In this paper two methods are proposed in order to determine the optimal or nearoptimal positioning of sensors which has to be carried out when vibration tests are being prepared. The optimal sensors location is proposed on the finite element model associated to the structure to be tested. The general context concerns the parametric identification of mechanical structures models using test results. The purpose of model updating is to converge the mathematical model, generally obtained by a finite element method, by adjusting the model parameters in agreement with test measurements. Missing displacement measurements increase the ill-posedness of this problem. The dynamic response of the structure is here described on a truncated modal basis. The first method of location of sensors emphasises the minimisation of the noise effect, the estimate of the modal coordinates is found in a least-squares sense. The second method is based on the observability gramian and the optimal sensors location has to ensure observability requirements. The smallest eigenvalue of the observability gramian represents the poorest case of information. The two methods are illustrated for a supported beam for a benchmark truss structure.