Shuttle Radar Topography Mission
SRTM data are remote sensing data of the Earth's surface, which were recorded during the STS -99 Shuttle Radar Topography Mission ( SRTM ) in February 2000 from space. They are used to create a uniform, high-resolution digital terrain model of the earth's surface.
With SRTM even impassable regions of the world were recognized and measured within 11 days. Were covered land between the 60th and 58th northern latitude south, due to the orbital inclination of the orbit, the poles are not included in these databases. Freely available are the data with a resolution of 90 m, and 30 m for North America.
- 3.1 Data Quality
- 3.2 Copyright and Licenses
Acquisition of data: SIR - C, X -SAR
The radar system of the SRTM mission was part of the equipment of the Space Shuttle Endeavour on the transport mission 99 It consisted of two components:
- SIR -C, developed by NASA produced data of approximately 100% of the measured surface with a horizontal resolution of about 30 meters and a height accuracy of about 6 meters. The data produced were evaluated by the Jet Propulsion Laboratory of NASA and the U.S. Geological Survey ( U.S. Geological Survey ) partly as a public domain free of charge on the Internet.
- The X-SAR from the German Centre for Aerospace, whose data have higher accuracy, but cover only 40 % of the earth's surface.
The amount of data collected by means of Synthetic Aperture Radar reflect the shape of the earth surface including vegetation and development, which leads to corresponding variations to the ground.
This type of remote sensing in contrast to the early days of Geodesy great benefits: Prior to the height and location of a place on the earth's surface by means of triangulation was determined using reference points. This is connected with a lot of effort, since the measuring points each must lie in sight. Far-off places had to thus be detected by means of intermediate steps, inaccessible terrain could not or only insufficiently be measured in part.
Data from the United States Geological Survey
Data Format and Resolution: SRTM -1, SRTM -3
The USGS has provided two different data sets available for the free use:
- The data with a resolution of one second of arc in length and width are available ( SRTM -1) for the U.S. territory. These data are offered in pieces of 1 × 1 degree with a size of 3601 × 3601 pixels.
- For the rest of the earth's surface only data at a reduced resolution of 3 arc seconds are offered ( SRTM -3). The size of 1 × 1 degree pieces here is 1201 × 1201 pixels.
- The user interface of the USGS also allows user-defined rectangular selection of any region for which the records are individually compiled and transmitted to the user. Note: At the equator corresponds to one second of arc about 30 meters or three seconds of arc about 90 meters. To the poles, the distance between the longitudes from (See Abweitung ).
The records are of 16-bit integers ( big- endian byte order ) are offered as templates on the FTP servers of the USGS. They are also available at various locations in different file formats. The level of presentation is in meters. Pixels for which there is no height information are provided with the value -32768.
The heights of the surface measurement refer to the worldwide uniform reference system WGS84 EGM96 geoid, the horizontal georeferencing is done with WGS84 geodetic datum. Pre-existing, nationally recognized level models had, in contrast, different reference systems, which further processing difficult.
Filename
The USGS file name (example: " N53E009 " Geographic for 53 ° north latitude and 9 ° East (East ) longitude ) refer to the length and width of the lower left corner of the data field shown, ie the image pixel (1,1).
Data Versions
The data initially published (version 1) have not only incorrect pixels without height information of other measurement errors and water areas are not constant height value. 2005, data were therefore re-edited with improved quality ( version 2). 2009, version 2 was revised again and, as version 2.1 out.
Copyright and Licenses
The data published by the USGS are available free of charge in the public domain for download and may be used without restriction: USGS -authored or produced data and information are in the public domain. The data are not published with one arc second resolution of areas outside the U.S. are subject to the control of the U.S. Department of Defense.
Data of the German Center for Aerospace: SRTM / X -SAR
The data captured by the X-SAR can be downloaded free of charge at DLR in DTED format. The openDEM project provides a free digital elevation model available that has been refined by the Processing of publicly accessible data, such as OpenStreetMap or viewfinder panoramas on.
Data quality
The resolution of the X-band height data of the DLR is 25 m horizontally and 1 m vertically. The horizontal accuracy is 20 m absolute, relative, and 15 m vertically 16 m absolutely, 6 m relative.
Copyright and Licenses
The X-band height data of the DLR itself may not be passed. From the data derived works (maps, etc. ) are subject, however, than the obligation to source naming no usage restrictions.
Weaknesses
For technical reasons and to reduce the amount of data the database is wrong:
- The amount of data indicates the surface of the earth again. This does not correspond to the height of the ground, see vegetation, buildings, etc.
- In some places, where the measurements were large errors or the received radiation was too small, the data "holes " in ( error pixel ). Can be several reasons to question: In areas with strong inclinations terrain the radar beams were too strong reflecting off the sensor. In very shallow water, ice or sand where the radar beams were too scattered to receive a sufficient amount of reflected radiation. Very dense and durable cloudiness prevented by ice particles contained ( for example, in thunderclouds) the "view".
- The horizontal grid of approximately 90 × 90 meters is no complete picture of the orography again. Points which lie between the grid points need to be interpolated for the further processing, ideally using higher resolution information.
- Due to the altitude reference model used have some programs from a false coastline because the zero meter mark the data does not have to coincide with the local sea level in reality. The keyword geoid are other explanations. Furthermore, some coastal areas are protected by a dike below sea level. The location quality of the shoreline has been improved in the new release of the data in 2005. The coastline cards were also published.
Correction of faulty pixels
To fill the positions for which no amount of information could be determined ( error pixel ), various processes are common:
- Interpolation from the existing values. This leads especially with larger gaps in mountainous regions to inaccuracies.
- Correction using data from other sources. The quality of the results of this method depends largely on the quality of these data. The deviation of the height values in the SRTM data by vegetation and development of conventionally determined height data should also be observed.
Software
The USGS provides for viewing the public-domain software dlgv32Pro, which is a limited freeware version of Global Mapper. In addition, there are several commercial and non-commercial software (freeware, shareware ) that can process SRTM data. As free software can import r.in.srtm GRASS GIS SRTM data with the module, just use the free program GPS track analysis SRTM data. The data are also available in GeoTIFF format that can be read by all major GIS and image processing software.
Examples
Faulty coastline: The picture shown is an area of the Elbe estuary. The red line marks the real bank of course, the same goes south of this line.
Faulty pixels: The picture shows the mountain peaks of the High Tatras are missing due to total reflection of radar waves. This effect can occur due to ice or snow surfaces as well as on steep mountain slopes. Likewise, water surfaces can lead to faulty pixels.
Height map of the Sierra Nevada of Maps -For -Free computed from SRTM data