We are in a marvellous age of technology, where we can pinpoint a needle in a haystack within inches. In the days before GPS and even Decca, there were transits. Transits can be good or bad; a good transit can be as accurate as any GPS. A bad transit is just bad.

I once went looking for a plane wreck. It hadn’t been dived for a long time, and we wanted to see what was left. It was a forty five minute boat ride but the transits were supposed to be good. We had to line up a telegraph pole with the third tree, out of four, in the hedge behind.

Something had happened to one of the trees; there were now only three, all spaced out a little… so which one was the original third tree? We used the sounder to look but found nothing, so we guessed at one of the trees…. and found sand. So a good transit had gone bad!

Decca was an improvement over transits when you were further offshore. Towards the end of its life, the last version of Decca was quite good, but eventually GPS took over. GPS was a little all over the place to start with, so along came Differential GPS (DGPS), which used a known fixed local transmitter to correct any errors. These errors were deliberate errors used by the Americans, so the rest of the world would be at a disadvantage. Once the Americans no longer added their “fudge factor”, the D was no longer needed.

So, what could possibly go wrong now? A few years ago, there was a news item on the BBC about a wreck called the Antoinette. It had become exposed as the sand shifted in the Camel Estuary. The report stated it was on the Doom Bar; it also stated that the bomb squad had been tasked with removing the remains of the wreck as they were a hazard to shipping. I headed over with my camera, but found nothing, just sand. I then headed back to Padstow to say ‘Hi’ to the staff at the lobster hatchery. While there, I could see something going on in the middle of the river. I was told it was the bomb squad about to blow a wreck up. But that was Town Bar not the Doom Bar – they are over a mile apart. Not the most accurate of reporting! That was quite recent though; when looking for old wrecks, with old information, the difference could be a lot worse.

Surely though, modern GPS is infallible? It is very good, as long as the numbers are accurate. They could be written down wrong, accidentally or deliberately. They could be from an alternative dataset. Most people use WGS 84, World Geodetic System, which originated in 1984. Some people don’t.

Myself and some friends wanted to dive the wreck of the St George, which lays twenty miles off shore in a depth of sixty five metres, and we wanted an accurate position. We asked around, got some accurate marks, and headed out. Once in the area, we watched the sounder as we went over the co-ordinates, but all we found was a flat sea bed. We repeated this several times, but still nothing. Time was running out to dive at slack water, but the sea bed was completely flat all around us.

Then one of us remembered that the person who gave us the co-ordinates liked to use OSGB 36, a different dataset. We quickly changed the GPS to OSGB and headed off. We arrived at the new location, and lo and behold, there was the wreck. So the co-ordinates were right… just an alternative right.

So, every form of position fixing has potential errors. Sometimes, there is nothing you can do about it… except guess!

Find out more about Mark and Atlantic Scuba at www.atlanticscuba.co.uk.