0.00083333 0 0 -0.00083333 8 55.00083333 file title : DEM of catchment Elbe from SRTM, BODC, WSD data data type : integer file type : binary columns : 10801 rows : 8401 ref. system : latlong ref. units : deg unit dist. : 1 min. X : 8 max. X : 17.0008 min. Y : 48 max. Y : 55.0008 pos'n error : unknown resolution : .00083333 min. value : -36 max. value : 1594 value units : m value error : unknown flag value : none flag def'n : none legend cats : 0 lineage: the map processed and composed by Dr. Klaus Baumgardt, Foerderkreis "Rettet die Elbe" eV ("Save the Elbe"), Nernstweg 22, D-22765 Hamburg, Germany buero@rettet-die-elbe.de http://www.rettet-die-elbe.de comment: "Save the Elbe" is an independent organisation for the protection of the environment of the river Elbe. It does not pursue commercial interests, all members work voluntarily. comment: the making of this map download from ftp://edcsgs9.cr.usgs.gov/pub/data/srtm/Eurasia/ of 40 tiles of srtm-3 data. Each tile, e.g. N48E014.hgt, covers an 1*1 degree area with 1201*1201 pixels. The data were imported to Idrisi for win v.2, with a macro command bilidris x N48E014.hgt 4814 1 1201 1201 lat/long deg 14 15.0008 48 49.0008 1 1 0 17 The images contain void pixels (see documentation below), and improbable low values around them, which are resulting from the mean filtering when the prime 1arcsecond pixels were aggregated to a 3arcsecond grid. Improbable values were identified and a buffer of pixels with "true" values formed around them. Then all non-buffer pixels were set to zero, and subsequently filled by a Thiessen-Polygon operation. Thus the most probable elevation values were formed, and pasted on the original image. The Idrisi macro runs like this: reclass x i 48141 4814yn 3 min0max1 # where min0max1.rcl is # 1 -32768 -4 # 0 -4 3000 # 1 3000 32768 # -9999 # continue convert x 4814yn 4814yn i 1 3 2 distance x 4814yn 4814di reclass x i 4814di 4814um 2 1 0.0008 0.0016 0 0.0016 2 -9999 convert x 4814um 4814um i 1 3 2 overlay x 3 4814um 48141 4814buf convert x 4814buf 4814buf i 1 3 2 # a tile with many void areas, e.g. with large water surface, it had to be cut into smaller windows. Else, Idrisi takes too long, or crashes, when performing the thiessen operation. Afterwards, the windows are concatenated again. # continue window x 4814buf 4814wia 1 0 0 599 599 window x 4814buf 4814wib 1 600 0 1200 599 window x 4814buf 4814wic 1 0 600 599 1200 window x 4814buf 4814wid 1 600 600 1200 1200 thiessen x 2 4814wia 4814thia thiessen x 2 4814wib 4814thib thiessen x 2 4814wic 4814thic thiessen x 2 4814wid 4814thid concat x 4814buf 4 4814thi 1 4814thia 1 0 0 4814thib 1 600 0 4814thic 1 0 600 4814thid 1 600 600 overlay x 3 4814thi 4814yn 4814fil overlay x 7 4814fil 48141 4814dem comment: from the corrected dem tiles, an image was concatenated, to cover the Elbe catchment. A rough outline was digitized, to cut the catchment and the border zones of adjecent river catchments. lineage: SRTM Documentation downloaded from data provider SRTM_Topo(last update 11/05/03) 1.0 Introduction The SRTM data sets result from a collaborative effort by the National Aeronautics and Space Administration (NASA) and the National Imagery and Mapping Agency (NIMA), as well as the participation of the German and Italian space agencies, to generate a near-global digital elevation model (DEM) of the Earth using radar interferometry. The SRTM instrument consisted of the Spaceborne Imaging Radar-C (SIR-C) hardware set modified with a Space Station-derived mast and additional antennae to form an interferometer with a 60 meter long baseline. A description of the SRTM mission, can be found in Farr and Kobrick (2000). Synthetic aperture radars are side-looking instruments and acquire data along continuous swaths. The SRTM swaths extended from about 30 degrees off-nadir to about 58 degrees off-nadir from an altitude of 233 km, and thus were about 225 km wide. During the data flight the instrument was operated at all times the orbiter was over land and about 1000 individual swaths were acquired over the ten days of mapping operations. Length of the acquired swaths range from a few hundred to several thousand km. Each individual data acquisition is referred to as a "data take." SRTM was the primary (and pretty much only) payload on the STS-99 mission of the Space Shuttle Endeavour, which launched February 11, 2000 and flew for 11 days. Following several hours for instrument deployment, activation and checkout, systematic interferometric data were collected for 222.4 consecutive hours. The instrument operated virtually flawlessly and imaged 99.96% of the targeted landmass at least one time, 94.59% at least twice and about 50% at least three or more times. The goal was to image each terrain segment at least twice from different angles (on ascending, or north-going, and descending orbit passes) to fill in areas shadowed from the radar beam by terrain. This 'targeted landmass' consisted of all land between 56 degrees south and 60 degrees north latitude, which comprises almost exactly 80% of the total landmass. 