We acquired five independent topographic images of the optic nerve head of eight normal eyes and eight eyes with primary open-angle glaucoma with a laser tomographic scanner. Each image had a field of view of 15 x 15 degrees with a resolution of 256 x 256 pixels. The pixel size was approximately 15 x 15 microns. The value of a pixel of a topographic image represented the height at this position. The mean height and the standard deviation over the five topographic images were calculated for each of the 65,536 pixel positions. The standard deviation of a single height measurement in normal eyes was 38.7 microns (range, 23.4 to 62.2 microns) for areas in the peripapillary retina and 42.6 microns (range, 24.4 to 53.7 microns) for measurements within the optic nerve head area. In glaucomatous eyes, the standard deviation was 41.2 microns (range, 23.2 to 59.6 microns) in the peripapillary retina and 49.4 microns (range, 28.1 to 72.8 microns) within the optic nerve head. There was no significant difference between the standard deviation of a single height measurement in normal and glaucomatous eyes (P = .34 within the optic nerve head area; P = .57 on peripapillary retina). No correlation was found between standard deviation of the measurements and pupil size or age of the subject.