Multiscale topographic position image of the Australian continent

Brief description

Digital elevation models (DEMs) reflect the morphology of landscape surfaces and attributes derived from these models, including slope, aspect, relief and topographic wetness index. DEMs have broad application in geomorphology, geology, hydrology, ecology and climatology. Here, we consider two important terrain attributes: topographic position index and topographic ruggedness. Topographic position index measures the topographic slope position of landforms. It compares the mean elevation of a specific neighbourhood area with the elevation value of a central cell. This is done for every cell or pixel in the DEM to derive the relative topographic position (e.g. upper, middle, lower landscape elements). Ruggedness refers to the roughness of the surface and is calculated as the standard deviation of elevations. Both these terrain attributes are scale dependent and will vary according to the size of the analysis window. Here, we generated a multiscale topographic position model over the Australian continent using a 3-second resolution (~90 m) DEM derived from the Shuttle Radar Topography Mission. The algorithm calculates topographic position scaled by the corresponding ruggedness across three spatial scales (window sizes): 0.2–8.1 km, 8.2–65.2 km and 65.6–147.6 km. The derived ternary image captures variations in topographic position across these spatial scales, giving a rich representation of nested landform features, with broad application to understanding geomorphological and hydrological processes, and mapping regolith and soils. Citation: Wilford, J., Basak, S. and Lindsay, J., 2020. Multiscale topographic position image of the Australian continent. In: Czarnota, K., Roach, I., Abbott, S., Haynes, M., Kositcin, N., Ray, A. and Slatter, E. (eds.) Exploring for the Future: Extended Abstracts, Geoscience Australia, Canberra, 1–4.

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Maintenance and Update Frequency: notPlanned

Statement: Based on an algorithm developed by Lindsay et al. (2015) we have generated multi-scale topographic position model over the Australian continent using 3 second resolution (~90m) DEM derived from the Shuttle Radar Topography Mission (SRTM). The algorithm calculates topographic position scaled by the corresponding ruggedness across three spatial scales (window sizes) of 0.2-8.1 Km; 8.2-65.2 Km and 65.6-147.6 Km. The derived ternary image captures variations in topographic position across these spatial scales (red local, green intermediate and blue regional) gives a rich representation of nested landform features that have broad application in understanding geomorphological and hydrological process and in mapping regolith and soils over the Australian continent. Topographic position is measured as the deviation of a central cell from the mean elevation within a specific window size or kernel. To account for the degree of variability within the analysis window the ruggedness is calculated as the standard deviation of elevations. The multi-scale topographic position model is then calculated as the deviation from mean elevation (DEV) and ruggedness parameter in the following equation: DEV(D)=(Zo - Zp)/Sv where D = size of the window measured in either map units or grid cells Zo is the elevation of the windows centre cell Zp window mean elevation. Sv ruggedness DEV is therefore a measure of relative topographic position scaled by the corresponding ruggedness. This formula is applied to every cell or pixel in the elevation grid across many window sizes to capture variations in relative topographic position from local through to regional landscape scales. The multi-scale topographic position model is based on the calculation of DEV values across three spatial scales (window sizes) of 0.2-8.1Km; 8.2-65.2km and 65.6-147.6Km. This generated three output images that were named according to the landscape scale they capture, including local, intermediate and regional, respectively. Incremental window sizes of 1 cell (0.09 km), 5 cells (0.43 km), and 10 cells (0.85 km) were generated for the local, intermediate and regional scales, respectively. The maximum deviation corresponding to these three scales is calculated and combined into a RGB ternary image. Each band in the ternary composite was histogram clipped to .5-99.5% and scaled to 8-bit 0-255 range values. Red, green and blues hues show variations in topographic position for regional, intermediate and local scales, respectively. For more information see the paper by Lindsay et al. 2015. Lindsay, J, B., Cockburn, J.M.H. and Russell, H.A.J. 2015. An integral image approach to performing multi-scale topographic position analysis, Geomorphology 245, 51–61.

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Purpose
Landscape evaluation; understanding landscape processes; natural resource assessment/management