However, instead of using a special camera to obtain an upward-looking viewshed for a location, TopoView and the Solar Analyst use DEMs to calculate viewsheds for each location in an area. This method determines the angle above the horizon where the sky is obstructed by topographic features. The viewshed is placed over a sun map, which depicts the sun's path, to calculate direct radiation. Each position along the sun's path corresponds to a time of day and year, and an amount of direct insolation originating from that sky direction. Obscured sky directions along the sun's path correspond to times when the location in shadow.
Direct radiation can only originate from visible sky directions.
The viewshed is then overlaid on a sky map to calculate diffuse radiation. Sky maps depict the entire sky and a known amount of diffuse radiation originates from each sky direction. Obscured sky directions correspond to directions from which diffuse radiation is blocked. The viewshed on the skyrnap allows for measuring only diffuse radiation originating from visible sky directions. Calculations account for latitude, elevation and orientation of the receiving surface, and atmospheric conditions.