Mapping the site

With the support of the Maritime Subdivision of the Saint-Malo Infrastructure Office and the Infomar firm from Saint-Vaast-la-Hougue, mapping of the zone of the two wrecks began in 2001 and continued through 2003. Using a sounder installed on La Traversaine – the vessel belonging to the Lighthouses and Beacons Office – and its automatic compensator providing real-time correction for fluctuations in sea-level, the team from the Maritime Subdivision created a particularly accurate survey of the entire archaeological zone. They decided on a one-metre-square map size, and mapped 92 squares, taking readings every thirtieth of a second. The resulting document contained every fluctuation in the level of the seabed at the site.
The involvement and availability of Subdivision staff also provided an opportunity to test new intervention processes in order to position the wreck site on the map of the Bay of Saint-Malo. An ingenious system was devised: weighted targets were precisely positioned with Differential GPS as they were dropped overboard; these were then triangulated on the ocean floor with respect to set points of the underwater site. The gridded site could be inserted, for the first time, in the general map of the bay and with respect to the surrounding rocks. The margin of error of this method – much lower than the usual technique of using weighted floats connected to the points to be coordinated and then mapped on the surface at low tide – was about one metre, or about the same as the one traditionally associated with DGPS.

Multimedia

Teddy Seguin (Adramar) © MCC / DRASSM
Weighted markers created by the maritime subdivision of the Ille-et-Vilaine Infrastructre Directorate. (Ref. : NAT02_M0056)
Teddy Seguin (Adramar) © MCC / DRASSM
Weighted markers being put in place by the maritime subdivision of the Ille-et-Vilaine Infrastructre Directorate. (Ref. : NAT02_M0057)
Teddy Seguin (Adramar) © MCC / DRASSM
The Traversaine, belonging to the maritime subdivision of the Ille-et-Vilaine Infrastructre Directorate, was used for the bathymetry of the Natière site. (Ref. : NAT02_M0063)
Erwan Froger (DDE 35) © MCC / DRASSM
Bathymetry of the Natière site carried out by Erwan Froger (Saint-Malo maritime subdivision of the Ille-et-Vilaine Infrastructre Directorate). (Ref. : NAT02_I0004)