1. An apparatus for assisting navigation in a vessel ( 2 ), comprising a) a sensor probe ( 3 ) for acquiring local images ( 5 , 10 ) that characterize the vessel ( 2 ) at the point where the particular local image is made, which sensor probe ( 3 ) can be moved along the vessel ( 2 ), b) a memory ( 8 ) for storing a sequence of local images ( 5 ) that is obtained in the course of the movement of the sensor probe ( 3 ) along the vessel ( 2 ), c) a data-processing unit ( 7 ) that is arranged to sort a further local image ( 10 ) of the vessel ( 2 ) into the sequence that is stored in the memory ( 8 ).