2.0 Data Set Characteristics 2.1 General SRTM data were processed in a systematic fashion using the SRTM Ground Data Processing System (GDPS) supercomputer system at the Jet Propulsion Laboratory. Data were mosaicked into approximately 15,000 one degree by one degree cells and formatted according to the Digital Terrain Elevation Data (DTED) specification for delivery to NIMA, who will use it to update and extend their DTED products. Data were processed on a continent-by-continent basis beginning with North America. NIMA is applying several post-processing steps to these data including editing, spike and well removal, water body leveling and coastline definition. Following these "finishing" steps data will be returned to NASA for distribution to the scientific and civil user communities, as well as the public. In advance of that, the unedited data are being released for public use subject to the caveats discussed below. 2.2 Organization SRTM data are organized into individual rasterized cells, or tiles, each covering one degree by one degree in latitude and longitude. Sample spacing for individual data points is either 1 arc-second or 3 arc-seconds, referred to as SRTM-1 and SRTM-3, respectively. Since one arc-second at the equator corresponds to roughly 30 meters in horizontal extent, the sets are sometimes referred to as "30 meter" or "90 meter" data. Unedited SRTM-3 data are being released continent-by-continent, with the definitions of the continents displayed in the file Continent_def.gif. By agreement with NIMA unedited SRTM-1 data for the United States and its territories and possessions are also being released and can be found in the directory /United_States_1arcsec./ Cells that straddle the border with neighboring countries have been masked with quarter degree quantiation such that data outside the U.S. have the void value. 2.3 Elevation mosaics Each SRTM data tile contains a mosaic of elevations generated by averaging all data takes that fall within that tile. Since the primary error source in synthetic aperture radar data is speckle, which has the characteristics of random noise, combining data through averaging reduces the error by the square root of the number of data takes used. In the case of SRTM the number of data takes could range from a minimum of one (in a very few cases) up to as many as ten or more. 3.0 Data Formats The names of individual data tiles refer to the longitude and latitude of the lower-left (southwest) corner of the tile (this follows the DTED convention as opposed to the GTOPO30 standard). For example, the coordinates of the lower-left corner of tile N40W118 are 40 degrees north latitude and 118 degrees west longitude. To be more exact, these coordinates refer to the geometric center of the lower left pixel, which in the case of SRTM-1 data will be about 30 meters in extent. SRTM-1 data are sampled at one arc-second of latitude and longitude and each file contains 3601 lines and 3601 samples. The rows at the north and south ecges as well as the columns at the east and west edges of each cell overlap and are identical to the edge rows and columns in the adjacent cell. SRTM-3 data are sampled at three arc-seconds and contain 1201 lines and 1201 samples with similar overlapping rows and columns. This organization also follows the DTED convention. Unlike DTED, however, 3 arc-second data are generated in each case by 3x3 averaging of the 1 arc-second data - thus 9 samples are combined in each 3 arc-second data point. Since the primary error source in the elevation data has the characteristics of random noise this reduces that error by roughly a factor of three. This sampling scheme is sometimes called a "geographic projection", but of course it is not actually a projection in the mapping sense. It does not possess any of the characteristics usually present in true map projections, for example it is not conformal, so that if it is displayed as an image geographic features will be distorted. However it is quite easy to handle mathematically, can be easily imported into most image processing and GIS software packages, and multiple cells can be assembled easily into a larger mosaic (unlike the pesky UTM projection, for example.) 