VORF - The UK Vertical Offshore Reference Frame: Enabling Real-time Hydrographic Surveying

M. Ziebart, J. Iliffe, J. Turner, J. Oliveira and R. Adams

Abstract:	The Vertical Offshore Reference Frame (VORF) is a project tasked to integrate all vertical datums and reference surfaces used in the UK and Eire; particularly the sea datums used as the basis for marine charting, the land datums used for expressing height above sea level, and the reference frame used for GPS/GNSS positioning. The project was commissioned by the UK Hydrographic Office in October 2005, the solution was designed and implemented by the Department of Civil, Environmental and Geomatic Engineering, University College London. VORF consists of a set of surfaces (Chart Datum, lowest astronomical tide, mean low water springs, mean sea level, mean high water springs and highest astronomical tide) modelled as a grid of heights above the GRS80 ellipsoid referenced to both ETRF89 and ITRF2000, and estimated at the epoch 2000.0. The surfaces were computed using a combination of a new GRACE-based UK and Eire gravity field, multi-mission satellite altimetry data, long term tide gauge observations, historical and current Admiralty tide gauge data, GPS observations and levelling networks. Grid post spacing is at the level of 1 km. The paper will describe the data sources and surface computation methods, as well as the resulting surface accuracies. One of the key drivers behind the development of VORF is that it enables GPS/GNSS observations of ship position and height to be used directly in hydrographic surveying without recourse to shore-based observations of the tide. Initial sea trials of VORF show that a bathymetric survey processed using VORF and GPS heighting is consistent with the traditional data reduction method using a shore deployed tide gauge. The test showed no significant bias and a scatter of 7-9 cm between the two surveys. The key significance here is that the VORF-enabled survey required no shore-deployed assets and could have been conducted in real-time saving significant amounts of money, simplifying the operation and reducing the latency in delivery of the bathymetric survey.
Published in:	Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007) September 25 - 28, 2007 Fort Worth Convention Center Fort Worth, TX
Pages:	1943 - 1949
Cite this article:	Ziebart, M., Iliffe, J., Turner, J., Oliveira, J., Adams, R., "VORF - The UK Vertical Offshore Reference Frame: Enabling Real-time Hydrographic Surveying," Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007), Fort Worth, TX, September 2007, pp. 1943-1949.
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