The distance from the center of one friction skin ridge to the center of the ridge next to it is quite variable in different regions of a given fingerprint. All Automated Fingerprint Identification Systems fingerprint reader subsystems fail to detect some of the true minutiae in a fingerprint. Missed and false detections will degrade the accuracy of ridge counts calculated from these data. In a small sample of fingerprints, this distance between ridges was measured. The values ranged from 0.2 mm to 0.85 mm on fingerprints of male subjects and from 0.2mm to 0.75mm on fingerprints from female subjects. The mean ridge to ridge distance for 731 measurements on the male subjects was 0.46mm. In 1,046 measurements on the female subjects, the mean value was 0.41mm. A method is described for using these values to calculate ridge counts between near neighboring minutiae, and estimates are made of the errors likely to result from the use of calculated ridge counts. 2 figures, 3 tables, 5 references. (Author abstract modified)