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

Sea Level Measurement and Interpretationunder test. The manufacturer could not solve the problemand a method of post-data-processing was devisedto improve the absolute accuracy of the data back tothe claimed 0.05%. This post-data-processing couldhowever only be carried out once all gauges in thenetwork had been calibrated in situ to establish theirindividual calibration factors.A refined method for in situ calibration was devised asa quality control tool. The method allows the followingto be reliably established on site:• Absolute accuracy• Measurement repeatability• Instrument datum offset.All gauges in the SAN tide gauge network are nowcalibrated every six months, using carbon graphitepoles of known length and a stainless steel target thatis suspended below the gauge.Data received from the gauge are very ‘spiked’. Thestability of the readings is also erratic. This spiking inthe data creates a problem when the time arrives forthe annual tidal prediction run. The spikes have to beedited out of the data by hand – each day’s data haveto be manually plotted, checked against the graphicsproduced by the Tech Tidal Assistant and then editedinto the analysis programme, before predictions can becalculated. This is a very unscientific, time-consumingprocess and human error comes into play.The quality of the lightning protection within the unitis not up to standard. A perfect example of this is thegauge in East London. It was struck by lightning andthis caused a fire in the gauge. Since the HydrographicOffice had placed the instrument box inside a watertightmetal box, the fire burnt itself out, owing to lackof oxygen. It would appear that the data logger wasnot damaged in the lightning strike/fire. The powersupply, junction-box telephone line and modem weredamaged.What the tidal department calls a ‘kick-start’ is thesolution to the problem of periodic unwillingness todownload data. Periodically, the power supply has tobe disconnected from the gauge, followed by a waitof 30 seconds and then restoration of the power supply.This problem is becoming more and more frequentand the down-time that it is creating in data analysis isbecoming problematical.In the last five years it has become evident that thetransducers are beginning to rust and this is gettingprogressively worse. The HO is unsure whether the rustis affecting the quality of the data; however, this is apossibility, as the transducers that have little or no rustare not creating as many problems.Kalesto radar gaugeThe OTT Kalesto radar gauge was tested by the TidalDepartment and under calibration from 12 to 15 April2002. After analysis of 1,443 readings (with a mean of2.4955 m, say 2.496 m) it was found that, in general:• 81% of the readings were within 2 mm of theabove-stated mean• 93% of the readings were within 3 mm of thismean• 97% of the readings were within 5 mm of thismean.An independent study to check-calibrate the Kalestoradar tide gauge was carried out by IMT on request bythe SAN Tidal Superintendent. The study showed that,during the calibration period, the Kalesto performedconsistently within the manufacturer’s claimed accuracyparameters over the 2–7 m range. The absoluteaccuracy of the gauge under test had a standard deviationof better than 3 mm over the 2–7 m range. Theindependent study confirmed the results achieved bythe SAN Hydrographic Office.The refined method for in situ calibration devised forthe SRD tide gauge as a quality control tool is used tocalibrate the Kalesto gauge every six months.The data received from the gauge have very little ‘spiking’in the graphics; this is due to the 17 s measuringinterval. The low spike density is evident in the graphicsand in the long run there will be very little editing ofthe data before the predictions can be produced, thusimproving on the accuracy of the Hydrographic Office’spredictions. The quality of data being sent to theGLOSS Fast Centre has also increased, not only in qualitybut also in frequency of data transfer. (Fast Data)This gauge is factory-fitted with an integrated lightningprotector to reduce the possibility of damage caused byexcess voltage (e.g. lightning or power surges.)Future prospectsThe SAN Hydrographic Office intends to upgradeits entire tide gauge network with the Kalesto radartide gauges. It is proposed to install a GPS receiverat Durban. It is also proposed to install a tide gaugeand GPS receiver on Marion Island, in connection withHARTRAO, thus restoring the South African GLOSS stationstatus to its original 100% capability.Currently, ‘fast data’ are being sent to the GLOSS FastCentre, at the University of Hawaii, from the Simon’sTown, Richards Bay, Port Nolloth, Port Elizabeth andEast London gauges. The remaining RSA stations willbe included in the fast-data streams once they havebeen upgraded to the Kalesto radar gauge.68IOC <strong>Manual</strong>s and Guides No 14 vol IV