1. Introduction. Nautical charts of many areas of the world carry notations adjacent to natural marine hazards such as ‘position doubtful’, ‘existence doubtful’, ‘position approximate’ and ‘reported (date)’. In spite of the development of precision survey and navigation systems, many of these notations must continue to be carried until the positions of the hazards are accurately determined or their existence disproved.

The expanse of the oceans obviously precludes costly systematic hydrographic surveys in all areas, particularly in remote offshore areas. Although international cooperation in the exchange of hydrographic information assists in meeting modern charting requirements, the task of charting the seas may never be complete.