Clear sky shortwave solar radiation varies in response to altitude and elevation, surface gradient (slope) and orientation (aspect), as well as position relative to neighbouring surfaces. While the measurement of radiation flux on a relatively flat surface is straightforward, it requires a dense network of stations for mountainous terrain. The model presented here uses a digital elevation model to compute potential direct solar radiation and diffuse radiation over a large area, though the model may be modified to include parameters such as cloud cover and precipitable water content of the atmosphere. The purpose of this algorithm is for applied work in forestry, ecology, biology and agriculture where spatial variation of solar radiation is more important than calibrated values. The ability of the model to integrate radiation over long time periods in a computationally inexpensive manner enables it to be used for modelling radiation per se, or input into other hydrological, climatological or biological models. The model has been implemented for commercially available GIS (viz. Arc Info and Genasys) and is available over the Internet.