Time to Revamp the Slope Algorithm and KISS?

Abstract—The slope and aspect algorithm, crucial for many applications of digital elevation models suffers from a number of variants that produce very similar results. While some early implementations used other approaches, most current algorithms use least squares to fit a surface to the analysis window, and then derive two partial derivatives to estimate the slope and aspect. Simplification of the least squares solution leads to the common Evans methods. It is time to standardize on a single slope algorithm, with four parameters that can be adjusted: the order of the polynomial, the size of the window, whether the computation can proceed with voids, and if it uses all available points in the window. Pre-filtering of the DEM should not be needed with current DEMs, but selection of a larger analysis window can smooth and generalize the slope and aspect grids and have the same effect as pre-filtering. Additional, related material at Guth, P. (2025). Using MICRODEM to Compare Slope Algorithms. Zenodo. https://doi.org/10.5281/zenodo.15156208