The paper describes a method by which ‘history’ in the form of tracks of all vessels appearing on the PPI is projected on a flat plotting surface, and the use that can be made of such information to ascertain either the true or relative tracks of all such vessels and also the effects of own ship alterations of course or speed or both on the relative tracks of other vessels.

In a recent survey of collisions and from experience on board ships it has been found that, in heavily congested waters and low visibility, even vessels with sophisticated radar equipment and associated plotting facilities and sufficient personnel may find themselves ‘saturated’. Vessels with simpler radar and plotting facilities and less staff consequently become saturated in less heavy traffic conditions.