This photo shows the colour difference as the water shallows over a sandbank of the beach at West Wittering. Colour has been used by nautical navigators since the first boats went to sea. In places where the water was too murky to use colour, like rivers, other techniques were developed. There are images dating back four thousand years that show ancient Egyptian boats plying the Nile with a man standing at the bow with a sounding pole. The pole gave an instant and very physical reading of depth.