  Motivated by the first detections of rotation signatures in the DG Tau jet
(Bacciotti et al. 2002), we examine possible biases affecting the relation
between detected rotation signatures and true azimuthal velocity for
self-similar MHD disc winds, taking into account projection, convolution as
well as excitation gradients effects. We find that computed velocity shifts are
systematically smaller than the true underlying rotation curve. When outer
slower streamlines dominate the emission, we predict observed shifts increasing
with transverse distance to the jet axis, opposite to the true rotation
profile. Determination of the full transverse rotation profile thus requires
high angular resolution observations (< 5 AU) on an object with dominant inner
faster streamlines. Comparison of our predictions with HST/STIS observations of
DG Tau clearly shows that self-similar, warm MHD disc wind models with lambda =
13 and an outer radius of the disc ~ 3 AU are able to reproduce detected
velocity shifts, while cold disc wind models (lambda > 50) are ruled out for
the medium-velocity component in the DG Tau jet.
