There are currently many aids to navigation which assist in making hazards visible and used sensibly they greatly enhance safety at sea A conspicuous exception to this is the means to avoid the hazard of grounding In this respect the navigator is still restricted by tunnel vision Information about a water depth hazard only reaches him via his echosounder when the ship is already half way across it. Needless to say he should have consulted the chart and have avoided known hazards.

A chart contains a compilation of what is known about the seabed It has never pretended to be a guarantee that no high spots are omitted or unrecorded Indeed care is taken to spell this out by pointing out in the Sailing Directions that it is possible for a battleship to lie undetected between the sounding lines when even a large scale echo-sounding survey has been carried out.

The almost universal use of sidescan sonar as an adjunct search tool of the area between the lines while surveying has greatly reduced the possibility of large targets remaining undetected, but it has not yet permitted a statement on the likely margin of error of the depth measurements incorporated in surveys, and hence eventually in charts This is because there is still a wide divergence between the depth sampling density along a track and across it Typically this is every 0 5 m and possibly 125 m between lines-a ratio of 1.250.

Swathe sounding has made it possible to reduce the ratio to 1.1 and, if the tracks are run with a suitable overlap, to 2 1 It has therefore become possible to be as certain of discovering hazards across track as along it During the last two years, experience has been gained showing that high spots were found which had, or would have, gone undetected in an ordinary echo-sounding survey even had a sidescan sonar been used.

It is too early to say that no hazards will remain undetected by swathe sounding, but it is possible to be specific about their locations and possible dimensions on the basis of the distribution of coverage density achieved by a swathe sounding survey.

The scale of data gathering is now so large that its storage has to permit efficient, rapid access, in order to process it This accessibility can be made available on ships, so it is possible for the navigator to have more detailed information available for examination when entering formerly unfamiliar waters This may lead to a fuller use of available water by reducing the sometimes very large under-keel tolerances now advocated in the light of current uncertainties, with the possible consequent reduction in congestion on busy routes, which is itself a potential navigation hazard.

CURRENTLY AVAILABLE SURVEY QUALITY INDICES

The chart is effectively a paper instrument abounding in safety features such as navigational aids information, only one of which is the selection of soundings shown Particularly, though not only, in Port Approaches, care is taken by chart producers to monitor unstable seabed areas and incorporate new finding.

This content is only available via PDF.
You can access this article if you purchase or spend a download.