The Hydrography Unit is responsible for supervising dredging operations and more generally for monitoring the condition of the channel and trenches. It guards against any changes, such as the accumulation of silt, following the natural depositing of sediments by the river or resulting from dredging operations.

Its principal activity is the production of hydrographic plans. Bathymetric surveys (depth readings obtained using an ultrasonic depth finder) are conducted by the hydrographers.

Three hydrographic survey vessels (the Guifette, the Octant and the Mercator), one small craft and items of data acquisition equipment (radioelectrical and optical positioning equipment, depth measuring equipment) enable the Loire Estuary’s marigraphic system to be maintained.