In order to establish rational use of ASTER GDEM and SRTM elevation datasets,the slope and slope length have been extracted from the datasets that were downloaded from the website and have been preprocessed.Furthermore,the extracted features were compared with Hc-DEM derived variables.The results showed that:(1) the Hc-DEM derived features were much better than the SRTM and ASTER GDEM derived in terms of all the topographic variables including elevation,slope,and slope length.(2) the variables extracted from ASTER GDEM were unrealistic for the areas with gentle slope,owing to the abnormally void values and irregular sinks,but in steepy area it is useful;(3) the topographic variables extracted from SRTM and Hc-DEM showed similar patterns,while the SRTM based variables featured reduced slopes and extended slope length,suggesting down-scaling may be required if it is applied in hydrological or erosional modelling.