A scheme for obtaining an improved boundary condition relevant to the perforated walls of transonic windtunnel is reviewed and the sources of possible errors involved in its practical application are assessed. Earlier work by the authors to implement the scheme provided measured boundary layer development and flow directions just outside the boundary layer in The City University transonic windtunnel. This work is shown to have been subject to errors from two main sources. First the yawmeter calibrations were not sufficiently accurate, and the remedy for this is presented. Second, lateral nonuniformity was present which produced significant flow angle variations. This nonuniformity is shown to be much reduced by windtunnel alterations, possibly the most important of these being an increase of the open-area ratio of the antiturbulence screens.