Disclosed is a bathymetric system including a multi-beam emitting and receiving device attached to a carrier vehicle, configured, in a first recurrence, to measure the amplitude of a first plurality of beams reflected off the seabed, and a computer to determine three-dimensional spatial coordinates of a first scan swath. In a second recurrence, the device emits a second incident beam and measures the amplitude of a second plurality of beams reflected off the seabed, the computer determines the three-dimensional spatial coordinates of a second scan swath, so that at least one probe point of the first scan swath is redundant with a probe point of the second scan swath, and the computer calculates an altitude variation of the carrier vehicle using altitude variations of the at least one redundant probe point between the first recurrence and the second recurrence.