In the UK national derived geological datasets are increasingly being produced, many of which are based on NEXTMap digital terrain model (DTM) or digital surface model (DSM) data. These include groundwater level and land stability datasets. Any DTM is a model of the land surface and under different conditions may have differing degrees of accuracy. This paper compares the NEXTMap data, derived from airborne Interferometric Synthetic Aperture Radar (IfSAR) data, with other frequently used datasets derived from contours and point data; in particular, the Integrated Hydrological Digital Terrain Model (IHDTM), a terrain model that was originally derived from Ordnance Survey (OS) 1:50 000 scale contours, and a DTM interpolated from Land-Form PROFILE data. This initial comparison of the DTMs has highlighted some issues with the NEXTMap data: first, that of elevation inaccuracy in woodland areas; and, secondly, the shadowing effect caused by the side-looking scanner. It also highlights the problems of using DTMs created from contour data in areas of low relief. The development of an uncertainty layer would enable a user to decide whether the DTM was appropriate in certain areas, and could also be incorporated into uncertainty models for the derived national datasets. \ud \u
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