ASSESSMENT OF CLASSICAL, ASTER AND SRTM DEMs IN NAIROBI REGION, KENYA
Keywords:ASTER DEM, SRTM DEM, Classical DEM, GPS, orthometric height, second order surface polynomial
A digital elevation model (DEM) is a 3-D representation of the earth’s topography. Vertical accuracy of a global digital elevation model (GDEM) is necessary for optimal application of satellite-based terrain elevation datasets. This study carries out an assessment of the vertical accuracy of Classical, ASTER (30 m) and SRTM (90 m) digital elevation models (DEMs) which are normally used for reconnaissance surveys, hydrological analysis, biomass estimation and geoid modelling among others. Classical DEMs are drawn from regional topographical maps while ASTER and SRTM DEMs are obtained from satellite-based remote sensing missions. The assessment is carried out by comparing orthometric heights from precise levelling at 18 points and heights derived from the DEMs over Nairobi County and its environs. The study found that the mean and standard deviation of the direct differences between precisely levelled heights and DEM heights are: 3.97 m and ±7.76 m respectively for classical DEM; 16.36 m and ±7.79 m respectively for ASTER DEM and -0.25 m and ±4.00 m respectively for SRTM DEM. The results indicate that SRTM DEM is the most accurate followed by the classical and ASTER DEMs in that order. We then modelled the differences between the DEM heights and the orthometric heights using a second order surface polynomial at 12 points; the polynomial was then applied to 6 test points in a cross-validation manner. The results from the polynomial improved accuracy of height determination in SRTM DEM but degraded accuracies in the classical and ASTER DEMs.