To calculate the radiation balance of small-scale heterogeneous terrain well known approaches were combined with the vegetation-boundary layer model HIRVAC (HIgh Resolution Vegetation Atmosphere Coupler). These modified approaches were transferred in a GIS (ArcView) environment to consider both topographic influences (terrain geometry, sky view factor) and the influence of different land uses (e.g. pasture and forest) on radiation components. The GIS application is based on the digital elevation model of the Saxonian land surveying office (SLVA) with a resolution of 25 meters and on the land use classification from the Institute of Cartography of the Technical University Dresden. The model was applied to the Eastern Ore Mountains (Saxony/Germany). To illustrate the results the area of the "Tharandter Wald" near Dresden was selected. The results show that the influences of topography and land use on radiation balance are equally important on a terrain with complex land surface patterns and a strong domination of small-scale topography. The shortwave insolation on a clear Spring day is, e.g., over areas with northern aspect up to 10 times lower than over southward orientated areas. The spruce areas in the forest are characterised by 2 times higher daily sums of net radiation than the adjacent clearcuts. The model results are in adequate agreement with measurements of the "Anchor Station Tharandter Wald" for the investigated period. The results can be applied for water budget modelling, forest management and for downscaling of satellite information.