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Vertical Reference FramesSimple Geometry or Measurement Sciences or a complex combination
of both?By
David PhilipThe views expressed in this presentation are based on my
own web-based research to date and are given in good faith as being factually and historically accurate.
All Sources of material are duly acknowledged. The answers to a number of emails sent to a number of busy experts were also very much appreciated
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How well do today’s vertical Datums, measurement technologies, and models match the practical realities nearshore
and offshore the UK?
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OUTLINE• Introduction and brief history• Land levelling –systematic errors• Vertical Datums• Tidal regime around UK• Sea Level Observations• Altimetry• Tidal Models and Tidal Data• Geoids• Accuracy
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A brief review of the third Dimension
• Five basic questions to be answered
• How?• What?• When?• Why?• Who?
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The main influences» Military» Scientific» Political» Commercial
– It should be appreciated that in the last 40 years there has been a revolution in measurement technology.
– Has our knowledge and appreciation of the vertical references kept pace with these changes or is it lagging further and further behind?
– Does our approach to the vertical dimension need to change?
– What is accuracy? Closeness to the “truth”.
– Is the “Truth” of 1918 the same as the “Truth” of 2009?
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MILITARY INFLUENCE• Military recognition that warship navigational safety relied on reliable
charts.• Charts show accurate “least” depths to a defined Chart Datum.
Measured depths need to be tidally reduced.• UK Hydrographic Office was established in 1795 and first
Hydrographer appointed.• An organisation of military officers and civilian scientists are the
Hydrographic “technical”experts.• Shared charting information benefits trade. Admiralty Charts made
available for Merchant Marine 1819• Hydrography had a traditional “international” approach to the rights
of passage over the high seas and the sharing of data.
• Is this gradually changing? Are organisations and nations becoming more and more territorial and is data sharing is reducing in some areas?
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HISTORICAL CONTEXT• First Hydrographic Chart completed and published 1801• First Admiralty Tide Tables published 1830 (4 Ports)• First North Sea co-tidal chart estimated (Rev. W.Whewell) 1836• First Hydrographic Tides Officer appointed 1912• J. Proudman and A. T. Doodson compute first “True” Co-tidal
Constituent Chart for North Sea 1924• A. T. Doodson and R.H. Cochran first “True” Co-tidal Constituent
Chart for English and Irish Channels 1932 • British Isles Co-tidal and Co-range chart 5058 published 1971
Included contributions from Belgium, Denmark, France, Germany, Netherlands, and Norway
5th Edition published 29th March 1996 Included offshore tidal data provided by Oil Companies.
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Why is a consistent Vertical Reference Frame needed?
On mainland UK all mapping heights are referred to Newlyn Datum. A few Islands have their own local Datum. Eg Shetland, Orkney, Isle of Man etc
Charts use Chart Datum. This varies from place to place as it is dictated by the range of tide. It is usually close to LAT but potentially in excess of 30 different types of Tidal Datums exist worldwide.
In the UK coastal zone there is data overlap with different conventions used by different agencies. (Hydrographic Office, Ordnance Survey, British Geological Survey)
Integrating data in the coastal zone requires some careful work by a professional surveyor.
UK Tidal data has never been adjusted to provide a homogeneous spatial vertical reference.
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UK land heighting history• The original level Datum of Liverpool used for the First Levelling of UK 1840-1860 was discarded
in 1921 but used 47 tidal stations around the English, Welsh, and Scottish coasts.• 2nd Geodetic Levelling completed in 1921 using 3 Tidal stations but had 0.8 feet misclose Newlyn
–Dunbar. All UK mainland heights based only on Newlyn Tide Gauge observations 1915-1921 (Epoch 1918).
• 3rd Geodetic levelling the systematic error was found to be even greater at 1.2 feet. A choice of two heighting systems could now be used but both are based only on Newlyn.
• UK “scientific” heighting on Newlyn Datum was based on the 2nd Adjustment of 3rd Geodetic levelling but has a systematic slope error of about 0.5 metres from south to north of the UK.
