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Sea Level Measurement and Interpretationor more stations. Levelling connections within thesenetworks then allow the heights of objects (e.g. mountains)to be related to MSL at the coast. For example,the UK national levelling network expresses heights interms of ‘Ordnance Datum Newlyn’ (ODN), which wasthe average level of the sea at Newlyn in southwestEngland during 1915–21. ODN can be thought of as animaginary datum plane extending over a large area (i.e.over the whole of Great Britain). The heights of benchmarks, for example, can be expressed in terms of ODNas can, therefore, the Chart Datum at the port.The concept of a national levelling network has undergonerevolutionary change during the last decade, primarilydue to the advent of GPS. However, it was alreadya defective concept from the point of view of sea levelstudies, for several reasons. First, sea level has risen atNewlyn since 1915, as it has done at many other placesaround the world, so ODN no longer represents thepresent average Newlyn levels. Second, the mean seasurface around a coast is not ‘flat’, i.e. it does not followthe geoid, but varies due to ocean currents, density differences,meteorological effects etc. Consequently, MSLwas never a perfect choice for a national datum plane.Third, rates of change of MSL are different at differentlocations, thereby complicating the time-dependenceof the network. Fourth, all national levelling networks(with the possible exception of that of The Netherlands,Finland and Sweden) contain multi-decimetric errors dueto systematic, instrumental errors in the levelling. Fifth,as levelling networks tended to be redefined at intervals,their redefinition in itself was a potential source of error,as ‘heights’ were redefined.Consequently, while interaction between sea level specialistsand national surveyors is inevitable, we advisemost sea level specialists to take great care with theconcept of a national levelling system.4.1.7 Chart DatumThe chart datum (or Admiralty Chart Datum in the UK)is the low-water plane below which the depths on anautical chart are measured and above which, tidal levelsare often presented for practical purposes, such astide tables for harbour operations. The chart datum isa horizontal plane over a limited area and the elevationof this plane will vary around the coastline, dependingon the tidal ranges at the places considered. In theUK, the chart datum at a port is the same as ‘LowestAstronomical Tide’ (Pugh, 1987).4.1.8 Working DatumsPractical working datums are often used in portswhere they describe sea level (or water depth) moreclearly than perhaps a scientifically rigorous referenceto a benchmark. Examples of such datums includethe levels of the sill of a lock or a shallow point in theentrance to a harbour. The sea level from a tide gaugethen indicates the depth of water above these hazards.Working datums often functioned as the first TGBMsfor Europe’s sea level records (e.g. the ‘Old Dock Sill’datum at Liverpool).4.2 Levelling Between LocalBenchmarksHigh-precision levelling will need to be made betweenall the marks of the local network at regular intervals.For GLOSS purposes, the recommendation is that theexercise be repeated at least annually, with results fullydocumented by the responsible agency. The exact frequencyof required levelling will depend on the geologyof the area. On unstable ground, more frequent levellingmay be necessary.Personnel familiar with the best practices of the techniqueshould perform levelling with a good-quality leveland staff. For example, if marks are far apart, it will benecessary to establish ‘staging points’ clearly identifiedand about 50 m apart on a hard surface. This can bedone by painting a small ring around the point and,on softer surfaces, by driving in a round-headed pin.The levelling instrument can then be set up between abenchmark and the first staging point and readings ofthe staff taken at the two positions. This is then repeatedthroughout the whole network. It is important thatthe pairs of readings be taken in the correct sequence,otherwise an erroneous height difference will result.Modern levelling instruments with built-in data loggerscan remove most of the tedious arithmetic associatedwith the use of a simple level.As with many other aspects of tide gauge operations,the main principle of levelling is that ‘practice makesperfect’. For advice on good levelling methods, thePSMSL website (www.pol.ac.uk/psmsl) contains a set ofnotes used by Prof. Charles Merry at the University ofCape Town GLOSS Training Course in 1998.4.3 Levelling Between Wider AreaMarksThe height of the TGBM should also be related to awider area network extending typically 10 km. Thisprovides a verification of whether the sea level measuredrelative to the TGBM is consistent with that ofthe surrounding area.First-order geodetic levelling is accurate to 1 or 2 mmover distances of a few kilometres and, therefore,annual campaigns can detect any vertical movementsof the TGBM with respect to the local benchmarks.30IOC <strong>Manual</strong>s and Guides No 14 vol IV
Sea Level Measurement and InterpretationLevelling over longer distances has been found to containsignificant systematic errors that can cause apparentspurious changes in the height of the TGBM. Forthis reason the PSMSL requires MSL data to be definedwith respect to the TGBM rather than with respect tonational datum levels.Consequently, while it is desirable in principle to performregular wide area levelling, their accuracy hasalways to be considered, especially as the areas consideredbecome wider. At a distance of some 10 km, theerrors involved in levelling become comparable to thoseachievable by means of space geodetic techniques.Therefore, while the choice of technology for the widerarea surveys is clearly evolving, the principle that therelative sea level measurements provided by the gaugedata are applicable to studies for the surroundingarea is still valid. Table 4.1 summarizes the accuracyobtained by the different techniques.Table 4.1 Accuracy of geodetic fixing of TGBMs.TechniqueAccuracy4.4 Geodetic Fixing of Tide GaugeBenchmarks4.4.1 IntroductionOver the past decade, advances in modern geodetictechniques have provided new methods for fixing tidegauge bench marks. These are the techniques of spacegeodesy, using the satellites of the Global PositioningSystem (GPS) and those of the Doppler Orbitography andRadiopositioning Integrated by Satellite (DORIS) system.Absolute gravity measurements provide collateral evidenceof vertical crustal movements. The space geodesymeasurements can be used to fix into a geocentric referenceframe the GPSBM, which should be connected tothe TGBM by levelling. Therefore, the MSL at the tidegauge can be defined in a global geocentric referenceframe. This furnishes an absolute measure of mean sealevel, rather than MSL relative to each local TGBM, oreven to the wider surrounding area. Measurements of sealevel are then defined in the same geocentric referenceframe as that used for satellite altimetry and can thereforebe directly compared with altimetric sea levels.Primary Levelling of LocalBenchmarksGPS from TGBM to SLR/VLBIReference FrameAbsolute Gravity near TideGauges and at SLR/VLBIStation0–1 km:
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