Photoclinometry has become a popular technique for the extraction of topography from digital images of planetary surfaces. The technique, however, is subjected to a number of error sources that can degrade results significantly. We present here a quantitative analysis of error sources in one-dimensional planetary photoclinometry. The technique is affected by at least seven error sources, which can be broken down into three categories: those arising from (1) the spacecraft image, (2) the planetary body, and (3) the scan line orientation. Spacecraft image error sources include noise, background offset, and quantization. Errors deriving from the planetary body include those induced by variations in photometric properties, such as albedo, by incorrectly compensated atmospheric effects, and by an incorrectly determined photometric function. Finally, errors will result for scan lines which do not lie perpendicular to topographic strike. We calculate slope errors for each of these sources, using the examples of Voyager imaging of Ganymede and Viking orbiter imaging of Mars. Slope errors are investigated for a variety of viewing and lighting geometries, slope angles, and slope orientations. The results can be broken down into nonsystematic and systematic errors. Nonsystematic errors are introduced by image noise and quantization and affect the slope calculations for each picture element independently. Other error sources are systematic; these errors are more insidious, since they may retain the general appearance of the topography while approximately scaling all relief by a multiplicative constant. We present derivations that allow the calculation of photoclinometric slope errors for any photometric function and also briefly discuss the implications of our results for two-dimensional photoclinometric techniques. ¿ American Geophysical Union 1991