Whilst in non-tidal waters it is valid to define the true wind as the wind when the vessel is stationary, but where there is a water flow (whether from tides or sailing on a river) this also has an effect on the wind experienced, and this is independent of the motion of the boat through the water. This is commonly factored out by defining the true wind as the wind experienced when the boat is drifting with the water (but moving with respect to the sea bed), and then defining the wind when the boat is stationary with respect to the sea bed as the ground or geographical wind.