|
|
|
You are here: Home > GA Metadata
|
||||
|
GA Metadata Related Links |
1 second SRTM Level 2 Derived Smoothed Digital Elevation Model (DEM-S) Version 1.0Note: This metadata describes the dataset in accordance with the ANZLIC (Australia New Zealand Land Information Council) Core Metadata Guidelines Version 2. Dataset citationANZLIC unique identifier: ANZCW0703014016 Title: 1 second SRTM Level 2 Derived Smoothed Digital Elevation Model (DEM-S) Version 1.0 CustodianCustodian: Geoscience Australia Jurisdiction: Australia DescriptionAbstract: The 1 second SRTM derived DEM-S Version 1.0 is a 1 arc second (~30m) gridded digital elevation model (DEM). The DEM-S represents ground surface topography, excluding vegetation features, and has been smoothed to reduce noise and improve the representation of surface shape. The dataset was derived from the 1 second Digital Elevation Model Version 1.0 (DSM; ANZCW0103013355) by an adaptive smoothing process that applies more smoothing in flatter areas than hilly areas, and more smoothing in noisier areas than in less noisy areas. This DEM-S supports calculation of local terrain shape attributes such as slope, aspect and curvatures that could not be reliably derived from the unsmoothed DEM because of noise. A full description of the methods is in progress (Gallant et al., in prep). ANZLIC search words:
Spatial domain:
Geographic extent name: AUSTRALIA INCLUDING EXTERNAL TERRITORIES - AUSAAT - Australia - Australia Geographic extent polygon: 113 -10, 154 -10, 154 -44, 113 -44, 113 -10, Note: The format for each Geographic extent name is: Name - Identifier - Category - Jurisdiction (as appropriate) See GEN Register
Data currencyBeginning date: 2000-02-11 Ending date: 2000-02-22 Dataset statusProgress: In Progress Maintenance and update frequency: Not Known Access
Access constraints: The data are subject to Commonwealth of Australia Copyright. A licence agreement is required and a licence fee is also applicable for packaged data (included in the purchase price). This data is strictly for Government use only. Order ProductData qualityLineage: 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 DEM and DSM. 4. Vegetation masks and water masks applied to the DEM to remove vegetation. 5. Adaptive smoothing applied to DEM to produce DEM-S. DSM processing This DEM was based on the 1 second SRTM-derived Digital Surface Model (DSM) that was itself derived from the 1 second Shuttle Radar Topographic Mission data. The DSM was produced by removing stripes, filling voids and re-flattening water bodies. Further details are provided in the DSM metadata (ANZCW0703013336). The vegetation removal used the DSM without voids filled so that vegetation height estimates would not be affected by interpolated heights and so that voids adjacent to vegetated areas could be filled using bare-earth elevations. DEM processing (vegetation offset removal) Vegetation offsets were identified using Landsat-based mapping of woody vegetation. The height offsets were estimated around the edges of vegetation patches then interpolated to a continuous surface of vegetation height offset that was subtracted from the DSM to produce a bare-earth DEM. Further details are provided in the DSM metadata (ANZCW0703013355). Adaptive smoothing The adaptive smoothing process was designed to smooth flat areas to a greater degree than steep areas, and to respond to the degree of noise so that very noisy flat areas are smoothed more than less noisy flat areas. The process operated over multiple resolutions, allowing smoothing over quite large distances in areas of very low relief. The smoothing was performed on overlapping tiles, with sufficient overlap that cells used in the final product were not impacted by edge effects. Simple version Smoothing The smoothing process was based on the amount of noise in the DEM. The noise was estimated from the local variation in the difference between elevation and the mean of nearby elevations. In essence, the smoothing process operated by comparing the variance of elevations in a 3x3 group of cells with the mean noise variance in the group. If the elevation variance was larger than the mean noise it was considered to be due to real topographic variation and the elevations were left unchanged, while if it was smaller it was considered to be due to noise and the elevations were replaced by the mean elevation in the group. This was applied at successively coarser resolutions, producing smoothing over large areas where the topographic variation was small compared to the noise levels. The algorithm actually used statistical tests to make the decisions, and combined the multiple estimates of elevation at different resolutions using variance weighting. Water bodies Water bodies defined from the SRTM Water Body Data as part of the DSM processing were set to the same elevations as in the DSM after the smoothing. The water bodies are also removed from the DEM (set to null) before the smoothing operation to prevent them affecting the land elevations unduly. One cell of water adjacent to land is retained