Abstract
AbstractCommon global digital elevation models (DEMs) at 1 to 3 arc‐second resolution use geographic coordinates for grid definition, and while cells are square (equal dx and dy spacing) near the equator, they become increasingly rectangular at higher latitudes. Some GIS software cannot handle rectangular grid cells, but minor modifications to slope, aspect, and hillshade algorithms can handle them. The algorithms can also handle grids with other projections, including Equi‐7, Lambert conformal conic, Lambert azimuthal equal area, and polar stereographic, converting aspect to true north instead of the potentially rotated grid. Computed slope varies with grid spacing with a logarithmic relationship and negative slope. Slope maps and distributions from rectangular DEMs, or DEMs where the actual grid spacing varies from the nominal values because points are far from the undistorted region, have characteristics equal to those of a Universal Transverse Mercator grid with spacing equal to the average dx and dy spacing of the DEM.
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