Manual on sea level measurement and ... - unesdoc - Unesco

Sea Level Measurement and Interpretationof course, it may provide useful ancillary information. Itis important that the benchmarks be clearly identified,by the inscription of a name or number. In addition,they should be unambiguously documented in the tidegauge metadata, with a description of the mark itself,photographs, national grid reference and a local map.4.1.1 Tide Gauge Benchmark (TGBM)The tide gauge benchmark (TGBM) is chosen asthe main bench mark for the gauge from the set ofapproximately five marks described above. The TGBMis extremely important, since it serves as the datum towhich the values of sea level are referred. The choiceof TGBM is somewhat subjective; in principle, it shouldbe the ‘most stable’ or ‘most secure’ mark of the set,although, if the area is largely stable, then the choiceshould be fairly arbitrary. Often the nearest mark to thegauge is chosen. Over a period of time it may be necessaryto redefine the TGBM, if the original is destroyedas a result of local development. The benefit of havinga set of five local marks, regularly interconnected byhigh-precision levelling, is that it allows a new TGBMto be defined in terms of the old one, if circumstancesrequire it.In some countries the historical practice has beennot to define one mark as the TGBM, but to use aweighted average of several marks. For GLOSS, it isrecommended that the single, unique TGBM approachbe adopted as the standard.4.1.2 GPS Benchmark (GPSBM)The GPS benchmark (GPSBM) is another special markof the available set that is the reference mark for GPSmeasurements near the gauge. In some busy ports, theGPSBM may be several hundred metres from the TGBMand the gauge. As with the other marks, it must beconnected by high-precision levelling to the TGBM atregular intervals. (See section 4.4.1 for details on GPSmeasurements at tide gauges).4.1.3 Gauge Contact Point (CP)The contact point (CP) of a tide gauge is a type of‘benchmark’, or vertical reference mark, associatedwith the gauge itself. After a geodetic connection hasbeen made between the TGBM and the CP, the gauge’ssea level data can be expressed in terms of the TGBMdatum. The essential point to note is that the CP comeswith the gauge; if a different type of gauge is installedat the site, it will have a different CP which will requirere-levelling to the TGBM.For conventional float and stilling well gauges, theCP is often located at the top of the well inside thetide gauge hut. Sometimes, in older stations, the CPis located in a most difficult and inaccessible locationfor levelling purposes and new stations should takecare to provide ready access. For acoustic gauges withsounding tubes, the CP is located at a point at the topof the gauge on the container holding the acoustictransducer. Similarly, for radar gauges, the CP will bea mark on the transducer. For ‘B’ gauges, the CP willbe at the top of the vertical supporting tube which isknown relative to the ‘B’ datum level.In the case of float gauges located in a tide gauge hut,the CP should not be used as the TGBM itself, as it isalways possible for the building and the well to graduallysettle over a long period. With a good set of localbenchmarks, this settling will be evident by check levellingbetween TGBM and CP.4.1.4 Tide Gauge Zero (TGZ)The tide gauge zero (TGZ) is the level for which thegauge would record zero sea level. In practice, the sealevel may not fall to this level. In a conventional floatgauge arrangement, the TGZ can be related to the CPafter dipping checks in the well have been performed.This is done using a calibrated tape set to zero at theCP. Measurements are made by lowering the tape untilit reaches the water and an electrical circuit is completed.The level of sea water in the well can then berelated to the CP and to all other local datums.4.1.5 Revised Local Reference (RLR) DatumThe revised local reference (RLR) datum at a gauge siteis a datum defined as a simple offset from the TGBM,such that values of sea level expressed relative to the RLRdatum have numerical values around 7,000 mm. Theconcept of the RLR datum was invented by the PSMSLso that long time-series of sea level change at a sitecould be constructed, even if parts of the time-series hadbeen collected using different gauges and different, butgeodetically connected, TGBMs. The approximate valueof 7,000 mm was chosen so that the computers of thetime (the late 1960s) would not have to store negativenumbers. The RLR datum is defined for each gauge siteseparately and the RLR at one site cannot be related tothe RLR at any other site, without additional knowledgeof connections between TGBMs at the different sites.When sea level data are contributed to the PSMSL, orto a sea level centre, it is essential that full informationon the geodetic relationships between TGBM and TGZetc. accompany the data. Without this information, itis impossible for the PSMSL to include such data in theRLR data set.4.1.6 National Levelling NetworkMost countries have, during the last one hundredyears, implemented national levelling networks that aredefined usually in terms of mean sea level (MSL) at oneIOC <strong>Manual</strong>s and Guides No 14 vol IV29