• The “scientific” levelling slope has been proved scientifically three times by different methods by UK Oceanographers
• It has also been proved scientifically from a number of geoid derivations using gravity data namely EDIN89, EDIN91, OX92, OSGM02, EDIN04.
• There is currently one exception in OSGM05 which indicates a much reduced slope but in comparison with 2nd Geodetic levelling
• “Warping” one data set to another eg OSGM02 merely perpetuates a known error. It does not make it right.
• What is the 4 parameter shift required to fit OSGM02 to Ellipsoid heights on ETRF89?
Can scientific reality be redefined by assertion?Does a lack of clarity cause confusion?
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29 UK Ports MSL 1960-19742nd Adjustment 3rd Geodetic Levelling
EGM2008 + “Warped” OSGM02Levelling, MSL, and the EGM2008 Geoid
29 East and West Coast Ports 1960-1974 (Epoch 1967)
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
5400000 5600000 5800000 6000000 6200000 6400000 6600000
Northing - metres
Hei
ght D
iffer
ence
- m
etre
s East Coast
West Coast
East Coast EGM2008
West Coast EGM2008
Linear (East Coast)
Linear (West Coast)
Linear (West Coast EGM2008)
Linear (East Coast EGM2008)
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Levelling and MSLNorth South UK Level Slopes
y = 5E-07x - 2.5864R2 = 0.8632
y = 3E-07x - 1.7577R2 = 0.7112
y = 2E-07x - 1.0228R2 = 0.4219
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
5400000 5600000 5800000 6000000 6200000 6400000 6600000
Northing - metres
Hei
ght D
iffer
ence
from
MSL
2Adj 3 GL Epoch 1967
1 Adj 2 GL Epoch 1918
EGM2008 + Warped OSGM02
1 Adj 2 GL Epoch 2000
Linear (2Adj 3 GL Epoch 1967)
Linear (1 Adj 2 GL Epoch 1918)
Linear (EGM2008 + Warped OSGM02)
Linear (1 Adj 2 GL Epoch 2000)
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TIDAL REGIMESDEEP OCEANSIn the DEEP oceans the tidal influence of sun and
moon are dominant with a small tidal range of 1 metre – major tidal constituents dominate.
SHALLOW SHELF SEASThe effects of friction caused by water depth,
seabed gradients, and seabed material create “non-linear “ Shallow water tides with extreme tidal ranges in enclosed basins. This generates compound and overtides with many constituents. Estuaries and rivers create further distortions.
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BATHYMETRY UKCS
BERR Atlas of UK MarineRenewable EnergyResources.
A Strategic Environmental AssessmentReport March 2008
Crown Copyright
Produced By: ABPmer, The Met Office,Proudman Oceanographic Laboratory
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PEAK FLOWS - SPRING and NEAP TIDEScourtesy BERR Atlas of UK Marine Renewable Energy Resources
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MEAN SPRING and NEAP TIDAL RANGEcourtesy BERR Atlas of UK Marine Renewable Energy Resources
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Fig 3.28 UKSeaMap
Seabed Landscapes in UK
Crown copyright
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SEA LEVEL MEASUREMENTSIt should be appreciated that sea level
observations are highly variable and very noisy.Two principal elements:-
1. Predictable Forces – 18.6 year cycleAstronomicalLocation Deep Ocean
Shallow shelf sea2. Un-predictable – annual seasonal cycle
Meteorological (Pressure, Wind)Temperature and salinity (El Nino)
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Sea Levels at Newlyn 1915-2005 : Analysis of Trends for Future Flooding Risks Courtesy Isabel B. Araujo and David T. Pugh
The 18.6 year Astronomical Cycle
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Maximum and Minimum TrendsCourtesy Isabel B. Araujo and David T. Pugh
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Newlyn Annual mean sea level (mm)Courtesy Isabel B. Araujo and David T. Pugh
Rising Sea Level
Sea level rise
1915 – 2000
0.15 metres
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UnpredictabilityDangers of short term trends -Management of change
Courtesy Isabel B. Araujo and David T. Pugh
Gauge change
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ALTIMETRY STATUS SUMMARY
