The invention concerns the field of devices and appliances for measuring the intensity and the direction of the magnetic field emitted by a plate or a structure, such as for example a ship's hull and it particularly concerns a method for determining the magnetisation and the magnetic field radiated by a ferromagnetic plate using magnetic field sensors distributed along said plate and at known positions relative thereto. The invention is characterised in that it consists in: producing a mesh of said plate by clipping into elementary surfaces; producing a first modelling of the magnetic field associated with measurements and based on the produced surface meshing of the plate, on its thickness, on measurements, on co-ordinates of measurement points and on the excitation B.sub.0=.mu..sub.0H.sub.0; producing a second modelling of the physical behaviour of the material forming the plate based on the mesh, its thickness, the excitation, and the reversible permeability of the material; calculating the permanent and induced magnetisation levels of each of the elementary surfaces based on said first and second modelling and on the measurements performed by said magnetic field sensors; calculating the magnetic signature of the plate in every point based on said permanent and induced magnetisation levels of each of the elementary surfaces, the surface mesh produced on the plate, its thickness, measurements, co-ordinates of the calculation points and the excitation B.sub.0=.mu..sub.0H.sub.0.