The projective structure depends only on spacetime location and 3-velocity, both of which are independent of 3-acceleration, as required for a well-defined standard.

The above argument for the necessity of geometric fields also holds for 3-velocity and 3-acceleration, denoted respectively by \(\xi^{\alpha}_{1}\) and \(\xi^{\alpha}_{2}\). The transformation law for the 3-acceleration is much more complicated than that of the 4-acceleration. However, analogous to the case of 4-acceleration, the transformation law of 3-acceleration is linear and is inhomogeneous in the 3-acceleration variable \(\xi^{\alpha}_{2}\). Consequently, there does not exist a unique s