Towards a Globally Consistent Nautical Chart Datum Definition in France

Abstract

The international Hydrographic Organisation recommends that Lowest Astronomical Tide (LAT) be adopted as the international Chart Datum. LAT is defined as the lowest tide level, which can be predicted in average meteorological situation and in any combination of astronomical conditions. Many hydrographic offices, like SHOM in France, have adopted this recommendation up to now. After a quick review of current methods based on tide predictions and observations in use to locate and recover chart datum, we will focus on modern techniques based on space geodesy and numerical modelling. A new approach to fix chart datum and rectify bathymetric surveys depth measurements will be presented. It is based on modern space techniques, including GPS, lowest astronomical tide calculation and mean sea level ellipsoidal heights. This subsequent chart datum can then be considered as heights related to a well-defined and maintained global geodetic reference frame like ITRF.