The ground beneath your feet: Digital elevation data for today and tomorrow

Abstract

Digital elevation models (DEMs) are fundamental data infrastructure that support a large range of applications in environmental modelling. The distribution of solar radiation, patterns of wetness and water movement, mobility of animals and evolution of soils are all intimately linked to topographic form. DEMs for the Australian continent have progressed from 1/10 th degree resolution in 1982 to 1/40 th degree in 1991, 9 second (1/400th degree) in 1996 and now 1 second (about 30 m) derived from the NASA Shuttle Radar Topographic Mission (SRTM). Each improvement in resolution and accuracy enables new applications as more detail and structure are revealed. The 1 second SRTM-derived DEMs for Australia are now publicly available through the Geoscience Australia National Elevation Data Framework (NEDF) portal (http://nedf.ga.gov.au). The raw SRTM data has been processed through a series of steps to provide several products suitable for different purposes: • DEM-S is a bare-earth elevation model with stripes and voids removed, offsets due to trees treated and noise substantially reduced in flatter areas by adaptive smoothing, which is suitable for most applications where connectivity of flow is not the primary concern such as calculation of slopes and topographic position • DEM-H is a hydrologically conditioned elevation model derived from DEM-S with drainage enforcement using 1:250,000 scale stream lines, which is suitable for applications where hydrological connectivity is of primary concern such as calculation of contributing areas and catchment delineation • DEM is the pre-cursor to DEM-S before adaptive smoothing, which may be useful when the retention of detail is more important than smoothness. There is also a cleaned digital surface model (DSM) that is the precursor to DEM-S before tree offsets are removed, which is restricted to Australian government use only. Most users of DEMs need information about the shape of the landscape, not just height, so we are producing a set of standard derived products including slope, aspect, curvatures, topographic position, relief, contributing area and wetness index. These derived surfaces are key inputs for continental scale prediction of soil properties and ecological patterns. Predictions of a suite of key soil properties on a 3 second grid for the Australian continent, using the 1 second terrain layers, are being developed and will contribute to the GlobalSoilMap.net global grid of soil properties. Catchments and stream lines derived from DEM-H will form the basis for future versions of the Australian Hydrological Geospatial Fabric (http://www.bom.gov. au/water/geofabric/index.shtml). Digital elevation data continues to improve as measurement technology improves. Airborne lidar already provides high precision terrain data with vertical accuracy of 20 cm or better at 1 m horizontal resolution or better. This data is available in parts of Australia where the better accuracy justifies the high cost, such as urban areas, coastal lowlands, wetlands and floodplains. We need to develop ways of using high resolution data covering limited areas in combination with lower resolution data covering the continent. New global DEMs are being acquired and will replace SRTM in the years ahead. The TANDEM-X satellite radar system will potentially produce global elevations at 2-3 times better resolution and accuracy than SRTM, and other systems will follow. Higher resolutions bring further technical challenges, not least in data volume: each 1 second DEM product for Australia is a 40 GB dataset and finer resolutions will be correspondingly larger.