Effects of DEM resolution on the calculation of topographical indices: TWI and its components

A variety of landscape properties have been modelled successfully using topographic indices such as the topographic wetness index (TWI), defined as ln(a/tan β), where a is the specific upslope area and β is the surface slope. Previous studies have shown the influence of scale on TWI values when converting standard-resolution DEMs to coarser resolutions. In this study a high-resolution digital elevation model (DEM) with a 5 m grid size derived from LIDAR (light detection and ranging) data was used to investigate the scale-dependency of TWI values when converting from high-resolution elevation data to standard-resolution DEMs. First, a set of DEMs was generated from an initial DEM by thinning to resolutions of 10, 25, and 50 m grid sizes to study the effects of lower grid size and decreased information content. Next, to investigate the impact of different information content on DEMs with the same grid size, the three lower resolution DEMs were all interpolated to the original 5 m grid size. In addition to comparing index distribution functions, a second objective was to evaluate differences in spatial patterns. Thus the values of TWI and its components as computed for the seven different DEMs were compared in three different ways: (1) distribution functions and their statistics; (2) cell by cell comparison of four DEMs with the same resolution but different information content; and (3) comparison of blocks of cells within different resolution DEMs with different information content. Like previous TWI studies, the computed specific upstream area decreased on average for higher resolution DEMs while computed slope values followed a narrower distribution. TWI variation between neighbouring cells in 50 x 50 m areas decreased largely with increasing grid size. A cell by cell comparison of the TWI values of the four 5 m DEMs with different information content showed a clear decrease in correlation with the TWI based on the original DEM with decreasing information content. The results showed considerable differences between topographic indices computed for DEMs of different grid resolution. Interpolating the DEMs to a higher resolution (i.e. a smaller grid size) provided more similar TWI distributions, but the pixel by pixel comparison showed that different information contents caused clearly different TWI maps.