Uncertainty and bias in electronic tide-gauge records: Evidence from collocated sensors

Abstract

Understanding noise and possible bias in tide-gauge sensors is important for determining the mean sea level, its fluctuations and their climatic, geophysical and engineering implications, but not an easy task. In the past, this problem has been examined through comparison of different sensors in the laboratory, or through correlations of neighbouring sensors. In this study we identified and studied 10 cases of harbours with fully collocated sensors. Transient differences were found between collocated records. Pressure gauges were found significantly more sensitive to noise than radar-type sensors, and with higher chances of long-term transient bias. The amplitude of the observed bias is important, of the same order of magnitude with tsunami waves in the open sea and with seismic ground displacements. Only 9% of the sensors analysed were found to satisfy the 1 cm accuracy criterion imposed by the Permanent Service for Mean Sea Level (PSMSL).

Assessment of Tide Gauge Biases and Precision by the Combination of Multiple Collocated Time Series

“…Assuming that both random errors e i (t) and e re f (t) are uncorrelated, the term e i (t) − e re f (t) in equation (2) follows a centered normal distribution with an unknown variance σ 2 i + σ 2 re f . The merge of the random errors e i (t) and e re f (t) in the differences ∆ y i (t) imply that, without assumption, DIFF methods can only assess the variance σ 2 i + σ 2 re f , which is just an upper bound to the tested gauge variance σ 2 i (Lentz 1993;Míguez Martín et al 2008b;Pytharouli et al 2018). To separate σ 2 i and σ 2 re f , a piece of additional information is needed: a third time series.…”

“…This linear model can be adapted to other types of biases. For example, longer time series analysis (several days, months, or years) may require to consider time-dependent biases such as trends and jumps (Pytharouli et al 2018).…”

“…The approach (a), also known as buddy-checking, is routinely used during calibration campaigns where a pair of tide gauges are compared over a tidal cycle, sometimes with the help of the socalled Van de Casteele diagram (Lennon 1968;IOC 1985). During the last decade, several studies have investigated the performances of radar gauges, pressures gauges, GNSS buoys or GNSS reflectometry based on this approach (Watson et al 2008;Míguez Martín et al 2008a, 2012Pérez et al 2014;Larson et al 2017;Pytharouli et al 2018). Even if this approach can provide bias estimates and general accuracy metrics, such as mean error or root mean square error (RMSE), it cannot rigorously separate the uncertainties of each gauge.…”

Precise water level measurements using low-cost GNSS antenna arrays

“…A range of different instruments are commonly used for monitoring water levels with variable cost and accuracy (GLOSS, 2012;Míguez et al, 2005;Pytharouli et al, 2018). With a budget of approximately USD 1000-10 000, it is pos-Published by Copernicus Publications on behalf of the European Geosciences Union.…”

“…sible to buy an acoustic gauge or a pressure gauge to monitor water levels with sub-centimetre accuracy -the level of accuracy required for the Global Sea Level Observing System (GLOSS) network (GLOSS, 2012). However, pressure gauges may suffer from drift over multi-year timescales (Míguez et al, 2005;Pytharouli et al, 2018), and acoustic gauges are difficult to install in remote regions because they require a structure to hang over the water surface. Radar and bubbler gauges are also commonly used to monitor water levels (see Woodworth and Smith, 2003, for a comparison), but these instruments are more expensive than pressure transducers or acoustic gauges.…”