VORF A model for surveying to the ellipsoid · TerraPos elevations referred to GRS-80 ellipsoid....

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VORF

A model for surveying to the ellipsoid

Chris Howlett

Head of Seabed Data Centre, UKHO

• What is VORF?

• How was VORF created?

• How is VORF being tested?

• How can VORF be used?

• Using VORF beyond Britain.

VORF

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•What is VORF?





VORF

• VORF – Vertical Offshore Reference Frame

• A set of surfaces each of which defines the separation of

one vertical datum from the WGS84 ellipsoid (GRS80).

• By combining surfaces, heights (depths) can be converted

from one vertical datum to another.

What is VORF?

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GRS80 Ellipsoid

Basic VORF functionality:

Transforming data between vertical datums

Example: Chart Datum to MLWS

MLWS

Existing survey

expressed in CD

Chart Datum

Sea floor

New data set in MLWS

What datums does VORF cover?

GRS80 Ellipsoid

Chart Datum

LAT

MLWS

MSL2000

MHWS

HAT

Newlyn

(OD other)

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16 Land Datums:

• Alderney (Channel Isles) Belfast (Northern Ireland Datum)

• Douglas (Isle of Man Datum) Foula (Island Datum)

• Guernsey (Channel Isles) Jersey (Channel Isles)

• Kirkwall (The Orkneys Datum) Lerwick (Shetland Isles Datum)

• Lundy (Island Datum) Newlyn (UK Mainland Datum)

• Poolbeg (EIRE Datum) Scalasaig (Colonsay Datum)

• St Kilda (Island Datum) St Marys (Scilly Isles Datum)

• Stornoway (Outer Hebrides) Sule Skerry (Island Datum)

What datums does VORF cover?

The surfaces are gridded at 0.008 degree intervals

(Approximately 900m in Latitude and, depending on the

Latitude, 500m in Longitude).

What resolution is VORF?

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What resolution is VORF?

Where the tidal regime

is complex or along

rivers ‘patches’ of

higher resolution grids

are used.

These are at 0.003

degree intervals

(approximately 100m

in latitude and 60m in

longitude).

• VORF covers the

entire UK and Irish

continental shelves

Where is VORF?

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• For each surface VORF contains two polygons. The area

between these is the area of applicability for that surface.

• Transformations are only permissible between surfaces (or

datums) where their areas of applicability overlap.

• This prevents transformations which have no meaning from

being carried out (e.g. Jersey datum to Stornoway datum).

Where is VORF?

Where is VORF?

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• What is VORF?

•How was VORF created?




VORF

• Tide Gauge Data

• GPS Data

• Satellite Altimetry Data

• Gravity Field Models

• Tide Modeling

How was VORF created?

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• Tide Gauge Data

Two ‘standards’ of tide gauges used:

Permanent Service for Mean Sea Level (PSMSL)

gauges and

Admiralty Tide Table (ATT) tide gauges.



• Permanent Service for Mean Sea

Level gauges

• High accuracy, long time series of

measurements

• Low density

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• Admiralty Tide Table gauges

• Low accuracy, short time series

of measurements

• High density

• Only definition of ‘Chart Datum’

mean tide time series with corrections for noise and bias

Correcting data for meteorological effects

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• Link to ETRF89

• Oddly, several tide gauges had

no connection to any of the

standard vertical datums.


Where it was considered necessary

to do so GPS observations were

made at these locations to provide

the necessary connections.

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• Satellite Altimetry Data

• High accuracy data but

degrades near shore as

the altimeter senses the

land.

• Satellite altimetry leaves

about a 15km gap

around any land.


• Gravity Field Models

• OSGM05 – the latest

gravity model for UK.

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Tide gauge

MSL

MLWSgeoid

Tidal modelling

Satellite altimetryInterpolate


POL tidal models:

CS3 12km horiz resolution

NISE10 3.5km horiz resolution

NISE10X Extended area of above

SCM 1.2km horiz resolution

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• CD is an imprecise datum.

Many CD levels were derived

from short term tidal

measurements and so CD

only approximates to LAT.

Tide stations around UK were

used to create a CD corrector

surface that was applied to

the LAT surface and so create

the CD surface.

