An optical inertial measurement method for determining a 6DOF pose in respect of a moving platform, the method comprising providing, in respect of said moving platform, a camera unit (10) comprised of at least three monocular cameras (14, 16, 18) connected rigidly together and configured such that their fields of view do not overlap and cover motion of said platform in each of three principal, substantially orthogonal axes; receiving video outputs from each of said cameras; determining individual point correspondences from said video outputs, and estimating therefrom, for each camera, a transform that includes translation values representative of direction of translation in the x and y axes, rotation about the optical axis and a scale factor, each transform being expressed with respect to a local 3D coordinate system associated with a respective camera; and mapping said translation values in the x and y axes to a global 3D coordinate system, having its origin defined by a point between said cameras, and multiplying said translation values by a respective scale factor so as to determine a 6DOF pose in respect of said camera unit.An optical inertial measurement system is also disclosed.