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DocFaqDEM
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Elevation data (DEM data) are provided in different file formats, among them the GeoTIFF and the ESRI Grid ASCII formats.
Examples of sources for DEM data are
-
GeoTIFF:
-
ESRI Grid ASCII format: (also known as
ARC/INFO ASCII GRID)- Germany: https://gdz.bkg.bund.de/index.php/default/digitale-geodaten/digitale-gelandemodelle/digitales-gelandemodell-gitterweite-200-m-dgm200.html (UTM32s/EPSG:25832)
- Germany/Bavaria: https://www.ldbv.bayern.de/produkte/weitere/opendata.html (UTM32/EPSG:25832)
- Spain: http://centrodedescargas.cnig.es/CentroDescargas/catalogo.do?Serie=LIDAR
DEM data files having one of these formats can be used in QMS after building a VRT wrapper for them. QMS offers a tool for the creation of such wrappers which can be called via the main menu entry Tool - VRT builder. Section "Installing Raster Maps or DEM Data" describes details on the use of this tool and the use of the wrapper files.
Remark: ESRI Grid ASCII files do not have a reference to a coordinate reference system (CRS). This information should be found in the metadata on the file server and must be added as value of the advanced optional -a_srs parameter in the VRT builder. If there is a .prj file attached to the DEM file, then the CRS info can be found there. In the case of the above-mentioned Bavarian data source, the CRS used is UTM32 having the CRS code EPSG:25832.
DEM data files are available for relatively small areas (tiles) to keep their size relatively small. The area covered can be found with the gdalinfo tool (on Windows provided with the QMS installation package and located in the installation directory). Here is an example:
gdalinfo DGM200_UTM32_EsriGridAscii.asc.vrt
Driver: VRT/Virtual Raster
Files: DGM200_UTM32_EsriGridAscii.asc.vrt
DGM200_UTM32_EsriGridAscii.asc
Size is 1854, 1857
Coordinate System is:
PROJCS["ETRS89 / UTM zone 32N",
GEOGCS["ETRS89",
DATUM["European_Terrestrial_Reference_System_1989",
SPHEROID["GRS 1980",6378137,298.257222101,
AUTHORITY["EPSG","7019"]],
TOWGS84[0,0,0,0,0,0,0],
AUTHORITY["EPSG","6258"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.0174532925199433,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4258"]],
PROJECTION["Transverse_Mercator"],
PARAMETER["latitude_of_origin",0],
PARAMETER["central_meridian",9],
PARAMETER["scale_factor",0.9996],
PARAMETER["false_easting",500000],
PARAMETER["false_northing",0],
UNIT["metre",1,
AUTHORITY["EPSG","9001"]],
AXIS["Easting",EAST],
AXIS["Northing",NORTH],
AUTHORITY["EPSG","25832"]]
Origin = (494800.000000000000000,5606600.000000000000000)
Pixel Size = (200.000000000000000,-200.000000000000000)
Corner Coordinates:
Upper Left ( 494800.000, 5606600.000) ( 8d55'35.42"E, 50d36'40.55"N)
Lower Left ( 494800.000, 5235200.000) ( 8d55'52.52"E, 47d16'12.93"N)
Upper Right ( 865600.000, 5606600.000) ( 14d 9'21.70"E, 50d29'49.19"N)
Lower Right ( 865600.000, 5235200.000) ( 13d49'28.32"E, 47d10' 6.91"N)
Center ( 680200.000, 5420900.000) ( 11d27'34.53"E, 48d54'53.26"N)
Band 1 Block=128x128 Type=Float32, ColorInterp=Undefined
NoData Value=-9999
Here, the coordinate system information is only available after the use of the additional parameter -a_srs: EPSG:25832!
The lines following the Corner coordinates marker in the output show the area coordinates.
The pixel size shown is measured in coordinate system units (here: in meters). It describes the precision of the DEM data in the file.
Warning: Sometimes DEM data are offered for bigger areas (whole countries). When using an ASCII file format (ESRI Grid ASCII or XYZ format, too) this may result in very big files and, as a consequence, in considerable degradation of QMS performance.
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