Studies of snake movement frequently report home range sizes, and these data are key to our understanding of snake ecology. However, despite the fact that many snakes occupy landscapes with topographic relief, snake home range sizes are consistently reported as planimetric (two-dimensional) estimates. We investigate the capacity for planimetric area measurements to underestimate snake home range sizes, and we explore how this may confound understanding of snake ecology. We use radiotracking data to estimate home ranges for Crotalus mitchellii and C. ruber in a topographically-variable landscape, model surface terrain in the estimated home ranges using digital elevation data and widely-available GIS software, and compare planimetric and topographic (three-dimensional) area measurements for these home ranges. The topographic measurements exceeded planimetric ones, on average, by 14% for C. mitchellii (8% for females, 19% for males) and 9% for C. ruber (10% for females, 8% for males); this suggests that terrain modeling can provide considerably truer estimates of snake home range sizes. We discuss how such increased precision might benefit snake ecology research, and we discuss recent papers that might have concluded differently had topographic home range measurements been used. Terrain modeling has become relatively simple with the advent of modern GIS software, and snake ecologists could begin using it routinely.