Abstract
Topographic depressions have an important role in hydrological processes as they affect the water balance and runoff response of a watershed. Nevertheless, research has focused in detail neither on the effects of acquisition and processing methods nor on the effects of resolution of nationwide grid digital terrain models (DTMs) on topographic depressions or the hydrological impacts of depressions. Here, we quantify the variation of hydrological depression variables between DTMs with different acquisition methods, processing methods and grid sizes based on nationwide 25 m × 25 m and 10 m × 10 m DTMs and 2 m × 2 m ALS-DTM in Finland. The variables considered are the mean depth of the depression, the number of its pixels, and its area and volume. Shallow and single-pixel depressions and the effect of mean filtering on ALS-DTM were also studied. Quantitative methods and error models were employed. In our study, the depression variables were dependent on the scale, area and acquisition method. When the depths of depression pixels were compared with the most accurate DTM, the maximum errors were found to create the largest differences between DTMs and hence dominated the amount and statistical distribution of the depth error. On the whole, the ability of a DTM to accurately represent depressions varied uniquely according to each depression, although DTMs also displayed certain typical characteristics. Thus, a DTM’s higher resolution is no guarantee of a more accurate representation of topographic depressions, even though acquisition and processing methods have an important bearing on the accuracy.
Highlights
Spatial information is widely used in fluvial applications, the potential of which is increasing owing to technological advances in topographic data acquisition
Depressions are removed from a grid digital terrain models (DTMs) prior to hydrologic analyses that are based on automated simulation of surface runoff [41,42,43]
airborne laser scanning (ALS)-DTM2 was the most accurate DTM compared with reference data with root mean squared errors (RMSEs) ranging from 0.176 to 0.406 m (Table 2)
Summary
Spatial information is widely used in fluvial applications, the potential of which is increasing owing to technological advances in topographic data acquisition. The effects of the acquisition method and grid size of a DTM on hydrologically interesting terrain derivatives, such as river network and watershed representation, slope and aspect, specific catchment areas and CTI-values (Compound Topographic Index), have been studied [10,11,12,18]. Depressions are removed from a grid DTM prior to hydrologic analyses that are based on automated simulation of surface runoff [41,42,43]. These analyses require hydrologically connected flow networks, in which the flow to the actual pour point of the watershed is not prevented
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