  A method for alignment of a vector magnetometer to a rigidly attached prism
is presented. This enables optical comparison of the magnetometer axes to
physical surfaces in an apparatus, and thus an absolute determination of the
magnetic field direction in space. This is in contrast with more common
techniques, which focus on precise determination of the relative angles between
magnetometer axes, and so are more suited to measuring differences in the
direction of magnetic fields. Here we demonstrate precision better than 500
$\mu$rad on a fluxgate magnetometer, which also gives the coil orthogonality
errors to a similar precision. The relative sensitivity of the 3 axes is also
determined, with precision of about 5$\times 10^{-4}$.