to prevent shoreline elevations from being raised to match the higher elevations further from the shore. Further information is provided in the User Guide (Geoscience Australia and CSIRO Land & Water, 2010). Positional accuracy: The horizontal positional error is the same as for the raw SRTM 1 second data, with 90% of tested locations within 7.2 m for Australia. See Rodriguez et al. (2006) for more information. Attribute accuracy: Accuracy of the 1 second DEM (before smoothing to form DEM-S) was tested using 1198 Permanent Survey Marks (PSM) distributed across the Australian Continent relative to the Australian Height Datum (AHD71). Results of this comparison show the absolute accuracy of the data as tested relative to AHD71 to be 7.582m at the 95th percentile with a RMS error of 3.868 in open, flat terrain. 99 percent of points are within a height difference of less than 9.602m. The smoothing process estimated the typical improvements in the order of 2-3m.This would make the DEM-S accuracy of approximately 5m. Relative elevation accuracy between adjacent cells is improved in DEM-S due to the reduction in noise levels; this has not been quantified but is evident in the comparison of slopes calculated before and after smoothing as shown in the User Guide (Geoscience Australia and CSIRO Land & Water, 2010). Height accuracy is likely to be poorer in areas where voids have been filled using the 9 second DEM, particularly in high relief areas. Logical Consistency: The DEM-S represents ground elevation with greatly improved relative elevations between adjacent grid cells in low relief areas due to the smoothing process. Slopes as small as 0.02% (2 m in 10 km) can be resolved in this DEM-S. The removal of vegetation involves estimation of vegetation height at the edges of vegetation patches, and interpolation of those heights across areas of continuous vegetation cover. Variations in vegetation height within large areas of vegetation are not captured by this method. The vegetation removal process guarantees that no elevations have been increased as part of the process. All void areas have been filled and there are no discontinuities due to tile boundaries. The SRTM editing rules relating to water bodies have been respected in the processing: lakes are flat, rivers decline continuously in a downstream direction and sea surfaces are at 0 m elevation. Flattened water bodies occupy the same areas as in the original SRTM 1 second data. Grid cells adjacent to water bodies are at least 1 cm above the water surface. Void areas within water bodies (small islands not represented in the original SRTM data) are at least 1 cm above the water surface over their entire area. Completeness: The DEM covers all of continental Australia and near coastal islands land areas including all islands defined by the available SRTM 1 second elevation and SRTM Water Body Data datasets. The following tiles containing fragments of mainland or pieces of islands were not supplied at 1 second resolution and are therefore missing from the DEM: (e.g. E112 S26; E113 S29; E118 S20; E120 S35; E121 S35; E123 S16; E124 S15; E125 S14; E132 S11; E133 S11; E134 S35; E141 S10; E142 S10; E143 S10; E146 S17; E150 S22; E152 S24) Note that the coordinates are of the southwestern corner of the tile. Contact information
Metadata informationMetadata date: 2011-12-08 Additional metadataMetadata reference XHTML: http://www.ga.gov.au/meta/ANZCW0703014016.html Metadata reference XML: http://www.ga.gov.au/meta/ANZCW0703014016.xml Conversion to floating point format The smoothing process alters all data values in the DEM by varying amounts and the result is a floating point data set capturing in some places very small but meaningful differences in elevation between adjacent cells. Ancillary data layers distributed with the data Five additional data layers provide information about the alterations made to the raw SRTM data to produce this DEM: - A water mask at 1 second resolution showing the cells that are part of the flattened water bodies - A void mask showing cells that were no-data in the raw SRTM and have been filled using the void filling algorithm - Vegetation masks at 1/8 x 1/8 degree resolution illustrating where vegetation was removed from the DEM and issues noted with the removal - Tile indexes for the DEM-S References: Gallant, in (in prep) An adaptive smoothing method for improving noisy DEMs. Geoscience Australia (2008) GEODATA 9 Second DEM Version 3 Geoscience Australia and CSIRO Land & Water (2010) 1 Second SRTM Derived Digital Elevation Models User Guide. Version 1.0. Geoscience Australia. Rodriguez, E., Morris, C.S., and Belz, J.E. (2006) A global assessment of the SRTM performance. Photogrammetric Engineering and Remote Sensing 72 (3), 249-260. Slater, J.A., Garvey, G., Johnston, C., Haase, J., Heady, B., Kroenung, G., and Little, J. (2006) The SRTM data "finishing" process and products. Photogrammetric Engineering and Remote Sensing 72 (3), 237-247. Authors:Gallant, J. Dowling, T. Read, A. Wilson, N. Tickle, P. |
|||
|
SPATIAL INFORMATION FOR THE NATION
|