Relative sea level trends: The relative sea level trends have been estimated by the PSMSL from the RLR tide gauge dataset of annual times series longer than 30 years with more than 70% of valid data. Only results from tide gauges for which a robust GPS velocity is available are displayed. (More about the PSMSL relative sea level trends).
Absolute sea level trends: The absolute (geocentric) sea level trends build upon the above-mentioned relative sea level trends by simply adding the vertical velocity of a nearby GPS station to correct for the vertical movement of the land upon which the tide gauge is grounded.
The vertical velocities used here come from the last GPS solution processed at the University of La Rochelle (the ULR5 solution). This solution is expressed in the latest realization of the International Terrestrial Reference Frame (ITRF), namely the ITRF2008. (More about the ULR5 solution).
As soon as other GPS solutions using the state-of-the-art data analysis strategies, as recommended by the IGS, are made available to SONEL with the same level of accuracy, they will be considered and proposed to the users.
Two working hypotheses are necessary when using GPS data to correct vertical land movements in tide gauge records. The first requires that the linear vertical land movement estimated from the GPS data is consistent over the multi-decadal to century timescale of the tide gauge record. The second hypothesis requires that the land motion detected by the GPS antenna is consistent with that affecting the tide gauge at the level of a few tenths of a millimetre per year. Both are necessary working hypotheses, which have been discussed extensively in the literature. (See for instance Santamaria-Gomez et al. (2012)).
By choosing to display the sea level trends that are estimated over the whole time series, trends from different time periods could be displayed on the same map.