping images). Table 1 (below) provides specifics on GCP use for each block project.