A method to estimate dispersion relations and warping associated with elastic wave propagation in a bar is presented. The method is based on Hamilton's principle. It is shown how the theoretical model together with strain measurements can be used to evaluate three dimensional (313) field quantities like displacements and stresses at an arbitrary position in the bar, as well as energy flux through an arbitrary cross-section of the bar. It is also shown how redundant measurements can be used to increase the accuracy. The method is general and can be applied to any mode of wave propagation, isotropic or anisotropic linearly elastic material, and any cross-sectional geometry. Here, it is applied to longitudinal waves in a split Hopkinson pressure bar with linear elastic isotropic material behaviour and square cross-section. In particular, axial displacement, axial stress and energy flux are evaluated at a free end of the bar in order to test the method. The method is also used to estimate the Poisson ratio of the bar material, by measuring axial and transverse strains at the same axial position.