How to use GPS

I've really got a bee in my bonnet about this one.

 

Here's an article that I submitted to DIVE magazine for publication. It never made it to the press and so divers will still miss wrecks for the lack of a little knowledge. The setting is Britain, but the message applies anywhere on earth.

How often have you received the following marks for a wreck, often badly written on the back of a beer mat:- 

50 47 32;  00 30 51.

What does this mean ?

Assuming that the figures are correct in themselves, it is likely that the first group is 50o north, but what about the second group; is it east or west (not trivial on British coastlines). OK, the guy in the pub talked about Littlehampton so he probably meant west. Now, did he imply 50o 47’ 32” as used by the Admiralty or 50o 47.32’ often used by navigators. In the worst case, the difference between positions in minutes / seconds and decimal minutes can exceed 1000 metres. OK if the worst comes to the worst you can check out both possible locations 50o 47’ 32” N  00o 30’ 51” W and  50o 47.32’ N  00o 30.51’ W, they are only 560 metres apart on this occasion. But what DATUM was he using ? Did he even appreciate that all GPS receivers need to be set to one of many hundreds of datums. Out of the box, most receivers default to WGS84, however, the older Admiralty Charts don’t use this datum. The last British British chart I looked at used OS 1936 and the Dutch use Euro 1950. The difference between datums can again be several hundred metres. 

More and more yachtsmen and navigators are using WGS84 and even the Admiralty are converting their charts, however, Shell, for example, uses the local datum, since it is vital for accurate surveying. In Brunei, Shell uses Timbalai 1948, not WGS84.

All modern GPS units can be set to the datum of choice, and degrees, decimal minutes or degrees, minutes and seconds on the SETUP menu.

What a shame. The guy in the pub was genuine, the co-ordinates were read off a GPS in good faith, but to answer my original question, 50 47 32;  00 30 51 is going to waste your time and money. You could be out by over a kilometre. 50o 47.32’ N  00o 30.51’ W (WGS84) will put you within 10 metres of the wreck. (Disclaimer: There is, to my knowledge, no wreck on these marks ! I have simply used the co-ordinates as an example).

Thanks are due to Bill Clinton, who, just before he left the presidency of the USA, decreed that the 'selective availability' be removed from the GPS signal, thus overnight improving accuracy from about +/- 70 metres to +/- 10 metres.

When giving or receiving co-ordinates, please record the datum and whether you are using decimal minutes.