The first insightful geometric and constructive proof of the existence of a triangulation of any smooth manifold was given by Hassler Whitney. We quantize Whitneys construction (in terms of the reach of the manifold) to prove the existence of a triangulation for any $C2$ manifold,so that we get an algorithm with explicit bounds. We also give a new elementary proof, which is completely geometric.
Joint work with Jean-Daniel Boissonnat and Siargey Kachanovich.