Evaluating the effects of generalisation approaches and DEM resolution on the extraction of terrain indices in KwaZulu Natal, South Africa

Jonathan T Atkinson,Willem P De Clercq,Andrei B Rozanov

doi:10.4314/sajg.v6i2.9

Abstract

Digital elevation model (DEM) data are elemental in deriving primary topographic attributes which serve as input variables to a variety of hydrologic and geomorphologic studies. There is however still varied consensus on the effect of DEM source and resolution on the application of these topographic attributes to landscape characterisation. While elevation data for South Africa are available from several major sources and resolutions: Shuttle Radar Topographic Mission (SRTM), Earth ENV and Stellenbosch University DEM (SUDEM). Limited research has been conducted in a local context comparing the extraction of terrain attributes to high resolution Digital Terrain Data (DTM) such as LiDAR (Light Detection and Ranging) that are becoming increasing available. However, the utility of LiDAR to topographic analyses presents its own challenges in terms of operational-relevant resolution, processing demands and limited spatial coverage. There is a need to quantify the impact that generalisation approaches have on simplifying detailed DEMs and to compare the accuracy and reliability of results between high resolution and coarse resolution data on the extraction of localized topographic variables. In this regional study, we analyse the accuracy on selected local terrain attributes: elevation, slope and topographic wetness index derived from DEMs from varying sources, at different spatial resolutions and using three generalisation algorithms, namely: mean cell aggregation, nearest neighbour and hydrological corrected topo-to-raster. We show that topographic variable extraction is highly dependent on DEM source and generalisation approach and while higher resolution DEMs may represent the “true“ surface more accurately, they do not necessarily offer the best results for all extracted variables. Our results highlight the caveats of selecting DEMs not “fit-for-purpose” for topographic analysis and offer a simple yet effective solution for reconciling the selection of DEMs based on neighbourhood size resolution prior to terrain analyses and topographic feature characterization .

Highlights

Digital Elevation Models (DEMs) provide a convenient and representative interpretation of the Earth’s surface and are generally considered to be the de-facto dataset(s) for a variety of terrain and spatial analyses
Gonga-Saholiariliva, Gunnell, Petit and Mering, (2011) point out, these simple global metrics may conceal substantial local variability between data sets; and alternative methods may be necessary to provide further insight into the variability between surface products. They offer an explanation for this observed trend by suggesting that even though error may be present in the various DEMs, including outliers, neither data scatter in the linear regression models nor variance in the vertical accuracy constitute sufficient criteria to achieve a full definition of DEM error because the error is expressed globally i.e. across the entire study site
This study endeavoured to demonstrate the importance of utilising DEM surfaces that are “fit-forpurpose” for describing the scale-dependent topographic relationships relevant to soil-landscape modelling for an area located along KZNs south coast region

Summary

Introduction

Digital Elevation Models (DEMs) provide a convenient and representative interpretation of the Earth’s surface and are generally considered to be the de-facto dataset(s) for a variety of terrain and spatial analyses. The topographic analyses of pedo-geomorphological processes using DEMs are well established and are central to the catena concept for soil formation (Hook and Burke, 2000). This is because the development and variation of soil properties is strongly influenced by the way water and soil materials interact with the land surface and often co-evolve with the local topography. High on the research agenda is the need for developing novel ways of exploring the complexity associated with deterministic characterisation and modelling approaches of topographical landforms (Bishop, James, Shroder and Walsh, 2012) and relating various soil properties to readily available spatial data such as digital elevation data (Mashimbye, De Clercq and Van Niekerk, 2014)

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