ESDU Aero W.02.04.09 plots against Reynolds number (based on l) values of the ratio l/k above which the location of the transition point is not affected. Here l is the wing chord or body length and k is the height of the roughness element. A curve is given for uniformly distributed roughnesses and a series of curves for isolated roughness elements at various distances downstream of the leading edge or nose. The data for the uniform roughnesses assume that transition is first affected when the Reynolds number reaches a critical value at which vortices are shed from the tip, while the data for the isolated roughnesses are based on experimental results obtained on a laminar flow section. The tests showed that the shape of the roughness had little effect on the onset of transition. The data should not be used when there is waviness (see ESDU Aero W.02.04.11) or lap joints nor when the roughness is near the nose or the natural transition position. The effects of interference between isolated grains is not considered and the data should be used with caution where the distance between individual grains is less than three or four diameters.