  Based on the three-ball inequality and the doubling inequality established in
[24], we quantify the strong unique continuation for an elliptic operator with
unbounded lower order coefficients. This result is then used to improve the
quantitative unique continuation from a set of positive measure obtained in
[24]. We also derive a uniform quantitative strong unique continuation for
eigenfunctions that we use to prove that two Dirichlet-Laplace-Beltrami
operators are gauge equivalent whenever their corresponding metrics coincide in
the vicinity of the boundary and their boundary spectral data coincide on a
subset of positive measure.
