The tide gauge observations are heights of sea level expressed with respect to an arbitrary (conventional) and local datum. Hence, the term relative sea level is often used (relative to the land upon which the tide gauge is grounded).
The sea level data available on SONEL are:
daily mean sea levels obtained from different tidal filters (e.g., Doodson, Demerliac);
monthly and annual mean sea levels, which contribute to the world data bank of the PSMSL.
Other kinds of water levels are available too, directly from our partner, the SHOM (REFMAR), whatever the French tide gauge network (RONIM, ROSAME... See news of 16/02/2011). These are:
hourly water levels, available 2-3 months after a thorough quality control;
Higher frequency water levels, available in real-time or near real-time.
Note : SONEL also provides data (meta-data and links to the data) for tide gauges all over the world as long as they are co-located with permanent geodetic GPS stations (white symbols on the map above). Indeed, for this geodetic aspect (continuous GPS at tide gauges) SONEL has been asked to act as a dedicated data assembly center for the GLOSS program from 2011 onwards (the next meeting of the GLOSS group of experts is planned for autumn 2011).
The files of average values
Only the daily, monthly and annual averages calculated from quality controlled (QC’ed) data are provided in the data files. However, the graphs show the averages calculated from QC’ed observations (in blue) and non-QC’ed observations (in red). The averages are calculated from the Demerliac filter and the Doodson filter, with the possibility of a choice for the user. An example is given on the image below, which is taken from the web page of the La Rochelle station.
To obtain further details:
on the calculation of mean sea levels and the filters;
on the quality controls carried out by SHOM.
The files of mean values can be downloaded individually from the SONEL Web page of the tide gauge, without authentication, by clicking on the button situated under the graph of the means chosen (filter, data sampling and average type: daily, monthly, annual). An example is given on the image opposite, which is taken from the web page of the La Rochelle station.
However, they are also available on the FTP server (ftp.sonel.org) which is very practical when rapid acces to the data from several stations is required. The FileZilla tool will give you access to the data.
The format adopted is that of the PSMSL, as are the calculation criteria for the monthly and annual means, in particular for the issue of observation gaps. The data files are presented in the form of a two-column matrix.
The dates are in decimal years (centered on the 15th day of the month for the monthly values and at noon for the daily values).
The daily, monthly and annual sea-level heights are expressed in millimetres.
This is how the names of the data files are defined:
The first letter in lower case :
"d" for the daily means, "m" for the monthly means and "y" for the yearly means
The acronym of the station, in capital letters, is made up of 5 letters (the internal identifier of the SONEL data base)
The extension ".slv", for the type of quantity "sea level".
Example: the daily means file of La Rochelle is called dLROCH.slv
Structure of the file system in the FTP server
|-- Demerliac => Name of the filter used
| |-- data -> Files of mean values
| `-- graphs -> Graphs displayed on the Web site
`-- Doodson => Name of the filter used
|-- data -> Files of mean values
`-- graphs -> Graphs displayed on the Web site
France is very rich in systematic observations of coastal sea level over long time periods. The oldest (published) series of measurements were performed by the astronomers La Hire and Picard at Brest, France, in 1679, over about ten days. They repeated the experiment in 1692, over several months, again in Brest. Their aim was to study the tides and their results showed the fundamental importance of observation to understand and determine the characteristics of tides in a given place for the purposes of prediction. As early as 1700, sea level observatories sprung up along the coast of France with the help in particular of the Académie des Sciences. The invention of an automatic mechanical recording device is attributed to Henry Palmer in 1831. In France, the French hydrographic engineer R. Chazallon (1802-1872) devised a mechanical instrument in the 1840s (1st installation in 1843 in Algiers), giving it the name of marégraphe. The geographical engineers also distinguished themselves in this field, particularly with the invention of the medimarametre at the end of the 19th century by Ch. Lallemand (1857-1938), and the development of a method to test the accuracy of mechanical tide gauges by C. van de Casteele (1903-1977), a method which bears his name in volume I of the Manual on sea level measurements and interpretation, Unesco (IOC 1985).