Vertical Offshore Reference Frame (VORF) Vertical ... - Iho-ohi.net

Vertical Offshore 

Reference Frame 

(VORF) 

Christopher Jones 

United Kingdom Hydrographic Office 

© Crown copyright 2007

Presentation Structure 

• What is VORF? 

• Brief overview of the technical 

development 

• Why is VORF needed? 

• Potential uses of VORF 

United Kingdom Hydrographic Office

What is VORF? 

• VORF = Vertical Offshore Reference Frame 

• A set of mathematical models of the major 

surfaces used in the current and future 

charting of UK home waters 

• A suite of software utilities allowing the 

transformation of mapping and positioning 

data between the VORF surfaces 


Current practice for bathymetric data processing 

Tide gauge 

observed tide 

(and time) 

Use co-tidal 

chart 

derived from 

survey vessel 

Tidal correction 

Sea Surface 

Issues: 

Complexity – onshore and 

offshore operations, time 

Latency – the process takes 

time and hence is expensive 


depth 

measurement 

(and time 

of observation) 

Charted depth 

LAT 

… or use 

seabed gauge 

Accuracy issues – co-tidal charts have limited resolution and are 

derived from limited data; seabed gauges are expensive 

Inconsistency – practices using Chart Datum are sometimes poorly defined 

and can lead to discrepancies

Bathymetric data processing with VORF and GPS 

survey vessel (+GPS) 

Sea Surface 

Tidal correction = 

h ε 

–VORF LAT 

Tidal correction 

depth 

measurement 

Charted depth = 

Depth measurement – tidal correction 

h ε 

LAT 

Charted VORFdepth 

LAT 

GRS80 Ellipsoid - accessible everywhere via GPS 


VORF surfaces: 

HAT 

MHWS 

MSL2000 

OD(Newlyn) 

(OD other) 

MLWS 

LAT 

Chart Datum 

GRS80 Ellipsoid 


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 


Brief overview of the technical 

development of VORF 


Technologies applied in 

development of VORF 

• Tide gauge data 

• GPS data 

• Satellite altimetry 

• Gravity field models 

• Tidal modelling 


Data sources: 

Tide . Gauge data via the 

Permanent . Service for 

Mean . Sea Level (PSMSL) 

• National Tidal and Sea 

Level Facility (NTSLF) 

stations 

• High quality continuous 

observations 

• BUT low spatial density 


Data sources: 

Tide Gauges Admiralty 

Tide Table (ATT) 

• Around 700 Standard 

and Secondary Port 

locations 

• Good spatial density 

• BUT occasionally low 

precision due to short 

term data series 


Chart Datum: 

• VORF aims to unify all 

these separate 

datums into one, 

seamless surface 

• Process involves 

verifying the link 

between CD and 

Ordnance Datum (the 

land-levelling levelling height 

datum) 


Technologies applied: Satellite Altimetry 

Ground Tracks 

ENVISAT 

TOPEX 

JASON 

GFO 


ERS1/2

Technologies applied: OSGM05 – the 

latest UK gravity field model 


Overview of VORF computation method 

Tide gauge 

MSL 

interpolate 

MSL (altimetry) 

geoid 

MLWS 

Tidal modelling 


Boundaries of VORF Model 

UK Continental Shelf 


VORF software functionality 

• Transformation between datums 

• Estimated error in transformations 

• Visualisation 

• User error detection 

• Point/file mode data import 

• Deals with complexity of searching for 

special cases such as rivers and 

impounded datums. 

• High speed data retrieval and 

processing. 



VORF Application

Ultra Rapid Point in 

Polygon (PiP) ) Benchmark 

Tests 

• 400,000 line segment polygon set 

• Conventional desktop PC (1 Gb RAM, 

3 GHz processor) 

• 8,000,000 queries carried out 

correctly in 16 seconds (including file 

reading) 

• University College London (UCL) has 

developed new concepts in high 

performance PiP tests 

• Technique based on quadtree 

subdivision of analysis space 



Project status 

• Demonstrator model delivered to UKHO in 

December 2006. 

• Currently being evaluated. 

• Meets the specification (10 – 15 cm 1σ) 1 

across ~80% of inshore area and ~100% 

of offshore areas. 

• Programme of improvements to the datum 

surfaces proposed (due for completion end 

2007).

Why is VORF needed? 

• Continuing developments in GPS 

• LIDAR and multibeam technology 

• Analogy with the Ordnance Survey 

heighting reference systems on land 

• To deal with the increased use of GPS- 

based hydrographic surveys submitted to 

UKHO 


How can VORF benefit the 

UKHO? 

• Cost and efficiency of surveys 

• Quality control 

• Enabling new technologies 

• Developing new products 


Additional uses of VORF….. 


GIS applications – coastal zone 

projects 

Chart Datum B 

Site of new wind farm? 

OD Stornoway 

Wildlife 

conservation area 

Chart Datum A 

Chart Datum C 

OD Newlyn 

SSSI 

VORF enhances the usability of UKHO data 


Specialist applications 

Tidal predictions at “virtual tide 

stations” – accessible via satellite web 

link. 

Ship equipped with VORF and GPS is its 

own tide gauge – compare observed 

reading to prediction, plot enhanced 

route/timing for approach to critical 

areas. 


Optimising the marine navigation 

space 

Greater More choices accuracy 

in for under route keel finding 

clearance Rapidly produce (extra 

1cm charts of clearance in 

yields different £10k tidal of 

cargo) regimes 


Summary of VORF advantages 

• VORF derives continuous surfaces, with fixed 

reference to ETRF89. 

• It provides a consistent interpolation between 

Chart Datums, and methodology for extrapolation 

offshore. 

• It eliminates some of the reliance on remote or 

expensive tidal observations. 

• It has the potential to be built in to real-time 

applications. 

• It fully exploits current and future GPS 

technology, and is the basis for future accuracy 

enhancements. 


Summary of VORF applications 

• Simplified data acquisition – more data for 

the same price, or same data cheaper. 

• More precise navigation – all bathymetric 

data plus specific hazards to navigation are 

brought into the same coordinate system as 

vessel. 

• Simplification of bathymetric data sets and 

integration with other data sources – 

increased pool of users. 


Conclusions 

• VORF is an enabling technology 

• Surveying without tide gauges – cheaper, faster, 

more accurate 

• New navigation and space management concepts 

• Fully integrated data products 

• SOLAS – improved navigation in critical areas 

• VORF will help UKHO in its development of marine 

charting and navigation products 



Read More….. 

• University College London (UCL) 

website: 

http://www.ucl.ac.uk/ge/research/gps_ge 

odesy_navigation/VORF 

• Hydro-International website: 

http://www.hydro- 

international.com/issues/articles/id696- 

Joining_Up_Land_and_Sea.html 

• Hydrographic Journal No. 125 July 2007

Vertical Offshore Reference Frame (VORF) Vertical ... - Iho-ohi.net

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