Courtesy Andersen O.B. and Knudsen P. (2009) DNSC08 mean sea surface and mean dynamic topography models
• 8 different altimetry satellites have been used to collect data. They have a range of accuracies, observation periods, repeat tracks, ground track spacing, and coverage limits but have been used in a combined solution
• Accuracy a priori estimates 0.6 to 11 centimetres• Observation periods 0.6 to 12 years• Repeat tracks 0 to 445• Ground Track spacing 6 to 340 kilometres• North/South coverage limits +/- 65 to +/- 82 Degrees
– ICESat coverage Arctic coverage +72 to + 86
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LATEST DEVELOPMENTS in DNSC08 global mss and mdt
• Latest result determined by the Danish National Space Center has a number of key attributes and limitations
1. First Global MSS without a polar gap.2. First with a 10 year temporal period.3. First to extend coverage to the poles.4. No significant voids or data gaps.5. MDT surface linked with EGM2008 Geoid6. Incomplete temporal averaging eg El Nino7. Longterm coverage limited in N/S Extent8. Geoid model assistance only needed to complete
polar coverage
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SATELLITE ALTIMETRYProcessing Focus
• Wet Tropospheric Correction• Dry tropospheric Correction• Ionospheric Correction• Air pressure Correction• Wind Correction• Tidal Correction
Items highlighted in RED are the most significant
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COASTAL ZONE is key area for altimetry improvements
• Loss of the radiometer correction information within 10’s of Kilometres of the coast degrades altimetry results.
• Improvements reported in 2007 using ECMWF Modelling to within10 Kilometres (Madsen, Hoyer, Tscherning)
• This is the key connection area between higher accuracy altimetry mean sea surface and port tide gauges
• This also coincides with weakest area for gravity data and also results in geoidal uncertainty
• Result – Dynamic Topograghy remains of very limited accuracy in the nearshore area
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IMPROVEMENTS IN SATELLITE ALTIMETRY Deep Oceans and Shallow Seas – a comparison
Courtesy R.D. Ray Thoughts on Shallow Water Tides and Altimetry
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OFFSHORE VALIDATION LOCATIONSaround UK – Offshore Pressure gauges and Dutch Platforms
Courtesy R. D. Ray Thoughts on shallow water tides and altimetry
? UK Platforms?
More Pressure Gauge data?
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Correlation between Tide Gauge Port ABERDEEN and satellite altimetry
Courtesy K.S.Madsen MSc Disertation
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CORRELATION
• A correlation greater than 0.8 would be described as STRONG
• A correlation less than 0.5 would be described as WEAK
• What would be a sensible correlation distance limit between an onshore tide gauge and an offshore location?
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Correlation between Tide Gauge Port CROMER and satellite altimetry
Courtesy K.S.Madsen MSc Disertation
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TIDE MODELSAccuracy and developments
• Two Basic Types– Empirical deduced model (Altimetry analysis)– Finite element hydrodynamic model (bathymetric nodes)
• Global Accuracy in “open deep ocean”Typical RMS 0.01m Worst case 0.03 metres
– Significant errors• shallow waters - non-linear tides• polar regions – sea ice, limited or lack of altimetry coverageRMS 0.10 - 0.15 metres
Global ModelsGOT00.2 , FES2004, TPX0.7New modesls EOT08a (Savcenko & Bosch) and GOT4.7 (Ray 2008)Notably the determination of shallow water tides
Regional Models of noteNW Mediterrranean – MOG2D, NW Europe POL 2nm , West of Portugal Mohid
The more modern the model the more attention paid to the “problem” areas. Still scope for significant further improvement
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Figure 1 : Variance differences of SSH at crossovers with EOT08a vs FES04 for Jason-1 and EnviSat missions over 2005. Blue and red colours indicate improvement and deterioration respectively
with EOT08a.
