Ocean Surface Topography Science Team pour OST-ST.


The Long Term Trend Components in Sea Levels from Tide Gauge and Satellite Altimetry Records project, conducted jointly with the University of La Rochelle, is sponsored by LEGOS (Laboratory of Geophysical Studies and Spatial Oceanography) and is associated with the Ocean Surface Topography Science Team (OST-ST) research programme.


Project Focus


The project focuses on several areas:

  • Combined analysis of long term trends in sea levels obtained from tide gauge and altimetry records;
  • Implementation of sophisticated statistical tools for optimum extraction of the low frequency variation in signals with error estimation
  • Research into "historical" observations of sea levels, pegged to current measurements, in order to supply new secular records.

This project fits into the LTT (Long Term Trend) theme of  GLOSS  per the objectives of the  PSMSL.




A study estimated the error margins when comparing tide gauge levels with altimetric measurements. These errors are sought in the altimetric data and their geophysical corrections, in the tidal measurement, and also in the method of comparing the two types of data as illustrated below.

Erreur d'interpolation commise lors de la comparaison des données altimétriques avec les mesures marégraphiques

Interpolation error made when comparing altimetric data with tidal measurements. This is a good example based on the construction of a theoretical series from 8 main tide waves corresponding to tidal ranges of less than 100cm, 150cm, 300cm and 600cm. We represented the interpolation error at the time of the satellite passage based on the tide gauge sampling and amplitude of the tide in the port in question. a. (resp. b.): Linear interpolation (resp. cubic spline interpolation) of the heights from the theoretical tide series on the dates of the satellite passage based on sampling.



The example below shows that cubic spline interpolation (figure 1.b) is indispensable when comparing tide gauge sites that have high tidal ranges. We also show that for medium tidal ranges (around 3 metres), taking readings every 30 minutes instead of every hour reduces the difference almost by a factor of four. A site affected by a low tidal range where the sea level is measured at least once every 20 minutes reduces the introduced error for the comparison.


Questions and Answers


The study in progress compares altimetric and tidal trends at a certain number of sites. The locations chosen for the study are also fitted with DORIS and GPS measuring stations. These technologies help determine vertical movements of the Earth's crust. The questions we want to answer are the following: Are the trends observed on the DORIS and GPS stations consistent with the altimetry-tidal trend differences (see table below)? Does processing of the standard altimetric data by AVISO give the same trends as regional processing at LEGOS's CTOH (Centre of Topography of the Oceans and Hydrosphere)?


The table below shows the trends obtained from the tidal measurements (corrected tides using the MAS tidal harmonic analysis programme developed by SHOM) and those calculated from the MSLA produced by AVISO (correction of the tide as well as the reverse barometer).

endances calculées sur la période 1/1/1994-31/12/2008 pour les sites multi-paramètres (DORIS / GPS / MAREGRAPHE)


Trends calculated over the period 1/1/1994 to 31/12/2008 for multi-parameter sites (DORIS/GPS/MARIGRAPH) close to the TOPEX/POSEIDON/JASON/JASON2 altimetric read-outs. The trends obtained from the differences between in-situ measurements and altimetric data give the vertical displacement speeds of the earth's crust.

The above table shows high variability in both the correlation and the standard deviation of signals, with correlation values of -0.1 to +0.9 and RMS values between 2.63cm and 5.97cm. The trends calculated from the two types of observations at each site show relative agreement. One objective of this project is to explain the observed differences for each site in order to enable a more robust estimate of sea levels.





Last updated: 12/12/2012

List of projects based on tide gauging:

List of programs, projects and reconstructions of tide gauge data