Morning Pete. That's good lateral thinking, but around the time I retired in 2000, I was in to Astro-archaeology (Prof Thom and star alignments around megalithic stone rows etc. in Scotland, mainly.) So, getting decent bearings at a site was worthwhile. GPS as available to the public was in its infancy, and I made do with an early Garmin. There's more on my old & now, non-maintained, website:- http://www.users.waitrose.com/~emes1938/ . More recently I've puzzled over the bearings given by my 7D MkII from sites on the North Devon Coast, east of Combe Martin such as the Holdstone farm area. I could see the Welsh Coast from where we were, but the bearing errors on my 7D MkII were large - making them useless. It was only when we got home, that I discovered that we'd been parked on an old iron workings quarry - now turned in to a very green carpark with a view over the Bristol Channel.