JASON 1
Courtesy - Performance estimation of recent tide models using altimetry
and Tide gauge measurements – L. Carrere, J-F Legeais, E. Bronner
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Figure 1 : Variance differences of SSH at crossovers with EOT08a vs FES04 for Jason-1 and EnviSat missions over 2005. Blue and red colours indicate improvement and deterioration respectively
with EOT08a.
ENVISAT
Courtesy - Performance estimation of recent tide models using altimetry and Tide gauge measurements –L. Carrere, J-F Legeais, E. Bronner
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Sources of UK TIDAL DATA and MSL
• 2 Primary sources of tidal data identified by Iliffe et al (2007) for VORF.• 75 P Stations Source PSMSL Monthly Average (1807-to date)
– 22 P’ Stations Source PSMSL Monthly Average
20 years of modern data (1960 onwards)• 385 Admiralty Tide Tables 2007
– 253 3 months or less data (majority only one month)– 25 3 months to 1 year– 107 more than 1 year– 51 in rivers and more than 2 kilometres from open sea– A further 10 identified as anomalous
A note of caution. These observations have been collectively labelled as gauges. For many Ports this may be valid but for a significant number it has been tide poles and human recorders that collected the data.
Integrated Coastal Zone Report identified that although ATT had Tidal Information for 800 ports around UK only 339 have all the constituent data.
No UK offshore pressure gauge (POL and others) data has been used. No offshore UK Platform data has been used
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DEFINING “The” GEOID• Choice of a normal gravitational potential field
(encompassing the reference ellipsoid)
• Choice of which specific level surface is "the" geoid. It approximates to mean sea level over the oceans
• Knowledge of what permanent tide system the geopotential model refers to (a choice of three)
• Knowledge of how the potential field acts inside topographic masses
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GEOIDS
• Astrogeodetic Geoid• Gravimetric Geoid• Quasi-geoid
• Local• Regional• Global
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Tidal Types of GeoidTide-free (or nontidal)—This geoid would exist for a tide-free Earth with all
(direct and indirect) tidal effects of the Sun and Moon removed.
Positioning (e.g.,from GPS) is given in the Tide Free system on WGS84 Ellipsoid. This requires that GPS/Leveling applications use a Tide Free geoid for Surveyors.
Mean Tide —This geoid exists in the presence of the Sun and the MoonA mean tide value includes both direct and indirect permanent tidal distortions.
Mean Tide system is given with respect to the IDEAL mean Earth ellipsoid. Its mainly for users working in oceanographic applications.
Zero Tide —This geoid would exist if the permanent direct effects of the Sun and Moon are removed, but the indirect effect component related to the elastic deformation of the Earth is retained. (Very theoretical)
Geodesists are supposed (as per an IAG resolution) to work in Zero Tide but generally they don’t apart from Finland and Sweden.
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GEOID TYPE DIFFERENCES
-30
-25
-20
-15
-10
-5
0
5
10
15
0 10 20 30 40 50 60 70 80 90
Geodetic Latitude
Diff
eren
ce -
cent
imet
res
Mean Tide - Zero Tide
Zero Tide - Tide Free
Mean Tide - Tide Free
Geoid semi-major axis f latenningTIDE FREE 6378136.58 1/298.257686ZERO TIDE 6378136.61 1/298.256452MEAN TIDE 6378136.71 1/298.252342
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EGM96Courtesy Chapter 11, of the EGM96 report
(NASA/TP-1998-206861),Best Estimates WGS84 Values
GM = 3.986004418e14 m3s–2 GM_o = 3.986004418e14 m3s–2 W_o = 62636856.88 m2s–2 U_o = 62636851.71 m2s–2 a_Ideal_TideFree = 6378136.58 m a = 6378137.00 m
1/f_Ideal_TideFree = 298.257686 (**) 1/f = 298.257223563
By computation Zeta_o = -52.8 cmBy computation Zeta_o = -52.9 cm
(**) Love number k_2=0.3 used.
Value adopted the rounded value: Zeta_o = -53 cm.
