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

Sea Level Measurement and InterpretationFor this timing correction, the reference was of coursethe Geónica sea level series, as it was the only one withGPS assignment of time during the experiment. Table1 shows the RMS errors of comparisons for each pairof sensors after adjustment for time. These range from0.7 cm (corresponding to Geónica and Seba, whichis reasonable, since they use the same type of radarsensor) and 1.9 cm (Aquatrak and Miros, mainly dueto an effect of temperature on the Aquatrak beinghigher than expected and not corrected). These valuesdecreased by about 10% when repeated with the filteredhourly values. Following Woodworth and Smith(2003), who state that RMS values below 1.4 cm forthe differences yield a precision better than 1 cm foreach sensor, most of the tide gauges would meet thiscondition (GLOSS standard) for 5-min data, and all ofthem would do so for hourly data.By computing the slope of the linear regression for eachpair of sensors, an idea of the sensitivity of the sensorto the tidal range was obtained. The most interestingconclusion in this context is that the bubbler gauge fromPOL was the one that recorded lower ranges (around0.5%). The explanation for this is that it has operatedassuming a constant salinity, which may not be realisticfor Vilagarcía harbour, which is located in an estuary(ría). More details of this comparison can be found inthe technical report of the experiment or the publicationincluded in the reference (Martín et al., 2005).Spectral analysisTo get an idea of the differences among the time-seriesfor each frequency band, a spectral analysis was madefor one month of data from all the sensors, as well asfrom the 5-min time-series. Figure 3 illustrates thesedifferences, showing that all the sensors have exactlythe same response to tidal frequencies, with differencesonly beginning to show up for frequencies correspondingto 30 and 100 min, for which it is very clear thatthe Miros sensor presents more energy. This might beimportant for the study of oscillations of these periods,such as seiches; so we thought the company shouldinvestigate it, which they did and found that it was dueto a problem, now solved, in an internal interpolationalgorithm of the sensor.Of course, the larger differences are found for frequencieslarger than 0.05 cycles/minute, and this was not asurprise for us, being due to the different measuringtechnique and the fact that some of the sensors (acoustic)measure inside a tube and others not (radar). Onthe other hand, for the computation of the 5-min data,different amounts of raw data were used, dependingon the sensor. Of course, differences in this range offrequency may still be significant if one needs to detectseiches or tsunamis, since we are talking about periodsof between 10 and 20 min. That is why a more detailedstudy of the response of the different sensors at higherfrequencies is required.Figure 3. Power spectral density for the 5-min time-series(December 2003–January 2004).Comparison of hourly values and daily meansTo complete the study concerning standard sea levelapplications, a harmonic analysis for all the sensorswas carried out, following the standard procedureemployed by the Puertos del Estado (filtering to hourlyvalues, and use of Foreman tidal programme). Table 2shows the results for the main harmonics, which arevery similar (differences similar tothe analytical error).Again, the POL sensor shows the lowest M2 amplitude,consistent with what was said above.The daily means were computed and compared, andtrends during the period of the experiment werefound to be consistent for all sensors, in spite of thelimitations in the operation of the sensors and in thecalibration of the datum, an aspect which was not fullyconsidered at this test site.Table 2. Main tidal constituents for each of the sensors,for the period December 2003–June 2004.H (cm)Tide AQU GEO MIR POL RAD SEB SRDO1 6.3 6.4 6.3 6.3 6.4 6.3 6.3K1 6.6 7.3 7.3 7.2 7.4 7.3 7.4N2 23.4 23.3 23.4 23.2 23.5 23.6 23.7M2 114.3 114.2 114.4 113.8 114.2 114.2 114.6S2 40.9 40.9 41.1 40.8 41.0 41.0 40.7G (deg)Tide AQU GEO MIR POL RAD SEB SRDO1 71.8 73.5 71.1 70.5 71.3 71.0 71.7K1 48.0 53.1 54.0 51.8 51.3 52.7 60.0N2 66.9 68.1 66.9 67.0 67.9 66.9 64.5M2 80.0 80.5 79.9 80.0 80.5 79.9 79.3S2 104.9 105.1 105.2 105.3 105.8 105.2 104.9IOC <strong>Manual</strong>s and Guides No 14 vol IV63