The Caledonian Orogeny marks a starting point for the evolution of the Scottish Highlands. There is debate as to the level of erosion that the Highlands have experienced since the Devonian and the extent to which the Highland landscape reflects Permo-Triassic rather than Caledonian events. Data on the position and elevation of the Caledonian and Permo-Triassic unconformities have been used to create topographic models of both surfaces. A variety of computer mapping packages have been used that allow the interpreter to control many of the mapping parameters, creating models of surfaces that honour the data and maintain realistic surface trends. The effects of these parameters have been tested in a series of sensitivity experiments. The modelled Caledonian erosion surface has proved to be a good indicator of the present-day surface, suggesting that the Highlands are an exhumed landscape. This indicates that there has been limited denudation of basement rocks since the end of the Devonian. The model of the Permo-Triassic erosion surface has lower altitude and less relief than the model of the Caledonian surface, suggesting onlap onto a positive Highland block. Palaeomagnetic results showing Permo-Triassic reddening and fissuring of Highland basement rocks are interpreted as reflecting re-occupation of an older surface.