WGS84 ellipsoid, whose origin is at the center of mass of Earth, as defined by WGS84 (G873)/ITRF94, and whose axes are aligned with the indicated reference frames.
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EGM2008Courtesy email N.Pavlis
Current Best Estimates WGS84 ValuesGM = 3.986004418e14 m3s–2 GM_o = 3.986004418e14 m3s–2 W_o = 62636855.69 m2s–2 U_o = 62636851.71 m2s–2 a_Ideal_TideFree = 6378136.58 m a = 6378137.00 m
1/f_Ideal_TideFree = 298.257686 (**) 1/f = 298.257223563
By computation Zeta_o = -40.62 cm
By computation Zeta_o = -40.89 cm
(**) Love number k_2=0.3 used.
Value adopted the rounded value: Zeta_o = -41 cm.Compare EGM96 Value : Zeta_o = -53 cm.
------------------------------------------------------------------------Difference : -12 cm
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Critical calibration of altimetry measurementsCourtesy email N. Pavlis
The primary reason for the change in the numerical value of Zeta_o from the EGM96 model (-53 cm) to the current best estimate (-41 cm), was the discovery by Ouan-ZanZanife (CLS, France) of an error in the Oscillator Drift correction applied to TOPEX altimetry data (see page 34 in http://www.amazon.com/Satellite-Altimetry-Earth-Sciences-International/d p/0122695453).
The erroneous correction was producing TOPEX Sea Surface Heights, biased by about 12-13 cm. Unfortunately, this error was discovered after the estimated Zeta_o and had created/released for the EGM96_WGS84 geoid grid back in the EGM96 days.
This error was also the reason why Rapp's (1995) best estimate of the equatorial radius (6378136.59 m in the Mean Tide system) is off by 12 cm from Bursa's (1999) estimate (6378136.71 m in the same Mean Tide system).
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Height Anomaly or Geoid undulationComparing Apples with Apples
HEIGHT ANOMALIES (with respect to an IDEAL reference ellipsoid,in the Mean-Tide Permanent Tide system).
The difference between a HEIGHT ANOMALY and a GEOID UNDULATION is proportional to the Bougueranomaly times the elevation (see Heiskanen and Moritz, 1967, Section 8-13).
Over ocean areas, height anomalies and geoidundulations are identical, provided of course that both have been computed with respect to the same ellipsoid, and both refer to the same Permanent Tide system.
Over land these differences become more significant .Ben Nevis about 0.11 metres, Mount Everest 4.4 metres
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Equatorial radius for scalingEllipsoid semi-major axis
There is "a" used as a SCALING parameter of the potential coefficients assigned for every geopotential model - 6378136.3 m value used for EGM2008 and EGM96
There is an "a" associated with the ellipsoid to which a set of geoidundulations refer. Changing the scaling parameter "a" associated with a model by a few decimeters has negligible effect on the geoidundulations computed from this model.
Changing the "a" associated with the ellipsoid to which a set ofgeoid undulations refer, amounts to introducing a zero-degree geoidundulation term. In this case, delta a = - delta_N.
