Seafaring has always been an occupation which exposes the seafarer to more than an ordinary degree of risk, and a significant part of that risk arises from his inability always to know his position with sufficient accuracy as and when needed. This lack of precision has always had a fundamental impact on the practice of navigation, the development of navigational aids, the production of charts and so forth, and to a much greater degree than is, perhaps, generally realized. As we are now approaching the time when a global capacity to fix position on demand and to a high degree of accuracy is becoming available it seems sensible that we should go back to first principles before forming a view as to its likely impact on future navigational practice.