What is a tide gauge?
The tide gauge is a recording device that measures the sea (or river) level at a specific location over a defined period of time. The principle is simple, but the recording and averaging of a large amount of data over a defined period of time was a technological feat at the time. For the tide gauge not only records the data (by plotting on paper like a barograph), but is also capable of calculating the average height of water. It has a sort of mechanical calculator built into the cylinder.
For this purpose, the tide gauge is connected by a cable to a float which is located underneath in a vertical tunnel. It is the position of this float that indicates the height of water.
The tide gauge of Marseille
It is a small discreet building located on the Kennedy cornice, in the southern bay of the Phocaean city. This location was chosen because the Mediterranean does not have many tides and it was easy by the Rhone valley to raise the altitudes all over France. Tide gauge measurements began in Marseille in February 1885. After twelve years of observing sea level variations, zero altitude was determined.
To materialize it, a physical point called the "fundamental marker" was sealed in the tide gauge premises at 1.661 m above the chosen zero.
This average level has been adopted as the French reference zero altitude, and adopted by SHOM for the calibration of our nautical charts and by IGN for all terrestrial charts.
What is the purpose of this Zero Hydrographic Point?
It is of immediate operational interest for :
- Hydrography (tide prediction, nautical charts, navigational safety, improvement of tide models...)
- Environmental applications (prevention plans for foreseeable natural hazards, storm surge and tsunami warning systems, validation of climate models, etc.)
- Calibration of satellite-borne radar altimeter measurements, evaluation and validation of space altimetry results
It is also used for longer-term studies:
- Understanding the processes that generate variations in mean sea level (tectonics, subsidence, hydrodynamics, etc.)
- Study of the influence of the tide on coastal ecosystems
- Study of the secular evolution of mean sea level
At the international level, for example, it is integrated into the GLOSS (Global Sea Level Observing System) programme and provides data to the PSMSL (Permanent Service for Mean Sea Level).
Is the sea level rising?
Yes, thanks to the cross-checking of data from different French tide gauges, it is possible to estimate the rise in the mean sea level in Marseilles since the end of the 19th century at 16 cm. This levelling value is consistent with that given by the GLOSS curve obtained with tide gauge measurements collected in other countries.
