Long reach manipulator systems (LRMS) can perform tasks in difficult to reach locations. They use a small and fast manipulator mounted on a large flexible structure that can vibrate, degrading the system performance. Control methods that use strain measurements of the flexible structure have been proposed to control the system’s position in spite of the supporting structure’s vibrations using a model of the structure that relates strains to displacements. Here the minimum number of strain sensors needed to accomplish this control and their optimal locations on the flexible supporting structure are determined. These locations have been selected to achieve high measurement resolution, to maximize the computational robustness and to minimize the error in the identification of the structure’s strain-displacement model. The results are validated with simulations and experiments using a six degree of freedom laboratory LRMS.