The direction of an automorphism of a totally disconnected locally compact group

We describe the asymptotic behavior of automorphisms of totally disconnected locally compact groups in terms of a set of `directions' which comes equipped with a natural pseudo-metric. The structure at infinity obtained by completing the induced metric quotient space of the set of directions recovers familiar objects such as: the set of ends of the tree for the group of inner automorphisms of the group of isometries of a regular locally finite tree; and the spherical Bruhat-Tits building for the group of inner automorphisms of the set of rational points of a semisimple group over a local field. Research supported by A.R.C. Grant DP0208137.