Thin films are key elements in the current development of nanotechnology, and their characterization has become an essential task. In this Letter, we report on a technique to reconstruct full 3D maps of dielectric thin films using the scattered light of decoupled surface plasmon polaritons. Patterned magnesium fluoride thin films were fabricated, and their 3D thickness map was fully reconstructed with high (<1 nm) precision. This technique can be applied and easily adjusted to identify inhomogeneities in wide areas (mm2-cm2) of dielectric samples with subnanometer precision, or to characterize the fabrication processes involved in the preparation of patterned multilayered systems.