  Suppose that \Delta, \Delta' are two buildings each arising from a semisimpe
algebraic group over a field, a topological field in the former case, and that
for both the buildings the Coxeter diagram has no isolated nodes. We give
conditions under which a partially defined injective chamber map, whose domain
is the subcomplex of \Delta, generated by a nonempty open set of chambers, and
whose codomain is \Delta', is guaranteed to extend to a unique injective
chamber map. Related to this result is a local version of the Borel-Tits
theorem on abstract homomorphisms of simple algebraic groups.
