Frequently, on maps of magnetic or electromagnetic data, strong interference due to surface relief can obscure anomalies which are unrelated to terrain. By the simultaneous use of topographic and geophysical maps, mining geophysicists attempt intuitively to remove terrain effects. In previous work (Richards et al, 1967), topographic and magnetic maps were used together to obtain direction and polarization of marine magnetic anomalies. In this paper, a simple linear filter to reduce terrain “noise” will be developed. The filter has two inputs: the geophysical and topographic maps. These are combined to form the output geophysical map.