EFFECTS OF VERTICAL RESOLUTION AND MAP SCALE OF DIGITAL ELEVATION MODELS ON GEOMORPHOLOGICAL PARAMETERS USED IN HYDROLOGY

The advent of digital elevation models (DEMs) has made it possible to objectively extract, calculate and store geomorphological parameters f or hydrological modelling at several scales. For a grid-based DEM, the threshold area used to extract the channel network is analogous to th e scale of the map produced. In addition to the map scale, the effects of the vertical resolution of the DEM on some frequently used geomorp hological parameters in hydrology are examined using high-resolution D EMs of two natural and two artificial catchments. The vertical resolut ion was varied between 1 cm and 1 m, the most common vertical resoluti on of DEMs. At a fixed map scale, the mean absolute percentage error i n the geomorphological parameters caused by a decrease in vertical res olution is within the range 0-5% for the medium-sized catchments and 0 -10% for the small catchments studied. Although it is true that a chan ge in vertical resolution may cause a change in the individual pixel s lope, area and topographic index (area/slope), particularly in low rel ief terrain, their cumulative distributions do not show any significan t change with the vertical resolution. The shape of the normalized wid th function is not very sensitive to the vertical resolution and the m ap scale. For small catchments order change may occur at different map scales for the different vertical resolution DEMs of the same catchme nt, causing a significant change in order-related parameters such as H orton ratios. It is suggested that the vertical resolution of the DEM of a catchment be considered satisfactory for most hydrological applic ations if the ratio of the average drop per pixel and vertical resolut ion is greater than unity. This ratio criterion could be used to defin e the minimum pixel area for reliable channel network definition for a ny given vertical resolution. The minimum pixel area places a lower bo und on the horizontal resolution with which a channel network can be e xtracted from a DEM. These results could potentially be used to assess the adequacy for hydrological purposes of existing and proposed digit al elevation databases.