[gdal-dev] Vertical datum conversion for SRTM data

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[gdal-dev] Vertical datum conversion for SRTM data

NikosAlexandris
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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Re: Vertical datum conversion for SRTM data

Even Rouault-2

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


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Re: Vertical datum conversion for SRTM data

Michael Smith

Egm96 grid shift file available from http://download.osgeo.org/proj/vdatum/egm96_15/egm96_15.gtx

 

Mike

 

 

-- 

Michael Smith

Remote Sensing/GIS Center

US Army Corps of Engineers

 

From: gdal-dev <[hidden email]> on behalf of Even Rouault <[hidden email]>
Date: Tuesday, November 21, 2017 at 6:18 AM
To: <[hidden email]>
Subject: Re: [gdal-dev] 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

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Re: Vertical datum conversion for SRTM data

Andre Joost
Am 21.11.2017 um 12:27 schrieb Michael Smith:
> Egm96 grid shift file available from http://download.osgeo.org/proj/vdatum/egm96_15/egm96_15.gtx
>
>

Note that OSGEO4W64 still ships an incorrect version of the file from
2010. According to http://download.osgeo.org/proj/vdatum/egm96_15/ the
file should be from 2014 (not 2010).

HTH,
André Joost


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