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

Note: This metadata describes the dataset in accordance with the ANZLIC (Australia New Zealand Land Information Council) Core Metadata Guidelines Version 2.


Dataset citation

ANZLIC unique identifier: ANZCW0703013336

Title: 1 second SRTM Level 2 Derived Digital Surface Model (DSM) v1.0


Custodian

Custodian: Geoscience Australia

Jurisdiction: Australia


Description

Abstract:

The 1 second Shuttle Radar Topography Mission (SRTM) derived Digital Surface Model (DSM) Version 1.0 is a 1 arc second (~30m) gridded 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 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. A full description of the methods is in progress (Read et al., in prep).

This 1 second DSM forms the source for the 1 second DEM with vegetation offsets removed (ANZCW0703013355), the smoothed DEM (DEM-S; ANZCW0703014016) released in August 2010 and drainage enforced version that is expected to be released in 2010-11.

ANZLIC search words:

  • CLIMATE AND WEATHER Climate change
  • ECOLOGY Landscape
  • HAZARDS Flood
  • LAND Topography
  • PHOTOGRAPHY AND IMAGERY Remote Sensing

Spatial domain:

locality map

Geographic extent name: AUSTRALIA EXCLUDING EXTERNAL TERRITORIES - AUS - 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

Geographic bounding box:
North bounding latitude: -10 °
South bounding latitude: -44 °
East bounding longitude: 154 °
West bounding longitude: 113 °

Data currency

Beginning date: 2000-02-11

Ending date: 2000-02-22


Dataset status

Progress: In Progress

Maintenance and update frequency: Not Known


Access

Stored data format:
DIGITAL - ArcGIS-grid ArcInfo grid Geographic WGS84
Available format type:
DIGITAL - ArcGIS-grid ArcInfo grid Geographic WGS84

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.

Free Data Download

Data quality

Lineage:

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).

Positional accuracy:

The horizontal positional error is the same as for the raw SRTM 1 second data, with 90% of tested locations within 7.2m for Australia. See Rodriguez et al. (2006) for more information.

Attribute accuracy:

Elevation accuracy is essentially the same as for the raw SRTM 1 second data, with 90% of tested heights within 9.8m for Australia, with improvements due to the removal of vegetation offsets. Errors in height are still mostly due to random variation (noise) that is spatially uncorrelated beyond distances of about 100m, but there are some broader scale errors. The noise component is typically about +/- 2m but in some areas is much larger. See Rodriguez et al. (2006) for more information.

Accuracy was tested using 1198 Permanent Survey Marks 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 7.6m at the 95th percentile with a RMS error of 3.9m in open, flat terrain. 99 percent of points are within a height difference of less than 9.6m.

The removal of striping artefacts improves the representation of the landform shape, particularly in low relief areas, but it is not clear whether this also produces an improvement in overall height accuracy. Some striping remains in the data at a much reduced level (mostly less than 0.3m amplitude). Additional artefacts including long-wavelength (~10km) striping have not been corrected.

The removal of vegetation offsets provides a significant improvement in the representation of the landform shape, particularly in low relief areas, and areas of remnant vegetation. Elevation accuracy varies in forested areas. Comparisons with several higher resolution datasets suggest that elevation accuracy varies depending on the height and structure of the existing vegetation, quality of vegetation input masks and local relief. Further details of these comparisons are provided in the User Guide (Geoscience Australia and CSIRO Land & Water, 2009).

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 DSM represents heights of the land surface or buildings or vegetation above the land surface. Due to random noise, the relative elevation between adjacent grid cells can be in error by several metres. The removal of striping has improved the representation of local landform shape, particularly in low relief areas.

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 0m 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 1cm above the water surface. Void areas within water bodies (small islands not represented in the original SRTM data) are at least 1cm above the water surface over their entire area.

Completeness:

The DSM covers all of continental Australia and near coastal islands land areas including all islands defined by the available SRTM 1 second elevation and Surface Waterbodies Data Base 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 DSM:(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 south western corner of the tile.


Contact information

Contact organisation: Commonwealth of Australia (Geoscience Australia) (GA)
Contact position: Manager Client Services
Mail address: Cnr Jerrabomberra Ave and Hindmarsh Dr
Mail address: GPO Box 378
Locality: Canberra
State: ACT
Country: Australia
Postcode: 2601
Telephone: 02 6249 9966
Facsimile: 02 6249 9960
Electronic mail address: sales@ga.gov.au

Metadata information

Metadata date: 2013-03-08


Additional metadata

Metadata reference XHTML: http://www.ga.gov.au/meta/ANZCW0703013336.html

Metadata reference XML: http://www.ga.gov.au/meta/ANZCW0703013336.xml

Horizontal Datum: WGS84

Vertical Datum: EGM96

Conversion to floating point format

As a by-product of the de-striping process the integer data was converted to floating point format to allow for the continuously varying nature of the striping. Areas where no de-striping was required will contain unaltered integer values, but represented in floating point format for consistency.

Ancillary data layers

Five additional data layers provide information about this DSM:

- A de-stripe mask indicating which ? x ? degree tiles have been affected by de-striping and which have not been de-striped

- A striping magnitude layer showing the amplitude of the striping at 0.01 degree (~ 1km) resolution

- 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

- Tile indexes for the DSM

References:

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.

Grohman, G., Kroenung, G., and Strebeck, J. (2006) Filling SRTM voids: The delta surface fill method. Photogrammetric Engineering and Remote Sensing 72 (3), 213-216.

Read, Gallant and Dowling (in prep) Destriping and void filling methods used in SRTM 1 Second processing. See http://www.clw.csiro.au/publications/waterforahealthycountry/index.html for progress.

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.

For technical queries please contact:

John Gallant

CSIRO Land and Water

Clunies Ross St

CANBERRA ACT 2600

e-mail: John.Gallant@csiro.au

Nerida Wilson

Geoscience Australia

GPO Box 378

Canberra ACT 2601

elevation@ga.gov.au

Authors:Dowling, T. Gallant, J. Read, A. Wilson, N. Tickle, P.
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