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

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Sea Level Measurement and InterpretationChilean Sea Level NetworkJuan Fierro, Chilean Navy Hydrographic and Oceanographic Service (SHOA)E-mail: jfiero@shoa.clIn 1941, the Chilean Navy Hydrographic andOceanographic Service (SHOA) initiated the establishmentof the national tide gauge network with theobjective of starting a systematic and permanentrecording of the sea surface level along the coast.Figure 2. Sea level station configuration.obtained has been of great value to the national andinternational scientific communities.Figure 1. Chilean sea level network.At the end of 1950, the network consisted of five tidegauge stations, and the experience gained during itsdeployment and operation was extremely important fora successful extension of the network.The great length of the Chilean coast and the accessibilityproblems in some areas have been the mainobstacles to extending the network in the short term.However, at the end of 1998, the sea level networkcomprised 19 permanent tide gauge stations, locatedon the mainland as well as in some islands and inAntarctica. Considered as a system, the operationof this network has allowed SHOA to provide usefulinformation to mariners sailing in Chilean waters andto contribute to engineering projects associated withcoastal-zone management. Moreover, the informationThe upgrading initiated in March 1999 considered thedeployment of 17 HANDAR model 555C data-acquisitionplatforms, as shown in Figure 1. The new platforms,with satellite data-transmission capability, replaced theold recording tide gauges, whose operation was basedon nitrogen gas pressure. The present tide gauge station’scharacteristics are shown in Figure 2.For the comparison of old and new gauges, six monthsof overlapping (concurrent) operation were considered.During this time, both systems operated in parallel andintercomparisons were performed. Sea level data andancillary data collected by the sea level network weretransmitted via the GOES satellite system. Incoming sealevel data were validated using a quality-control packageprovided by the University of Hawaii. Plots of newcomputations of yearly residuals showed fewer errorsin comparison with previous findings.The standard configuration of every sea level stationincluded a water-column pressure sensor, an atmo-72IOC <strong>Manual</strong>s and Guides No 14 vol IV
Sea Level Measurement and Interpretationspheric pressure sensor, and seawater temperature andair temperature sensors. Recently, an ultrasonic windsensor was added to some stations on the northernpart of the Chilean coast.Sea LevelSensorGPSAntennaAir TempSensorSolar pannelWaterTemperature SensorInternalCircuitryGPSConnectionFigure 4. Submerged sensors.• High cost compared to simpler systems• Requires continuous maintenance for the DRUCKsensor• Requires levelling campaigns on a regular basis toensure accurate data• Modem communication with each DCP is not reliableFigure 3. Sea level station components.BatterryThe sea level sensor used is a submersible pressuretransducer, with an operating range from 0 to 70g/cm 2 (equivalent to 13.7 m) of water column. Thesignal conductor cable holds a ventilation tube thateliminates the atmospheric pressure effect over thewater column.During the first semester of 2005, SHOA has performedfurther upgrades to its network, by installing a VaisalaDirect Readout Ground Station (DRGS) at SHOA’s headquartersat Valparaíso (Figure 5). This DRGS receivercollects information from our remote data collectionplatforms (DCP). The DRGS is fully user-configurablethrough the Data Management System (DMS) using aThe sea level height is recorded at two-minute intervalsand the other environmental parameters once everyhour. The main components of the DCP are shown inFigures 3 and 4.The development of this modern data collection platform(DCP) network has several benefits which may besummarized as:• Flexible configuration• Easy installation, adaptable to different structureson the field• Long autonomy• Powerful data acquisition software• Better accuracy: 1 cm with DRUCK sensor complieswith GLOSS requirementsNevertheless, according to the experience accumulatedby SHOA, this system has a few drawbacks, summarizedas follows:Figure 5. DRGS antenna at SHOA headquarters.IOC <strong>Manual</strong>s and Guides No 14 vol IV73
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