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

Sea Level Measurement and InterpretationAPPENDIX I.GLOSS Requirements for Tide Gauges(update from the third edition of the manual)The aim of any tide gauge recording should be to operatea gauge which is accurate to better than 1 cm atall times; i.e. in all conditions of tide, waves, currents,weather etc. This requires dedicated attention to gaugemaintenance and data quality control. In brief, the majorrequirements for GLOSS stations are (IOC, 1997):• A sampling of sea level, averaged over a period longenough to avoid aliasing from waves, at intervals oftypically 6 or 15 minutes, or even 1 minute or less ifthe instrument is to be used also for tsunami warning(IOC 1997 states: ‘but in all circumstances theminimum sampling interval should be one hour’,which these days is an insufficient sampling formost agencies);• Gauge timing be compatible with level accuracy,which means a timing accuracy better than oneminute (and in practice, to seconds or better, withelectronic gauges);• Measurements must be made relative to a fixed andpermanent local tide gauge bench mark (TGBM).This should be connected to a number of auxiliarymarks to guard against its movement or destruction.Connections between the TGBM and thegauge zero should be made to an accuracy of a fewmillimetres at regular intervals (e.g. annually);• GLOSS gauges to be used for studies of long termtrends, ocean circulation and altimeter calibrationneed to be equipped with GPS receivers (andmonitored possible by other geodetic techniques)located as close to the gauge as possible;• The readings of individual sea levels should bemade with a target accuracy of 10 mm;• Gauge sites should, if possible, be equipped forrecording tsunami signals, implying that the site beequipped with a pressure sensor capable of 15-secondsor 1-minute sampling frequency, and possiblyfor recording wave conditions, implying 1-secondsampling frequency;• Gauge sites should be also equipped for automaticdata transmission to data centres by means of satellite,Internet etc., in addition to recording datalocally on site;• Sea level measurements should be accompanied byobservations of atmospheric pressure, and if possiblewinds and other environmental parameters,which are of direct relevance to the sea level dataanalysis.Regular (e.g. daily) inspection of data will informoperators when a gauge is malfunctioning, andlead to overall better long-term data sets. Datafrom gauges in polar or other remote locations willinevitably be inspected less frequently, unless satellitedata transmission can be installed. Similarly, datafrom the relatively few gauges recording only onpaper charts will be slow to reach centres for qualitycontrol; these must be considered priorities forupgrading to meet modern standards.Operators of gauges must always be aware of possiblesystematic jumps in sea level time-series whenone form of recording is replaced by a ‘better’one. All gauges have systematic errors, but thoseerrors will be irrelevant for time-series work if thesame technique is used throughout. New-technologygauges are, by definition, less well understoodthan old ones, and they must always be operatedalongside the older ones until sufficient experiencehas been acquired.52IOC <strong>Manual</strong>s and Guides No 14 vol IV