![]() Vertical datum transformations are defined using a GTX file. ![]() PROJ uses pre-defined grids for datum transformations. GDAL using the PROJ library to carry out CRS transformations. If you load original cartosat.tif, cartosat_orthometric.tif and srtm.tif in QGIS, and inspect the elevation values, you can see the the cartosat_orthometric and srtm elevations match almost exactly now since they are referenced to the same geoid.Ībove image contains a visualization of CartoDEM Version-3 R1 provided by National Remote Sensing Centre, ISRO, Government of India, Hyderabad, India. gdalwarp -s_srs " proj=longlat datum=WGS84 no_def" -t_srs " proj=longlat datum=WGS84 no_defs geoidgrids=egm96_15.gtx" cartosat.tif cartosat_orthometric.tif So now the conversion between the 2 is a matter of specifying the source and target srs.īelow is the command used for converting Cartosat-1 DEM to orthometric elevation. SRTM = WGS84 ellipsoid with EGM96 elevation proj=longlat datum=WGS84 no_defs geoidgrids=egm96_15.gtxĬartosat-1 = WGS84 ellipsoid with WGS84 elevation proj=longlat datum=WGS84 no_def So we can reference them using a proj strings as follows: Converting Between Vertical CRSsīoth the datasets have the same horizontal srs but a different vertical srs. Replace that with your filenames before running the commands. The commands in the sections below assume your filename is cartosat.tif. For example, if your file is in C:\Users\ujaval\Downloads\, run the following command first cd c:\Users\ujaval\Downloads ![]() One installed, open the shell, then cd to the directory containing your files. Install GDAL utilities following these instructions for your platform. The article has been updated with the new syntax. Since the release of GDA元/PROJ6, this syntax is no longer supported. We need to supply it with a source spatial reference system (s_srs) and a target spatial reference system (t_srs) values.Ī previous version of this article used EPSG codes to specify a compound CRS, such as EPSG:4326 5773. ![]() The GDAL command needed to convert the dataset is gdalwarp. SRTM data is distributed with referenced to the EGM96 geoid model (and hence have orthometric elevations) An easy interface to download SRTM 30m data is the 30-Meter SRTM Tile Downloader. National Aeronautics and Space Administration ( NASA). During its 11-day mission, the space shuttle Endeavour orbited the Earth 16 times and captured Earth’s topography at 1 arc-second (30 meters) for over 80% of the Earth’s surface. National Geospatial-Intelligence Agency ( NGA) and the U.S. Shuttle Radar Topography Mission (SRTM) is an international project spearheaded by the U.S. The source data at 30m resolution can be downloaded from BHUVAN. The Cartosat DEM is referenced to the WGS84 ellipsoid model (and hence have ellipsoidal elevation). It is derived from the Cartosat-1 stereo payload launched in May 2005. Here I will show a real example of converting between 2 elevation datasets.Ĭartosat-1 Digital Elevation Model (CartoDEM) is a National DEM developed by the Indian Space Research Organization (ISRO). GDAL command-line tools are a free and easy way to do these conversions There exists several models which define the geoid shape across the entire surface of the earth.Īs both of these are mathematical models, it is possible to convert elevations from one reference to another. Geoid is a much more complex model as it attempts to model the gravitational force of the earth which is not uniform. Elevations derived from space-borne platforms are usually ellipsoidal elevations.Įllipsoids are a simple model of the earth’s surface and are defined using semi-major axis (a) and semi-minor axis (b). Ground based surveys will produce orthometric elevationsĮllipsoidal elevation measures height of a point from a reference ellipsoid (such as WGS84). Orthometric (or geoid) elevation measures height of a point from mean-sea-level (MSL). A common reason for this being each dataset being referenced to a different surface. When working with elevation data, sometimes you may discover that 2 datasets from different providers have very different elevation values for the same location.
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