Relating Data to a Seamless Vertical Reference Surface Good morning. Introduce myself and where I work. Questions at end. I am here to give a presentation on some of the technical issues involved with Seamless Data. I will be followed by Bob Croft who will provide examples of how this data fusion has worked within the UKHO. Ruth Adams, UK Hydrographic Office, UK

Why Seamless Data? Hydrographic ECDIS surveying applications Defence capabilities Coastal Zone Management User choice of output datum Marine boundary delimitation

Combined Land and Marine Data

What is a seamless vertical datum? Vertical datum = surface to which heights (or depths) of points are referred. A seamless vertical datum is one which does not vary over time or location.

Current Vertical Datums in use by UK Hydrographic Office This diagram shows the different types of vertical datum we use in the UKHO. The key one is Chart Datum (approximately LAT) – and off this hang the other datums such as Mean Sea Level, Mean High Water Springs (which we use as coastline), Mean Low Water etc.

Variable Vertical Datum But Chart Datum is not seamless! It varies spatially. For example the difference between Chart Datum and Ordnance Datum Newlyn is: -2.80m at Torquay -1.50m at Lowestoft -6.10m at Barry +0.60m at Barnstaple Chart Datum varies around the UK, and, indeed, the world. You can see from this that it varies quite considerably around just the UK. We are investigating modelling the difference between chart datum and WGS84 Datum, or rather to the geoid at present. It is not a simple model to construct, and there is quite a paucity of data in places.

A Seamless Surface? A stable reference surface which could be considered suitable for this is WGS84 Datum … Although a regional realisation may be more appropriate

Vertical Datums The ultimate aim is to refer all heights and depths to WGS84 Datum. Currently land applications use MSL or a local land datum (for example) for their heighting whereas marine applications use Chart Datum and MHWS. It is relatively straight-forward to transform land detail to WGS84 Datum but data referred to Chart Datum is not easy to convert. Vertical datums are a considerable challenge for us. Ultimately, we would like to refer all depths to WGS84 Datum to cerate a seamless database. It is relatively straightforward for land data as geoidal models can be used to define the separation. However, Chart Datum, which we refer all soundings, is not a coherent surface. It is certainly not easy to model.

The Challenge The challenge is not so much to develop a seamless vertical reference surface, but to develop a way of relating current and future datasets to it in a seamless manner.

How do you Develop a Vertical Datum Separation Model? It depends on available information… For example What bathymetric and topographic data is held? Are there ellipsoidal heights at Chart Datum points? Is there an accurate geoid model? And it depends on the accuracy needed

UK Example - VORF Connections are known between Chart Datum and ODN (Ordnance Datum Newlyn) at tide stations onshore. Using a geoid model, can obtain relationship between Chart Datum and ETRS89 (a European realisation of WGS84 Datum)

Vertical Datum Separation Model Contours show CD to OS vertical datum (ODN) separation. Red is 6 metres separation; yellow is 0 metres separation with 0.5 metre interval. Slide 8 – VERTICAL DATUM SEPARATION MODEL The result of the ICZ project was the production of this contour chart showing the CD to OD(N) vertical datum separation. Despite inaccuracies, this was a useful exercise in showing us what could be carried out. However, it is not currently good enough for navigation. More work to refine the model is required.

How Good is this Model? Still relies on co-tidal models – only guaranteed to be accurate to +/- 0.5m in vertical accuracy. Does this reflect the current minimum sea surface? What does the Mariner need? Slide 9 – HOW GOOD IS THE MODEL The project relied on existing co-tidal information which is not necessarily an accurate reflection of the current minimum height of the sea surface. Trial data from Milford Haven was used to validate the model. This proved that the method used for projecting data offshore using co-tidal models did not provide the sufficient accuracy, due to inherent errors in the co-tidal model, which is only good to ±0·5 m. There are doubts to its validity when extended more that 10kms offshore. The mariner ideally requires to know the minimum sea surface. As CD does not now necessarily represent the minimum sea surface, then so this model will not.

The Way Ahead UKHO will develop a new minimum sea surface related to ETRF89. UKHO will develop a Chart Datum to ETRF89 Datum separation model. UKHO will also develop a Chart Datum to minimum sea surface corrector surface. Slide 10 – THE WAY AHEAD In an effort to improve our understanding of the relationship between the different surfaces, the UKHO is aiming to develop a new minimum sea surface related to ETRF89. A modern LAT. UKHO will also develop a CD to ETRF89 datum separation model. This will lead on to the development of a CD to minimum sea surface corrector surface.

