VORF. A model for surveying to the ellipsoid

Transcription

1 VORF A model for surveying to the ellipsoid Chris Howlett Head of Seabed Data Centre, UKHO VORF What is VORF? How was VORF created? How is VORF being tested? How can VORF be used? Using VORF beyond Britain. 1

2 VORF What is VORF? How was VORF created? How is VORF being tested? How can VORF be used? Using VORF beyond Britain. What is 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. 2

3 Basic VORF functionality: Transforming data between vertical datums Example: Chart Datum to MLWS MLWS New data set in MLWS Chart Datum Existing survey expressed in CD Sea floor GRS80 Ellipsoid What datums does VORF cover? HAT MHWS Newlyn (OD other) MSL2000 MLWS LAT Chart Datum GRS80 Ellipsoid 3

4 What datums does VORF cover? 16 Land Datums: Alderney (Channel Isles) Douglas (Isle of Man Datum) Guernsey (Channel Isles) Kirkwall (The Orkneys Datum) Lundy (Island Datum) Poolbeg (EIRE Datum) St Kilda (Island Datum) Stornoway (Outer Hebrides) Belfast (Northern Ireland Datum) Foula (Island Datum) Jersey (Channel Isles) Lerwick (Shetland Isles Datum) Newlyn (UK Mainland Datum) Scalasaig (Colonsay Datum) St Marys (Scilly Isles Datum) Sule Skerry (Island Datum) What resolution is VORF? The surfaces are gridded at degree intervals (Approximately 900m in Latitude and, depending on the Latitude, 500m in Longitude). 4

5 What resolution is VORF? Where the tidal regime is complex or along rivers patches of higher resolution grids are used. These are at degree intervals (approximately 100m in latitude and 60m in longitude). Where is VORF? VORF covers the entire UK and Irish continental shelves 5

6 Where is VORF? 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? 6

7 VORF What is VORF? How was VORF created? How is VORF being tested? How can VORF be used? Using VORF beyond Britain. How was VORF created? Tide Gauge Data GPS Data Satellite Altimetry Data Gravity Field Models Tide Modeling 7

8 How was VORF created? Tide Gauge Data Two standards of tide gauges used: Permanent Service for Mean Sea Level (PSMSL) gauges and Admiralty Tide Table (ATT) tide gauges. How was VORF created? Permanent Service for Mean Sea Level gauges High accuracy, long time series of measurements Low density 8

9 How was VORF created? Admiralty Tide Table gauges Low accuracy, short time series of measurements High density Only definition of Chart Datum Correcting data for meteorological effects mean tide time series with corrections for noise and bias 9

10 How was VORF created? Link to ETRF89 Oddly, several tide gauges had no connection to any of the standard vertical datums. How was VORF created? Where it was considered necessary to do so GPS observations were made at these locations to provide the necessary connections. 10

11 How was VORF created? 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. How was VORF created? Gravity Field Models OSGM05 the latest gravity model for UK. 11

12 How was VORF created? MSL Tide gauge Interpolate Satellite altimetry geoid MLWS Tidal modelling How was VORF created? POL tidal models: CS3 NISE10 NISE10X SCM 12km horiz resolution 3.5km horiz resolution Extended area of above 1.2km horiz resolution 12

13 How was VORF created? How was VORF created? 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. 13

14 How was VORF created? MHWS MSL CD LAT ETRF89 0 Developing VORF (in one slide) MSL Land datums Tide gauge observations Satellite altimetry: sea level measured directly from space Sea surface topography = MSL - geoid Interpolate sea surface Tide gauge topography MSL Tidal modelling Ellipsoidal height of mean sea level at coast CD MLWS MHWS LAT Altimeter measurements geoid HAT Ellipsoidal height of Ellipsoidal satellite from height of GPS open oceans Ellipsoidal height of tide gauge from GPS Ellipsoid 14

15 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 VORF What is VORF? How was VORF created? How is VORF being tested? How can VORF be used? Using VORF beyond Britain. 15

16 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. How is VORF being tested? 16

17 How is VORF being tested? How is VORF being tested? 17

18 CHP Surveys CHP provides test sites for VORF values. How is VORF being tested? 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

19 How is VORF being tested? 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. How is VORF being tested? NAVOCEANO GPS Buoy 19

20 How is VORF being tested? 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

21 VORF What is VORF? How was VORF created? How is VORF being tested? How can VORF be used? Using VORF beyond Britain. Current practice for bathymetry data processing Tide gauge survey vessel observed tide (and time) Use co-tidal chart derived from Tidal correction Problems: Complexity onshore and offshore operations, time Latency the process takes time and hence is expensive depth measurement (and time of observation) Charted depth Low accuracy co-tidal charts have limited resolution and are derived from limited data; seabed gauges are expensive Inconsistent practices using Chart Datum are poorly defined and can lead to discrepancies LAT or use seabed gauge 21

22 bathymetry data processing with VORF and GPS survey vessel (+GPS) Tidal correction = h ε VORF LAT Tidal correction depth measurement Charted depth = Depth measurement tidal correction h ε LAT Charted depth VORF LAT GRS80 Ellipsoid - 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. 22

23 TerraPos elevations referred to GRS-80 ellipsoid. (From Dave Mann, Gardline Geosurvey) 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 /08/ :00 02/08/ :00 02/08/ :00 02/08/ :00 03/08/ :00 VORF What is VORF? How was VORF created? How is VORF being tested? How can VORF be used? Using VORF beyond Britain. 23

24 VORF beyond Britain 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. Differences in LAT values (with respect to MSL) between ATT data and CSR4.0 model 24

25 VORF beyond Britain BLAST an EU funded project may harmonize vertical tidal models in the North Sea area effectively expanding VORF to Norway, Belgium, Holland and Denmark. Questions? 25