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VORF for DUMMIESPage 1 Page 2
If you’ve been asked to use “VORF tides” in your survey and you’re uncertain of the implications, this is written to reassure you. There is one fundamental thing to remember about VORF; say it to yourself in the morning, and when you turn in after your watch; display it above your desk:
There is no such thing as a VORF tide.
But it’s called VORF tides!
Only by mistake, because one of the most commonly used VORF tables is the one that relates GNSS height to Lowest Astronomical Tide. VORF does indeed produce vertical correction tables for a number of statistical tidal levels, e.g. LAT and HAT, but it is absolutely not about tide gauges, or dynamic tidal curves (the height of tide at any given moment).
The UK Hydrographic Office describes it like this: “VORF will 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 (page 6, paragraph 2.7).”
So what is VORF?
The Vertical Offshore Reference Frames, “deliver the capability to transfer heights and depths from one vertical reference system to another.” In practical terms, as far as most survey work is concerned, this means that VORF is little more than a set of files, each file containing a grid of height corrections to apply to GPS-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; just look it up (you will use your software to do that automatically) and apply the correction to your WGS84 height. Job done
Obviously there’s a lot more under the hood that you can research starting with the links below, but to use VORF, all that you need is the files, which are the final product of all that work.
Why is VORF needed?
The purpose of the VORF corrections is to make it possible to carry out accurate bathymetric surveys using GPS instead of tide gauges.
Work that involves installations both on land and on the sea-bed needs bathymetric measurements that can be tied in to the vertical datum used ashore, as well as being relatable to surfaces such as LAT. Traditional bathymetric survey using tide gauges and pressure measurements cannot provide this, because the tide gauges are tied into local chart datum, which is different all around the coast. VORF allows depths recorded relative to GPS-derived heights (and therefore fully compatible with land survey) to be mapped as required to a new, accurate, model of LAT – or several other tide-related surfaces.
Nautical charts are not referred to the same vertical reference datum as land maps. In fact, because the chart datum is essentially Lowest Astronomical Tide, they are not even consistent around the coast, because LAT itself is not consistent. Over 700 datum points around the UK have been used as the LAT reference for different Admiralty charts.
VORF provides a set of highly accurate surfaces that describe the true shape of LAT and four other statistical tidal levels at any point in the area covered, and not just at the 700+ marked datum locations. VORF publishes (among others) correction files for the following surfaces, all relative to ETRF:
VORF correction files are purchased from the same source as Admiralty charts.
LAT different from Chart Datum?
A Chart Datum (CD) which approximates the level of Lowest Astronomical Tide is used as the reference for charted depths because the primary purpose of charts is to aid safe navigation, and using a depth reference which approximates LAT means that a navigator can simply calculate safety margins by comparing charted depth with the draft of the vessel. However the height of LAT is not a constant, but actually varies smoothly and continuously around the coast, depending fundamentally on the corresponding variation of the tidal range – by simple arguments it follows that the larger the tidal range, the higher and lower the tidal heights will rise and fall respectively, thus the lowest predictable tide (astronomically speaking, i.e. LAT), will accordingly fall lower. The production of a chart therefore required meticulous observation of tides in the area at a specific point or points, which then became the datum points chosen for depth references.
So, rather than being referenced to the continuous surface of LAT, chart depths have historically been referenced to a large but finite set of datum points. (LAT is, after all, just a predicted tidal level – therefore it can be subject to change following the tidal analysis of new tidal observations at any particular location. However Chart Datum, once reliably established, remains ‘fixed’, hence the description, “CD is approximately LAT”). The VORF project had to confirm the vertical offset between ETRF and 700+ datum marks, which involved several expeditions to remote parts. VORF therefore includes correction files for both the LAT surface, and a “Chart Datum” surface that was produced by, “... developing a corrector surface which ‘warped’ the LATETRF surface to coincide precisely with the known Chart Datum definitions.”
How were the VORF surfaces computed?
A major part of the project was to tie in all of the disparate chart datum points. This makes it possible to relate VORF depths to charted depths on any given chart.
To tie in the 700+ chart datum levels, the project used:
To compute the actual shape of the tidal surface, the project used:
The VORF project was carried out by University College London, in collaboration with the UK Hydrographic Office.
WGS84 or ETRF. Which?
Technically, the VORF project used ETRS89, which is linked to ITRF. However, the difference between WGS84 and ETRS89 is less than the expected accuracy of GPS, so that the two can for most purposes be regarded as equivalent.
“New WGS84 realisations (since G730, 1994) are coincident with ITRF at about 10-centimetre level. For these realisations there are no official transformation parameters. This means that one can consider that ITRF coordinates are also expressed in WGS84 at 10 cm level. However, the most recent G1674 realization adopted ITRF2008 coordinates for more than half of the reference stations and velocities of nearby sites for the others. Thus, ITRF2014, ITRF2008 and WGS84 (G1674) are likely to agree at the centimetre level, yielding conventional 0-transformation parameters.” – https://fsp.support/quicklinks/datums.php
How does VORF assist with offshore survey?
Traditional offshore survey techniques before GPS would use the local LAT datum, and would involve taking accurate soundings from a vessel while taking synchronised readings from a tide gauge on the coast or on a fixed offshore object. Subtracting the tide gauge reading from the sounding would give an absolute depth relative to the tide gauge’s vertical datum. If an ROV were being used, then the sonar readings from the ROV, together with the ROV’s highly accurate pressure sensor, would give depth below surface, and the tide gauge reading would again be subtracted in order to correct the results to the local chart datum. This method can still be used with an AUV, and no vessel needs to be present to take soundings from the surface.
The results from a traditional survey of this kind therefore give vertical measurements that are relative to local Chart Datum.
Modern survey is performed using GPS, which gives vertical measurements relative to the WGS84 ellipsoid. Bathymetric measurements taken using height reference from GPS are therefore already compatible with land measurements, with the exception that the Ordnance datum is not at the same height as the WGS84 ellipsoid, so there is a fixed vertical offset. The VORF corrections, applied to survey results from GPS, can relate the results to the new highly accurate LAT surface. Or VORF can even be used to map the results to Chart Datum, if that is what is required.
So are tide gauges needed in order to use the VORF models?
Emphatically, no! Using tide gauges is the old-fashioned way that was needed before GPS. One purpose of VORF is to be able to use accurate GPS positions relative to the WGS84 ellipsoid, and correct those vertical measurements to one of the VORF reference surfaces. To repeat the words of the Hydrographic Office: “VORF will 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.”
 Actually, there is software developed by the VORF project that interpolates the VORF data to produce smooth seamless surfaces.