Complex Datum Surfaces Around the Coast Coastline ODN CD Geoid Corrector Surface between CD and Minimum Sea Surface VORF Model ICZM Model ( Corrector Surface ) OSGM 02 CD-ODN diff from ATT ETRF89 Minimum Ref. Surface Min. Sea Surface Slide 11 – COMPLEX DATUM SURFACES AROUND THE COAST (2) This slide revisits those complex surfaces and how they relate to each other. This time I have included ICZM and VORF. It shows that we already know the relationship between OSGM02 and ETRF89. We know the relationship between the ICZM model, but as I said before this needs to be refined. The VORF model will give us the relationship between the minimum sea surface and ETRF89 and therefore to the other surfaces.

The UKHO Model This could be done using modern satellite data – expected accuracy +/- 0.2m 1 sigma. It is planned to match the area covered by OSGM02 SW 012°W, 48°N SE 004°E, 48°N NE 004°E, 62°N SE 012°W, 62°N Work is in hand to put out the contract and it is anticipated we will have a working model for research purposes by early 2007. Slide 16 – THE UKHO MODEL The plan is to use modern satellite data which has an estimated accuracy of ±0·2m at 1 sigma (probability of this 68·2% of the time), to match an area covered by OSGM02. The work is in the process of being put out to contract at the moment. Phase 1 of the contract is to produce a model usable for research purposes.

Developing the VORF 1.Develop a mean sea surface using satellite altimetry and geoid modelling 2.Use tidal modelling to derive minimum sea surface

Satellite Altimetry - Ground Tracks TOPEX/POSEIDON ground track Along track spacing 6-7 km Across track spacing 315km Additional data from other satellites ENVISAT ERS2 Future satellites – 200km wide swath Slide 17 – SATILLITE ALTIMETRY GROUND TRACKS On to the method. To get the minimum sea surface offshore up to 6-7 km from the coast, it is proposed to use the Topex/Poseidon satellite altimetry ground tracking data. This data was used in the earth gravity model EGM96. The overall coverage of this data is a point measured every 6-7 km along track and spaced 315km apart. The reason the points are spaced 6-7 km apart is that the distance travelled over the ground by the satellite when a 1 second logging rate is set. The satellite passes over the same point every 10 days and has been doing this for about the past 10 years or so. It is estimated that the data has a resolution of around 15mm. In future technology will be capable of observing a 200km swath, effectively closing the gap. A sort of multibeam type altimetry! “Can be likened to the deployment of thousands of tide gauges across the ocean”

Tidal Modelling Once this MSL has been established the new minimum sea surface will be derived using tidal models provided by Proudman Oceanographic Laboratory, POL. Slide 22 – Tidal Modelling Once this MSL has been established the new minimum sea surface will be derived using tidal models provided by Proudman Oceangraphic Laboratory, POL.

VORF Project Latest Notice placed in the OJEU, inviting expressions of interest in the VORF project. Tenders were submitted and have been evaluated. Contract has been let with UCL. Slide 24 – VORF PROJECT LATEST A notice was placed in the Official Journal of the European Union (OJEU) inviting expressions of interest in the VORF project Tenders were submitted and have been evaluated. Contract has been let with UCL. This contract is purely a research project at this stage to determine if it is possible, how accurate it will be and to provide UKHO with an idea of the utility of such a model.

Other Vertical Datum Separation Models The UKHO methodology is only one of the many ways to develop a vertical datum separation model

Canadian Example The Canadian Hydrographic Service have used satellite altimetry and tidal models to develop a Chart Datum model offshore

US Example In Tampa Bay NOAA developed a Chart Datum model using a hydrodynamic tidal model and a known relationship to NAD83 (a US realisation of WGS84).

Keep the Goal in Mind Don’t create a high accuracy, expensive model if a lower accuracy one will suffice – keep in mind the purpose of the model What area will the model cover? Have neighbouring nations already developed similar models which can be extended? Is the aim to re-create Chart Datum or develop a model of the minimum sea surface? What resources are available? What storage requirements will the model have? How will the model be maintained?

Conclusions Seamless vertical data is vital to future hydrographic requirements. Chart Datum is not seamless and data must be referred to another vertical datum. This can be via a vertical datum separation model. The challenge is to develop this model so that historic, current and future data can be merged.

Thank you for listening Good morning. Introduce myself and where I work. Questions at end. I am here to give a presentation on some of the technical issues involved with Seamless Data. I will be followed by Bob Croft who will provide examples of how this data fusion has worked within the UKHO. Ruth Adams UK Hydrographic Office