For non-omnidirectional sonic anenometers like the Kaijo-Denki DAT 600 TR61A probe, it is shown that separate planar-fit rotations must be used for the undisturbed (open part of the sonic anemometer) and the disturbed sector. This increases the friction velocity while no effect on the scalar fluxes was found. In the disturbed sector, irregular values of <-w'u'> <0 were detected for low wind velocities. Up to a certain extent these results can be transferred to the CSAT3 sonic anemometer, Campbell Scientific Ltd. This study was done for data sets from the Naqu-BJ site on the Tibetan Plateau.