<P> Each mark indicates the edge of the safe water channel in terms of port ( left - hand ) or starboard ( right - hand ) . These directions are relative to the direction of buoyage ; this is usually a nominally upstream direction . In a river , the direction of buoyage is towards the river 's source ; in a harbour , the direction of buoyage is into the harbour from the sea . Where there may be doubt , it will be labelled on the appropriate chart . Often the cardinal mark system is used instead , when confusion about the direction would be common . </P>
what is the main purpose of the lateral system of red and green buoys and markers
indicates the edge of the safe water channel in terms of port ( left - hand ) or starboard ( right - hand )