The present system and method determine a position on an external surface of an object. The system comprises a magnetic field measurement module and an ultrasound measurement module. A control unit stores a magnetic field map of the object comprising a first plurality of reference areas of the external surface of the object and associated reference magnetic field value. The control unit also stores an ultrasound map of the object comprising a second plurality of reference areas of the external surface of the object and associated reference ultrasound characteristic. The processing unit determines: a subset of the reference areas for which the associated reference magnetic field value is substantially equal to a measured magnetic field value, a subset of the reference areas for which the associated reference ultrasound characteristic is substantially equal to a measured ultrasound characteristic, and determines the position on the external surface of the object.
