# [gdal-dev] Vertical datum conversion for SRTM data

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## [gdal-dev] Vertical datum conversion for SRTM data

 Dear list, can GDAL handle the following exercise? Of interest is the Mean Sea Level, based on NASA’s SRTM. I am new in this topic. Is the following equation correct? SRTM(MSL) = SRTM(WGS84) - MSL(WGS84) [*] Is it “correct” to recommend the https://hydrosheds.cr.usgs.gov/ data set for Mean Sea Level related studies? I refer specifically to i) 5.1 Void-filled digital elevation model  and ii) 5.2 Hydrologically conditioned elevation [**] Thank you, Nikos -- Basic (notes to self) - Types of vertical datums: tidal, gravimetric, geodetic - Obviously, if source is seal level -> tidal, if geoid -> gravimetric, if ellipsoid -> geodetic - MSL is a (type of a) vertical datum From SRTM(EGM96) to SRTM(MHHW) The report describes the conversion of NASA’s SRTM, referenced to the EGM96 geoid, to refer to the local mean higher-high water (MHHW) tidal datum, say SRTM(MHHW). The conversion involves: 1. get MSL tidal, resolution. 2-arc-minute [0] 2. get MHHW(MSL), res. 2-arc-minute [1] 3. upsample MSL(TP) and MHHW(MSL) (based on NN) to SRTM's res. 3-arc-second [2] 4. geo-reference SRTM(EGM96) & MSL(TP) to WGS84; outputs: SRTM(WGS84) & MSL(WGS84) [3] 5. convert from "tidal" to "geodetic": MHHW(WGS84) = MHHW(MSL) + MSL(WGS84) | [4] 6. SRTM(MHHW) = SRTM(WGS84) - MHHW(WGS84) [5] Details [*] Equation re-built based on the steps described here [**] chrome-extension://oemmndcbldboiebfnladdacbdfmadadm/https://hydrosheds.cr.usgs.gov/webappcontent/HydroSHEDS_TechDoc_v10.pdf[0] begin with the global 2-arc-minute mean sea surface MSS_CNES_CLS_11 (Aviso 2014) based on 16 years of satellite altimetry observations (1993-2009), referenced to the Topex-Poseidon ellipsoid, and here called MSL(TP) (MSL for mean sea level); [1] Employ a global MHHW grid, MHHW(MSL), referenced to the MSL tidal datum, provided by Mark Merrifield of the University of Hawaii, and developed using the model TPXO8 at 2-arcminute resolution (Egbert et. al, 2002) [2] Upsample these grids to 3-arc-second SRTM resolution and georeference using nearest-neighbor interpolation [3] Convert both SRTM(EGM96) and MSL(TP) to reference the WGS84 ellipsoid [4] Convert MHHW grid to reference WGS84 through the operation: MHHW(WGS84) = MSL(WGS84) + MHHW(MSL) [5] Convert SRTM to MHHW reference through the operation, SRTM(MHHW) = SRTM(WGS84) - MHHW(WGS84) _______________________________________________ gdal-dev mailing list [hidden email] https://lists.osgeo.org/mailman/listinfo/gdal-dev signature.asc (235 bytes) Download Attachment
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## Re: Vertical datum conversion for SRTM data

 On mardi 21 novembre 2017 10:25:42 CET Nikos Alexandris wrote: > Dear list, can GDAL handle the following exercise? > > Of interest is the Mean Sea Level, based on NASA’s SRTM. I am new in this > topic. Is the following equation correct? > > SRTM(MSL) = SRTM(WGS84) - MSL(WGS84) [*] > > > Is it “correct” to recommend the https://hydrosheds.cr.usgs.gov/ data > set for Mean Sea Level related studies? I refer specifically to i) 5.1 > Void-filled digital elevation model and ii) 5.2 Hydrologically conditioned > elevation [**]   Not completely sure this answers your questions ,but this is related.   Quoting end of http://www.gdal.org/gdalwarp.html:   """ (GDAL >= 2.2) To transform a DEM from geoid elevations (using EGM96) to WGS84 ellipsoidal heights:   gdalwarp -overwrite in_dem.tif out_dem.tif -s_srs EPSG:4326+5773 -t_srs EPSG:4979 """   This assumes the egm96_15.gtx grid is in the proj.4 resource directory.   -- Spatialys - Geospatial professional services http://www.spatialys.com_______________________________________________ gdal-dev mailing list [hidden email] https://lists.osgeo.org/mailman/listinfo/gdal-dev