During gait the motion of the residual femur within a trans-femoral socket may be estimated using video recorded data from two ultrasound transducers attached to the socket wall. This paper reviews possible measurement errors and identifies the magnitude of the inaccuracies. Inaccuracies due to equipment limitations and those due to human error are identified and quantified. Ranges of flexion/extension and abduction/adduction of the residual femur within the socket during gait have been estimated with a cumulative level of inaccuracy of <1 degree.