[en] The problem of orientation or runout errors being formulated, it is firt noted that these errors necessarily include the form errors of the measured part. The second problem is the correct asseement of a simulated reference when the reference surface is of imperfect form. The logical choice would be perfect form having the least maximal distance to the physical one. But this choice being a difficult one, the best approximation in the least square sense if often chosed. The effect of this reference perturbation is studied.