Errors were estimated for GIS spatial data derived via monoscopic interpretation of 230 mm (9-inch) aerial photographs. Positional and area error were observed for a range of photo scales, camera tilts, and relief. Average positional error was observed between 4 and 104 meters and varied positively with tilt and terrain range and inversely with scale. Maximum average errors were approximately 3 to 4 times overall averages. Average errors in polygon area were observed over a range of polygon sizes, from 40 to 600 meters. Average percent area error ranged from 1.3 to 10.7 percent and was positively related to tilt and terrain range and not related to scale or polygon size. The magnitude of these errors and the availability and cost of correction technologies suggest analytical methods for removing relief and tilt distortion should be used where possible.