  We consider the classical ballistic dynamics of massless electrons on the
conducting surface of a three-dimensional topological insulator, influenced by
random variations of the surface height. By solving the geodesic equation and
the Boltzmann equation in the limit of shallow deformations, we obtain the
scattering cross section and the conductivity {\sigma}, for arbitrary
anisotropic dispersion relation. At large surface electron densities n this
geodesic scattering mechanism (with {\sigma} propto sqrt{n}) is more effective
at limiting the surface conductivity than electrostatic potential scattering.
