Zero hydrographic
One calls zero hydrographic , or zero of the charts the reference of level for measurements of depth at sea (or datum-line of the Sonde S) on a Sea chart.
On the French charts, the zero hydrographic one is the level theoretically reached by the low tides (Marée of coefficient 120 - or extreme astronomical low tide; in English LAT: Lower Astronomical Tide). It is thus about the leaves low tides. It is obtained by calculation and can be very exceptionally exceeded (for example by anticyclonic situation and blowing ground wind). This choice thus almost always guarantees a depth higher than the depth indicated on the chart.
Is this convention now universally (?) adopted. That was not always the case. Until a certain time, on the English charts, the zero were close the height of low-sea of average spring tide (coefficient 95). The real depth could thus be, in period of spring tides, lower than that of the chart.
The zero of the sea charts are different from the zero of the terrestrial charts, reference level of the Altitude S. In France, the charts of IGN have as a reference the mean level of the Mediterranean measured by the marigraph of Marseilles.
See too
- altimetric Sea level
- Referent
External bonds
- Zero hydrographic and marine altimetric references in France (SHOM)
- Zero Hydrographic: towards a total determination
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