Issues of using digital maps for catchment delineation

Digital elevation models (or maps) (DEMs) are increasingly becoming available to consulting engineers and academic researchers for delineating catchment boundaries. There are two important questions to be answered in using DEMs for generating those boundaries: first, how good are the computer-generated boundaries and second, what map resolution should be used from a host of data products on the market? This paper describes some explorations of these two topical issues through a case study for the Brue catchment in the south-west of England. The study concludes that computer-generated boundaries failed to delineate accurately or reliably the catchment border with all the available digital maps tested. This was mainly because the computer cannot pick up man-made features (highways, ditches, etc.), in addition to data quality and algorithm problems. With respect to map resolution, it has been found that although the discrepancy of the delineation increases with the grid size (i.e. poor resolution maps would generate lower-quality boundaries), there is a threshold which defines a zone where the clear relationship between the map resolution and discrepancy becomes very unstable. This is a very important conclusion since it indicates that higher map resolution may produce poorer results than lower map resolution, which is quite serious in practical projects for map users who face increased map costs, longer computing time and potentially poorer results. In the end, the paper proposes an integrated technique that uses the strengths of both manual and computer methods to produce an optimised boundary.