[gdal-dev] Vertical datum conversion for SRTM data
Nikos Alexandris
nik at nikosalexandris.net
Tue Nov 21 01:25:42 PST 2017
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 <http://sealevel.climatecentral.org/uploads/research/Global-Mapping-Choices-Report.pdf>
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)
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