Square visibility maps (blue area) for a given observer (green outlined pixel). All pixels (white + signs) that are within the horizon lines (black line) can potentially contribute additional heat to the observer position if a direct line of sight exists. All pixels (black + signs) that are outside the horizon line, or coincide with the observer, do not contribute to the temperature at the observer position. (a): NOLOD visibility map using the full resolution of the input DTM. (b): LOD visibility map applying two LODs with full resolution for the first LOD at ±1 pixel around the observer and a third of the resolution for the second LOD.