This paper explores the quantitative relation between the reliability of slope aspect, gradient, and form mapped from a gridded DEM and the sampling interval (SI) of elevations. Grid DEMs initially interpolated from digitised contours at 10 m were sampled to five other resolution levels. The topographic variables mapped at these SIs were compared with those at 10 m. It is found that the reliability of mapped slope aspect and form is not significantly affected by SI. By comparison, the reliability of slope gradient is more susceptible to SI, especially if it is derived from a gently rolling terrain. Around 90% of the variation in the mapped slope aspect and gradient are accounted for by the inaccuracy of DEMs. A lower percentage exists for slope form. The stability of the mapped topographic variables can be reliably predicted from SI and terrain complexity.