When the curve of reference velocity, vr, has been plotted against time, this curve is used to relate all traverse measurements to the same reference flow rate, qo (preferably that which corresponds to the mean of velocity measurements at the fixed point). For comparatively small changes of the reference velocity, the velocity, vi,t, measured at any point i at time t can be transposed by multiplication by the ratio of velocity, vr,o, at the reference point corresponding to flow rate, qo, at velocity, vr,t, at this reference point at time t: