1 second SRTM Level 2 Derived Digital Surface Model v1.0

Brief description

The 1 second SRTM derived DSM Version 1.0 is a 1 arc second (~30m) gridded digital surface model (DSM) that represents ground surface topography as well as features above the ground such as vegetation and man-made structures. The dataset was derived from the Shuttle Radar Topography Mission (SRTM) data acquired in February 2000, supported by the GEODATA 9 second DEM in void areas and the SRTM Water Body Data. Stripes and voids have been removed from the 1 second SRTM data to provide an enhanced and complete DSM for Australia and near-shore islands. This 1 second DSM forms the source for the 1 second DEM with vegetation offsets removed (ANZCW0703013355) and for smoothed and drainage enforced versions that are expected to be released in 2010. The grid spacing is 1 second in longitude and latitude (approximately 30 metres).

Lineage

Maintenance and Update Frequency: asNeeded

Statement: Source data 1. SRTM 1 second Version 2 data (Slater et al., 2006), supplied by Defence Imagery and Geospatial Organisation (DIGO) as 813 1 x 1 degree tiles. Data was produced by NASA from radar data collected by the Shuttle Radar Topographic Mission in February 2000. 2. GEODATA 9 second DEM Version 3 (Geoscience Australia, 2008) used to fill voids. 3. SRTM Water Body Data (SWBD) shapefile accompanying the SRTM data (Slater et al., 2006). This defines the coastline and larger inland waterbodies for the SRTM DSM. De-striping SRTM data contains striping artefacts oriented approximately NE-SW and NW-SE that vary in amplitude from about 0.2m to nearly 4m. The wavelength of the striping is approximately 800m. Stripes were detected in the elevation data using a 2-dimensional Fast Fourier Transform. Peaks in the spectra were visually identified and manually delineated using a tool designed specifically for this purpose. Striping occurred everywhere except where relief was high enough to obscure striping. Spectral analysis was performed on sub-tiles to account for spatial variation in the intensity and direction of striping. Fourier transform was applied to overlapping sub-tiles covering 1536 x 1536 cells (0.43 x 0.43 degrees). Central 1024 x 1024 cells were retained, each comprising one sixteenth of a 1 x 1 degree tile (900 x 900 cells) with a 62-cell overlap on each edge to provide smooth transitions between sub-tiles. Void filling Voids (areas without data) occur in the data due to low radar reflectance (typically open water or dry sandy soils) or topographic shadowing in high relief areas. Delta Surface Fill Method (Grohman et al., 2006) was adapted for this task, using GEODATA 9 second DEM as infill data source. The 9 second data was refined to 1 second resolution using ANUDEM 5.2 without drainage enforcement. Delta Surface Fill Method calculates height differences between SRTM and infill data to create a "delta" surface with voids where the SRTM has no values, then interpolates across voids. The void is then replaced by infill DEM adjusted by the interpolated delta surface, resulting in an exact match of heights at the edges of each void. Two changes to the Delta Surface Fill Method were made: interpolation of the delta surface was achieved with natural neighbour interpolation (Sibson, 1981; implemented in ArcGIS 9.3) rather than inverse distance weighted interpolation; and a mean plane inside larger voids was not used. Water bodies Flat water bodies in the original 1 second data were modified as part of the de-striping process and were re-flattened afterwards. SRTM Water Body Data was converted to a 1 second resolution grid then adjusted to match the extent of equal-height pixels in original SRTM 1 second data. Grid cells within that water mask were set to the original SRTM height. Edit rules for land surrounding water bodies SRTM edit rules set all land adjacent to water at least 1m above water level to ensure containment of water (Slater et al., 2006). Following de-striping, void filling and water flattening, the heights of all grid cells adjacent to water was set to at least 1cm above the water surface. The smaller offset (1cm rather than 1m) could be used because the cleaned digital surface model is in floating point format rather than integer format of the original SRTM. Some small islands within water bodies are represented as voids within the SRTM due to edit rules. These voids are filled as part of void filling process, and their elevations set to a minimum of 1cm above surrounding water surface across the entire void fill. For rest of the lineage and metadata information please consult the User Guide (Geoscience Australia and CSIRO Land & Water, 2010).