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MHWS

MSL

LAT

ETRF890

CD

Sea surface topography

= MSL - geoid

Developing VORF (in one slide)

Ellipsoid

geoid

Tide gauge

Ellipsoidal height of tide gauge from GPS

MSL

Tide gauge

observationsEllipsoidal height of

mean sea level at coast

Satellite altimetry: sea level measured

directly from space

Ellipsoidal height

of satellite from

GPS

Altimeter

measurements

MSL

Ellipsoidal height

of open oceans

Interpolate sea surface

topography

HAT

LAT

MHWS

MLWS

CD

Land datums Tidal modelling

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Method assessment:

Interpolating/predicting sea level at a point

Knowing sea level at these points

we needed to work out the sea level

at some other point

Fundamental mathematical

building block in VORF

Test:

1. Remove known stations from dataset

2. Use the others to predict the value

3. Compute known – predicted

4. Repeat this all around coast

5. Compile statistics

RMS prediction error around

whole UK coast = 7 cm

• What is VORF?


•How is VORF being tested?



VORF

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How is VORF being tested?

• VORF values were checked against those in ATT (used in the

creation but being checked to ensure no large differences exist).

• VORF range used to plot co-tidal chart (range contours only) and

compared against co-tidal chart.

• Offshore tide gauges used to compare with VORF values.

• New offshore gauges have been laid in areas where differences

are too large.


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CHP Surveys

CHP provides test sites

for VORF values.


Onshore tide gauges

• CHP contractors measure tide gauges with GPS equipment and

confirm VORF separation values in their areas.

• Once check confirms VORF is correct, VORF is used for survey

and the check is added to VORF test data.

• Collation of all VORF test data underway.

• Testing scheduled for completion by January 2010.

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Offshore tide gauges

• Ideal solution offshore is to deploy a buoy that contains a high

precision GPS receiver.

• Log 60 days of water level data.

• Harmonic analysis of water level heights to obtain values for MSL

and LAT.

• Direct comparison of WGS84 heights to LAT.

• Suitable buoys do exist but are expensive and the risk of loss

quite high.


NAVOCEANO GPS Buoy

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Compromise offshore solution:

Sea floor tide gauge deployed for 60 plus days.

Deploying vessel records GPS height of water surface for 8

hours (including a high and low water event).

Harmonic analysis used to generate MSL and LAT from tide

gauge data.

GPS water level curve used to ‘fit’ MSL and CD to the ellipsoid.

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• What is VORF?



•How can VORF be used?


VORF

LAT

Charted depth

depth

measurement

(and time

of observation)

survey vesselTide gauge

observed tide

(and time)Tidal correctionderived from

Use co-tidal

chart

Current practice for bathymetry data processing

Problems:Complexity – onshore and

offshore operations, time

Latency – the process takes

time and hence is expensive

Low accuracy – co-tidal charts have limited resolution and are

derived from limited data; seabed gauges are expensiveInconsistent – practices using Chart Datum are poorly defined

and can lead to discrepancies

… or use

seabed gauge

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LAT

Charted depth

depth

measurement

Tidal correction

bathymetry data processing with VORF and GPS

GRS80 Ellipsoid

VORF

LAT

survey vessel (+GPS)

hε

Tidal correction =

hε– VORF LAT

Charted depth =

Depth measurement – tidal correction

- accessible everywhere via GPS

How can VORF be used?

• No software development is being carried out. Hence any future

user will have to get VORF values from UKHO (Seabed Data

Centre).

• Only VORF values will be provided, the model will remain the

property of and within UKHO.

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TerraPos elevations referred to GRS-80 ellipsoid.

(From Dave Mann, Gardline Geosurvey)

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54.5

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55.5

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56.5

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57.5

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58.5

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02/08/2008 12:00 02/08/2008 15:00 02/08/2008 18:00 02/08/2008 21:00 03/08/2008 00:00

C-Nav Raw C-Nav Smoothed TerraPos Raw TerraPos Smoothed

Need post

processed

GPS

Smoothed

data is OK

No need to

apply

measured

heave/pitch/

roll

• What is VORF?




•Using VORF beyond Britain.

VORF

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• UCL have just completed a feasibility study for VORF in the

Gulf. There is sufficient data to create a model. Its

accuracy will be less than around UK but the tidal ranges

are less allowing the absolute accuracies to remain

comparable.

VORF beyond Britain

Differences in LAT values (with respect to MSL) between ATT data and

CSR4.0 model

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• BLAST – an EU funded project may harmonize vertical tidal

models in the North Sea area effectively expanding VORF

to Norway, Belgium, Holland and Denmark.

VORF beyond Britain

Questions ?

VORF A model for surveying to the ellipsoid · TerraPos elevations referred to GRS-80 ellipsoid....

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