  We extend results of Dos Santos Ferreira-Kenig-Sjoestrand-Uhlmann
(math.AP/0601466) to less smooth coefficients, and we show that measurements on
part of the boundary for the magnetic Schroedinger operator determine uniquely
the magnetic field related to a Hoelder continuous potential. We give a similar
result for determining a convection term. The proofs involve Carleman
estimates, a smoothing procedure, and an extension of the Nakamura-Uhlmann
pseudodifferential conjugation method to logarithmic Carleman weights.
