Comparison of methods for deriving a digital elevation model from contours and modelling of the associated uncertainly

Topographic maps are the most common sources of Digital Elevation Models (DEMs).On these maps elevation data are explicitly displayed along contour lines. However such an input is not fully representative of the terrain shape which is known to vary continuously and smoothly in space. Moreover, in many instances the DEM is not the final objective. In our case, we intend to use it along with its derivatives as secondary information in interpolation of soil attributes. So DEM errors will propagate into the target variable. Therefore assessment of the amount and spatial distribution of these errors is crucial. Often, what is important is not the absolute DEM accuracy but the reproduction of the terrain shape. In this respect quantitative assessment criteria should be complemented by other qualitative criteria that account e.g. for artefacts and inconsistencies. The topographic data source is an 1:50000 topographic map with a 20 m contour interval. The study area has an area of 14.8 km and is composed of two contiguous watersheds located in a mountainous and lithologically contrasting zone of Burundi. Correlations between soil attributes and DEM-derived topographic attributes are significant, but they are too weak to be readily accounted for in soil attributes interpolation. Also propagation of DEM uncertainty into DEM derivatives is too pronounced and precludes the use of these as secondary information in soil attributes interpolation. Therefore we suggest changing the DEM into topographic units and use these as well as lithologic units as categorical soft information.