Calculates the minimum time taken to walk from a given location within a Digital Elevation Model (DEM) to all other locations within the DEM. It assumes that walking speed depends only on the gradient of the terrain, using a simple model (Rees 2004a), and implements Dijkstra’s least-cost algorithm (Rees 2004b). It assumes that all areas within the DEM are equally navigable, so it will happily cross bodies of water, fences etc.