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1 second SRTM Level 2 Derived Digital Elevation Model 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: ANZCW0703013355

Title: 1 second SRTM Level 2 Derived Digital Elevation Model v1.0


Custodian

Custodian: Geoscience Australia

Jurisdiction: Australia


Description

Abstract:

The 1 second SRTM derived DEM Version 1.0 is a 1 arc second (~30m) gridded digital elevation model (DEM). The DEM represents ground surface topography, and excludes vegetation features. The dataset was derived from the 1 second Digital Surface Model (DSM; ANZCW0103013355) by automatically removing vegetation offsets identified using several vegetation maps and directly from the DSM. This product provides substantial improvements in the quality and consistency of the data relative to the original SRTM data, but is not free from artefacts. Man-made structures such as urban areas and power line towers have not been treated. The removal of vegetation effects has produced satisfactory results over most of the continent and areas with defects are identified in the quality assessment layers distributed with the data and described in the User Guide (Geoscience Australia and CSIRO Land & Water, 2009). A full description of the methods is in progress (Read et al., in prep; Gallant et al., in prep).

Smoothed and drainage enforced versions are under development, and are expected to be released in 2011.

ANZLIC search words:

  • ECOLOGY Landscape
  • LAND Topography Models

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:

These datasets are available freely, with the user being required to agree to the disclaimer at the time of order before the order can be processed. Creative Commons Attribution Australia Licence means data can be shared (copied, distributed, transmitted) or adapted provided you acknowledge the author or licensee. Consult the Creative Commons website for more information. http://creativecommons.org.au

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 Topography 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 DEM and DSM.

4. Vegetation masks and water masks applied to the DEM to remove vegetation.

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.

Vegetation offset removal

The processing of vegetation offsets to produce the DEM relies on Landsat-based mapping of woody vegetation to define where the offsets are likely to occur. The mapped extents of woody vegetation were adjusted using an edge-matching process to better represent the extents of areas affected by vegetation offsets in the SRTM DSM. Vegetation was processed across approximately 40% of Australia as shown in the vegetation mask ancillary dataset and in the User Guide (Geoscience Australia and CSIRO Land & Water, 2009).

Vegetation offset processing involves detecting vegetation patches, measuring the height offset around the edges, interpolating the height offset across the vegetated areas and subtracting the offset from the DSM. The heights of the offsets are estimated by measuring height differences across the boundaries of the vegetation patches. The method provides good estimates of the offsets in flat landscapes with well-mapped vegetation boundaries. The effect of sloping terrain is accounted for in the estimation of the offsets, but the results are less reliable in hilly terrain. Estimates of the offsets can also be very poor where the mapped vegetation extents do not match the extents of vegetation offsets as seen by the SRTM instrument. The estimation of the vegetation offsets can also be under or over-estimated if vegetation and topographic patterns coincide, such as trees on hilltops or dune ridges, or in inset floodplains or swamps.

The height offsets at vegetation edges are interpolated within vegetation patches to estimate the effects within the patches. The best results tend to be in small patches such as remnant tree patches. In continuously forested areas with few edges for estimating the offsets the heights are likely to be less reliable, and there is no information at all on variations of the height offset within continuous forests.

The removal of vegetation has been quite effective overall but there are many areas that contain either untreated or incompletely treated vegetation effects.

The methods will be fully described in Read, et al. (in prep) and Gallant, et al. (in prep).

Please consult the User Guide (Geoscience Australia and CSIRO Land & Water, 2009) for the rest of the lineage and metadata information.

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 7.582m 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 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 DEM represents elevation. Due to random noise, the relative elevation between adjacent grid cells can be in error by several metres.

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 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 DEM 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/ANZCW0703013355.html

Metadata reference XML: http://www.ga.gov.au/meta/ANZCW0703013355.xml

Reference system

- Horizontal datum: World Geodetic System 1984 (WGS84)

- Vertical datum: Earth Gravitational Model 1996 (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

Four 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

References:

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

Geoscience Australia (2008) GEODATA 9 Second DEM Version 3

Geoscience Australia and CSIRO Land & Water (2009) 1 Second SRTM Derived DSM and DEM 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.

Sibson, R. (1981) A brief description of natural neighbour interpolation. In V. Barnet, editor, Interpreting Multivariate Data, pages 21-36. John Wiley & Sons, Chichester.

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:

John Gallant

CSIRO Land and Water

Clunies Ross St

CANBERRA ACT 2600

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.
SPATIAL INFORMATION FOR THE NATION