6 Philipp Rupprecht photography
- Challenge & approach
- NLGEO2018, the new Dutch quasi-geoid model
- NLLAT2018, the new Dutch LAT model
Outline
7
The problem…
onshore offshore
NAP
Typically, the separation chart datum/ height reference surface is a spatially varying function only known at the tide gauges!
tide gauge
LAT
9
The problem…
onshore offshore
NLGEO2004
LAT
LAT reduction matrix MSL
GEONZ97
reference ellipsoid
LAT reduction matrix = difference LAT – “MSL” in open sea and LAT - NAP along the coast (smooth transition)
10
Main objective NEVREF project
To obtain accurate realizations of the quasi-geoid and lowest astronomical tide surface, including
the necessary transformations from/to all common land and marine vertical reference surfaces
Required with cm accuracy …
…to obtain this one with accuracy of 1 dm!
11
Approach
hQG
Tide Gauges /
GNSS
Coastal-waters-inclusive continuous (CWIC) 3D description of LAT
Hydro. model
+
Gravi- metric data
Radar Altimetry
QGLATζ
13
411,947 altimeter-derived along-track QG height differences
453,334 terrestrial gravity anomalies
8,205 airborne gravity disturbances
94,137 shipboard gravity anomalies
7,179 spatially interpolated gravity anomalies
• Altimeter data corrected for dynamic topography from Dutch operational storm-surge model;
• Colored noise in altimeter-derived QG height differences accounted for;
• New terrestrial gravity data in Limburg, Germany, Belgium;
• New airborne data in German Bight; • Shipboard data re-processed; • Remove-compute-restore:
– GOCO05S as a-priori gravity field; – RTM correction applied using the TS
(tesseroid) software;
14
• Quasi-geoid is parameterized using spherical radial basis functions.
• Parameters are estimated using weighted least-squares.
• Variance component estimation is applied for proper weighting of all datasets.
• Systematic errors in gravity datasets are accounted for.
• Full noise covariance matrix of estimated quasi-geoid.
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GNSS NAP geometric gravimetric( ) quasi-geoidh Hε ζ ζ= − − = −
- Acquired during 1996–1997 (5th primary leveling);
- Reprocessed by L. Huisman using Bernese Software Version 5.2;
- ITRF2005;
- Tide free mean crust;
- 5 mm (5 days data) hσ ≈
- Acquired within 1 month after GNSS meas.;
- Correction to NAP heights resulting from 5th primary leveling applied;
- Mean tide zero tide (Mäkinen 2008);
-
0.50
0.40
0.35
0.25
0.10
cent
imet
ers
0.45
0.30
0.20
0.15 Hσ =
Validation - Netherlands
17
Validation - Netherlands
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
cent
imet
ers
Nr of points: 82 Mean: -1.93 SD: 0.66 Min: -4.1 Max: -0.62
Differences [centimeters] -3.5 -2.5 -1.5 -0.5 0.5 -4.5
14
12
10
8
6
4
2
0
Occ
urre
nces
geometric gravimetricε ζ ζ= −
< 1 cm (better than target accuracy)
21
Validation - Netherlands
x true false
0.70
0.66
0.62
0.58
0.54
0.50
cent
imet
ers
0.68
0.64
0.60
0.56
0.52
εσ
abs( ) 2 εε σ>
geometric gravimetricε ζ ζ= −
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Corrector surface
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
cent
imet
ers
geometric gravimetric( CS)ε ζ ζ= − +
Differences [centimeters] -1.5 -0.5 0 -0.5 1.5 -2.0
14
12
10
8
6
4
2
0
Occ
urre
nces
Nr of points: 82 Mean: -0.04 SD: 0.50 Min: -1.91 Max: 1.04
-1.0 1.0
x x
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Tide free mean crust (GNSS)
Zero crust over zero geoid to mean crust over mean geoid
Corrector surface
NLGEO2018 (gravimetric part)
NLGEO2018
26
LAT computations: setup Mean atmospheric fluxes (wind + pressure) ERA-Interim
Sea level = tides + mean surge + mean baroclinic
Mean horizontal density gradients (salinity + temperature fields)
Internal tide
Assimilating tidal water levels @ 31 tide gauges
in deep water
Vertical reference of hydrodynamic model NAP
29
LAT w.r.t. quasi-geoid - validation Obs – Mod LAT
10
5
0
-10
-15
-20
-25
-5
cent
imet
ers
Mod
el-d
eriv
ed L
AT
Observation-derived LAT
Region Nr Rms (cm) Range (cm) Mean (cm) Std (cm)
North Sea 47 10.6 57.6 -2.7 10.4
Wadden Sea 18 24.0 59.5 -18.7 15.4
All 92 14.9 77.6 -7.2 13.1
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LAT w.r.t. quasi-geoid – Dutch waters
Region Nr rms (cm)
North Sea 19 6.6
Wadden Sea 12 14.8
All 31 10.5
0
-0.5
-1
-1.5
-2
-2.5
met
ers
31
LAT in the Wadden Sea?
In tidal flats, LAT = no-water level! LAT ≠ smooth
0
-0.5
-1
-1.5
-2
-2.5
met
ers
32
In the Wadden Sea LAT pseudo LAT 0
-0.5
-1
-1.5
-2
-2.5
met
ers
To obtain pseudo-LAT, we added 2 m of water to open boundary conditions & assimilated tidal water levels
33
In the Wadden Sea LAT pseudo LAT 0
-0.5
-1
-1.5
-2
-2.5
met
ers
NOT valid outside Dutch waters!!!
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