According to Bursa et al. (1999), using the value of 0.3 for thesecond-degree Love number
a 1/f
Tide-Free : 6378136.58 m 298.257686
Zero-Tide: 6378136.61 m 298.256452
Mean-Tide: 6378136.71 m 298.252342
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GEOID DIFFERENCES EGG08 and EGM96Courtesy T. Gruber “Evaluation of the EGM Gravity Field by means of
GPS Levelling and Sea Surface Topography solutions”
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GEOID DIFFERENCES EGG08 and EGM2008Courtesy T. Gruber “Evaluation of the EGM Gravity Field by means of
GPS Levelling and Sea Surface Topography solutions”
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GEOID DIFFERENCES EGG08 and EIGEN-5CCourtesy T. Gruber “Evaluation of the EGM Gravity Field by means of
GPS Levelling and Sea Surface Topography solutions”
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UK FBM Distribution H1’s and H2’sUK 188 FBM Distribution
5400000
5600000
5800000
6000000
6200000
6400000
6600000
200000 300000 400000 500000 600000 700000 800000 900000
Easting
Nor
thin
g
H1 FBM'sH2 FBM's
LISKEARD
DUNCANSBY HEAD
DUNBAR EAST
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FBM's used in 2ND and 3RD GEODETIC LEVELLING
5400000
5600000
5800000
6000000
6200000
6400000
6600000
0 100000 200000 300000 400000 500000 600000 700000 800000 900000
Easting - metres
Nor
thin
g - m
etre
s
H1 England Wales
H2 England Wales
H1 Scotland
H2 Scotland
H1 and H2 Not common to 2GL and 3GL
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Levelling Slopes v EGM2008Common H1 and H2 FBM's
0.400
0.500
0.600
0.700
0.800
0.900
1.000
5400000 5600000 5800000 6000000 6200000 6400000 6600000
Northing - metres
Hei
ght D
iffer
ence
3rd GL England and Wales
3rd GL Scotland
2nd GL England and Wales
2nd GL Scotland
Latitude 53 Degrees North
Linear (3rd GL England and Wales)
Linear (3rd GL Scotland)
Linear (2nd GL England and Wales)
Linear (2nd GL Scotland)
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Geoid Height DifferencesGlobal and Local
• Mean Difference EGM2008 - OSGM02+ 0.564 +/- 0.06 metres
• Mean Difference EGM2008 - EGM96+ 0.287 +/- 0.16 metres
• Mean Difference EGM2008 - OSU91A- 0.289 +/- 0.25 metres
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GEOID MODEL COMPARISONSNorth - SouthGEOID Height Differences
EGM2008, OSGM02, EGM96, OSU91A82 UK H1 FBM's 106 UK H2 FBM's
-1.200
-1.000
-0.800
-0.600
-0.400
-0.200
0.000
0.200
0.400
0.600
0.800
1.000
5400000 5600000 5800000 6000000 6200000 6400000 6600000
Northing
Hei
ght D
iffer
ence
- m
etre
s H1 EGM2008 - OSGM02H1 EGM2008 - EGM96H1 EGM2008 - OSU91AH2 EGM2008 - OSGM02H2 EGM2008 - EGM96H2 EGM2008 - OSU91APoly. (H1 EGM2008 - EGM96)Poly. (H2 EGM2008 - OSGM02)Poly. (H1 EGM2008 - OSGM02)Poly. (H2 EGM2008 - EGM96)Poly. (H1 EGM2008 - OSU91A)Poly. (H2 EGM2008 - OSU91A)
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GEOID MODEL COMPARISONSWest - East
GEOID Height DifferencesEGM2008, OSGM02, EGM96, OSU91A
82 UK H1 FBM's, and 106 UK H2 FBM's
-1.200
-1.000
-0.800
-0.600
-0.400
-0.200
0.000
0.200
0.400
0.600
0.800
1.000
250000 350000 450000 550000 650000 750000 850000
Easting
Hei
ght D
iffer
ence
- m
etre
s
H1 EGM2008 - OSGM02H1 EGM2008 - EGM96H1 EGM2008 - OSU91AH2 EGM2008 - OSGM02H2 EGM2008 - EGM96H2 EGM2008 - OSU91ALinear (H1 EGM2008 - OSGM02)Linear (H1 EGM2008 - EGM96)Linear (H1 EGM2008 - OSU91A)Linear (H2 EGM2008 - OSGM02)Linear (H2 EGM2008 - EGM96)Linear (H2 EGM2008 - OSU91A)
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SEA SURFACE TOPOGRAPHYDYNAMIC OCEAN TOPOGRAPHY• The mean dynamic ocean topography (DOT) is the difference between the time-
averaged sea surface and “the” geoid.• All geoid slopes are 'horizontal‘ equipotential surfaces with respect to gravity. • Any tilt of the the ocean sea surface relative to the horizontal measures the strength
of surface 'geostrophic' currents or wind driven currents. • The DOT is a measures of the long-term-averaged strength of ocean currents, the
'steady-state' circulation. Eg. The Gulf Stream, • The DOT can be deduced by geodeticaly differencing an accurate altimetric mean
sea surface and an accurate geoid.• The DOT can also be constructed by combining in-situ oceanoraphic data
(temperature and salinity of seawater, direct measurements of current velocity, etc). • A third way is by combining the geodetic estimate (altimetry and geoid) with the
traditional oceanographic estimate.
