1

NEVREF research team

2 Philipp Rupprecht photography

3 Philipp Rupprecht photography

4 Philipp Rupprecht photography

5 Philipp Rupprecht photography

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

8

The problem…

onshore offshore

reference ellipsoid

LAT

NLGEO2004

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

12

NLGEO2018, the new Dutch quasi-geoid model…

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.

15

NLGEO2018 - gravimetric quasi-geoid

16

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)

18

19

20

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= −

22

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

23

Tide free mean crust (GNSS)

Zero crust over zero geoid to mean crust over mean geoid

Corrector surface

NLGEO2018 (gravimetric part)

NLGEO2018

24

NLGEO2018

25

NLLAT2018, the new Dutch LAT model…

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

27 Source: http://www.kvibe.com/wp-content/uploads/2015/05/

28

LAT w.r.t. quasi-geoid 0

-0.5

-1

-1.5

-2

-2.5

met

ers

-3

-3.5

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

30

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!!!

34

NLLAT2018

35

NLLAT2018

36 Thank you! https://snapsbox.com/images/2018/01/29/

lamborghini-terzo-millennio-promo.jpg