3.1 DEM File (.HGT) The DEM is provided as 16-bit signed integer data in a simple binary raster. There are no header or trailer bytes embedded in the file. The data are stored in row major order (all the data for row 1, followed by all the data for row 2, etc.). All elevations are in meters referenced to the WGS84 EGM96 geoid as documented at http://www.nima.mil/GandG/wgsegm/. Byte order is Motorola ("big-endian") standard with the most significant byte first. Since they are signed integers elevations can range from -32767 to 32767 meters, encompassing the range of elevation to be found on the Earth. In these preliminary data there commonly will be data voids from a number of causes such as shadowing, phase unwrapping anomalies, or other radar-specific causes. Voids are flagged with the value -32768. 4.0 Notes and Hints for SRTM Data Users 4.1 Data Encoding Because the DEM data are stored in a 16-bit binary format, users must be aware of how the bytes are addressed on their computers. The DEM data are provided in Motorola or IEEE byte order, which stores the most significant byte first ("big endian"). Systems such as Sun SPARC and Silicon Graphics workstations use the Motorola byte order. The Intel byte order, which stores the least significant byte first ("little endian"), is used on DEC Alpha systems and most PCs. Users with systems that address bytes in the Intel byte order may have to "swap bytes" of the DEM data unless their application software performs the conversion during ingest. 4.3 SRTM Caveats As with all digital geospatial data sets, users of SRTM must be aware of certain characteristics of the data set (resolution, accuracy, method of production and any resulting artifacts, etc.) in order to better judge its suitability for a specific application. A characteristic of SRTM that renders it unsuitable for one application may have no relevance as a limiting factor for its use in a different application. In particular, data produced by the PI processor should be considered as "research grade" data suitable for scientific investigations and development and testing of various civil applications. No editing has been performed on the data, and the elevation data in particular contain numerous voids and other spurious points such as anomalously high (spike) or low (well) values. Water bodies will generally not be well-defined - in fact since water surfaces generally produce very low radar backscatter they will appear quite "noisy" or rough, in the elevations data. Similarly, coastlines will not be well-defined. 5.0 References Farr, T.G., M. Kobrick, 2000, Shuttle Radar Topography Mission produces a wealth of data, Amer. Geophys. Union Eos, v. 81, p. 583-585. Rosen, P.A., S. Hensley, I.R. Joughin, F.K. Li, S.N. Madsen, E. Rodriguez, R.M. Goldstein, 2000, Synthetic aperture radar interferometry, Proc. IEEE, v. 88, p. 333-382. DMATR 8350.2, Dept. of Defense World Geodetic System 1984, Its Definition and Relationship with Local Geodetic Systems, Third Edition, 4 July 1997. http://164.214.2.59/GandG/tr8350_2.html Lemoine, F.G. et al, NASA/TP-1998-206861, The Development of the Joint NASA GSFC and NIMA Geopotential Model EGM96, NASA Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A., July 1998. Other Web sites of interest: NASA/JPL SRTM: http://www.jpl.nasa.gov/srtm/ NIMA: http://164.214.2.59/nimahome.html STS-99 Press Kit: http://www.shuttlepresskit.com/STS-99/index.htm Johnson Space Center STS-99: http://spaceflight.nasa.gov/shuttle/archives/sts-99/index.html German Space Agency: http://www.dlr.de/srtm Italian Space Agency: http://srtm.det.unifi.it/index.htm U.S. Geological Survey, EROS Data Center: http://edc.usgs.gov/ Note: DTED is a trademark of the National Imagery and Mapping Agency lineage: stop comment: bathymetric data from the BODC, http://www.bodc.ac.uk/ were pasted on the srtm-image. However, the BODC grid is rather coarse (1 arcminute), so this part of the map shall give just an impression, what kind of sea area the Elbe is discharging to. lineage: Reproduced from the GEBCO Digital Atlas published by the lineage: British Oceanographic Data Centre on behalf of IOC and IHO, 2003. comment: in the course of deepening the shipping channel from Hamburg harbour to the North Sea, planning information had to be provided to the public. A digital map of the estuarine water and marshland areas was downloaded, converted to Idrisi, and pasted on the srtm-image. The shipping channel, harbour basins, shallow water and wadden zones can be marked on the dem. lineage: Lower Elbe bathymetry from Wasser- und Schifffahrtsdirektion (German Waterways Administration) http://www.cux.wsd-nord.de/htm/zustimm.asp comment: from the idrisi DEM elbumdem.img, a hillshaded image was derived, and both exported as tiff-files. In an image processing software, the hillshade was semitransparently overlayed to the coloured elevation picture, to yield a shaded elevation picture. A world-file is added, to use it in Arcview or Arcexplorer.