• This most significant deepwater “dynamic ocean topography” features are of the order of + 0.8 to -1.8 metre at maximum.
• On the shallow continental shelf seas these features are much less significant and one or two orders of magnitude less.ie 0.02 metres to 0.2 metres
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SEA SURFACE TOPOGRAPHY UK and IRELANDor DYNAMIC OCEAN TOPOGRAPHY
• 320 GPS measured Tide Gauges• Around Britain
• TG – DNSC08MSS• Mean = 1.24 cm, • Std = 6.8 cm
• Comparison by• Marek Ziebart, UCL London,
• Courtesy Andersen et al. The DNSC08 global Mean sea surface and Bathymetry. Presented EGU-2008, Vienna, Austria, April, 2008.
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ACCURACY
• A quote from FIG Guide on the Development of a Vertical Reference Surface for Hydrography. No.37 September 2006
• Section 3.3 Sub para Accuracy• The accuracy of the model will depend on many
factors and the total error budget should be kept in mind at all times. All data contains errors and hence all models have an error associated with them. The key factor is knowing the errors. The greatest danger is to use data without knowing how accurate it is. Poor data is not useless data, but it should be treated according to its accuracy.
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ACCURACY SUMMARYTypical Range Worst
Altimetry – oceans 0.010 to 0.015 0.03Altimetry – Shelf Seas 0.100 to 0.150 0.30Altimetry – nearshore not available ???Tides – oceans 0.020 to 0.050 0.05 Tides – Shelf Seas 0.100 to 0.150 0.25Tides – Nearshore 0.100 to 0.200 0.25Geoid – Oceans 0.070 to 0.100 0.10Geoid – Shelf Seas 0.100 to 0.250 0.30Geoid - Nearshore no gravity data ???
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The Accuracy of UK Co-tidal Charts
• FIG Publication No.37 Page 16 Quote• For example, the UK co-tidal model has a quoted accuracy of +/- 0.5
m in the vertical for the ranges and therefore any separation model developed using these can be no more accurate than this.
• This figure is actually the 2 sigma figure although unstated. It is based purely on an estimate and has no statistical basis.
• As a North Sea user of the co-tidal chart for 35 years the figure does not match my practical experience.
• Observed offshore tides match predictions very well in calm and settled conditions.
• My estimate with statistical evidence is better than +/- 0.1 metres (one sigma) in the North Sea and West of Shetland
• The unpredictable part is the daily and seasonal meteorological surge effects induced by wind and pressure systems and even the variations in temperature and salinity.
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CONCLUSION• With today’s enhanced GPS we at last have a 3-D measurement
system capable of providing high accuracy, quickly and cost effectively.
• Our improving knowledge of the geoid means we are now “approaching” the true figure of the earth both locally and globally
• There are still key areas where gravity and altimetry data is missing or of poor quality.
• The perpetuation of historical heighting errors is confusing, inefficient, ineffective, and extremely wasteful.
• A broader, more measured, and balanced solution is required
There are very real dangers in jumping to the wrong conclusions too quickly.
New technologies take time to fully develop and mature.
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RECOMENDATION• There is a pressing need to maintain and
perpetuate the Hydrographic tradition of a collective approach to real quality and genuine accuracy compatible with the most modern measurement technology.
• Well considered collective solutions are more cost effective to implement and maintain and reduce the need for future change.
• At a time of dramatic technological developments the need to consult broadly and look outward rather than inward is key.
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FINAL QUESTIONS?
• Do scientists make good surveyors?or
• Do surveyors make good scientists?or
• What is Hydrography ?
Mathematics and Measurement Sciences
