The present system and method determine a position on an external surface ofanobject. The system comprises a magnetic field measurement module and anultrasound measurement module. A control unit stores a magnetic field map oftheobject comprising a first plurality of reference areas of the external surfaceof theobject and associated reference magnetic field value. The control unit alsostoresan ultrasound map of the object comprising a second plurality of referenceareas ofthe external surface of the object and associated reference ultrasoundcharacteristic.The processing unit determines: a subset of the reference areas for which theassociated reference magnetic field value is substantially equal to a measuredmagnetic field value, a subset of the reference areas for which the associatedreference ultrasound characteristic is substantially equal to a measuredultrasoundcharacteristic, and determines the position on the external surface of theobject.
