# [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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