  It is expected that ion trap quantum computing can be made scalable through
protocols that make use of transport of ion qubits between sub-regions within
the ion trap. In this scenario, any magnetic field inhomogeneity the ion
experiences during the transport, may lead to dephasing and loss of fidelity.
Here we demonstrate how to measure, and compensate for, magnetic field
gradients inside a segmented ion trap, by transporting a single ion over
variable distances. We attain a relative magnetic field sensitivity of \Delta
B/B_0 ~ 5*10^{-7} over a test distance of 140 \micro m, which can be extended
to the mm range, still with sub \micro m resolution. A fast experimental
sequence is presented, facilitating its use as a magnetic field gradient
calibration routine, and it is demonstrated that the main limitation is the
quantum shot noise.
