HYDROGRAPHIC OFFICE SEEKS THE HELP OF YACHTSMEN

Howard Letty

I have been in touch with the Hydrographic Office for some time on the question of how yachtsmen worldwide might provide helpful information concerning differences between positions derived from GPS observations and their apparent position according to certain charts.

In a UK Notice to Mariners to be published at about the same time as this issue of Flying Fish, this cooperation will be formally requested. I believe that this is an important development which will eventually lead to much better charts and safer navigation in some of the more remote parts of the world.

Not every yachtsman is aware that only 60 percent of Admiralty Charts are referenced to datums whose relationship to the datum used by GPS has been established. (The datum used by GPS is WGS 84 -- World Geodetic System 1984). Put simply, on only 60 percent of Admiralty charts can a GPS position be plotted accurately.

With the remaining 40 percent, over 1200 charts in all, the relationship between the chart's datum and WGS 84 is not known. Thus the GPS is more accurate than the charts! Many of us have experienced this phenomenon -- indeed a GPS position may differ by a mile or more from the same position plotted by means of charted landmarks. Some, though not all, of such charts carry a cautionary note warning that the discrepancies may be significant.

Many charts of the Caribbean, North African coast, Eastern Mediterranean, East Africa, Pacific and Far East come into this category.

A full explanation of the situation and details of the relevant charts and areas will be given in the forthcoming Notice to Mariners, together with a copy of the form that the Hydrographic Office would like yachtsmen to use in reporting discrepancies between a chart `fix' and their GPS-derived position. An announcement is also to be made to the British yachting press.

Those cruising in the areas of the `40 percent' are asked to read the relevant Notice to Mariners. The many OCC members without ready access to UK Notices can get copies of the form by writing to: HDC - Geodesy, Hydrographic Office, Ministry of Defence (Navy), TAUNTON TA1 2DN, Somerset, UK. It is hoped that, as a result of cooperation by yachtsmen and other mariners, a significant improvement in the accuracy of many charts will be made possible.

(396 words)