Vertical Offshore Reference Frames

Vertical Offshore Reference Frames (VORF) is a set of high resolution surfaces which together define the vertical datum for hydrographic surveying and charting in the United Kingdom and Ireland. The following surfaces are included:
All the surfaces are modelled with respect to the terrestrial reference frame used for satellite navigation (GNSS) positioning, ETRS89. Thus VORF directly permits the use of high precision GNSS in hydrographic survey, and also allows the capability of transforming vertical data between the different datums.
Use.
A major output of the VORF project was the gridded vertical correction files which deliver the capability to transfer heights and depths from one vertical reference system to another, “allowing the direct use of depth data from surveys which is referred to a WGS84 compatible datum rather than Chart Datum and thus enabling Hydrographic surveyors to survey without the need to measure tides”.
This is accomplished via a set of files, each file containing a grid of height corrections to apply to GNSS-derived heights to translate them to one of several VORF models. There is higher resolution in estuaries and inlets, but for most of the areas covered, there is a single height correction for each roughly 900 by 500 metre rectangle. VORF correction files are purchased from the same source as Admiralty charts.
Development.
The VORF project started in 2005 as a collaborative research project, sponsored by the UKHO, with a consortium comprising Proudman Oceanographic Laboratory, DTU, and led by University College London. It was delivered to the UKHO in 2008. 
VORF was developed using satellite altimetry, tide gauge observations, geoid and tidal modelling, and GNSS observations. VORF meets its target inshore accuracy of 10 cm in most of the domain of applicability.