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Non-magnetic compass? Is there such a thing?

Discussion in 'General Equipment Chat & Advice' started by Malcolm_Stewart, Jul 20, 2021.

  1. Malcolm_Stewart

    Malcolm_Stewart Well-Known Member

    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.
     
  2. Malcolm_Stewart

    Malcolm_Stewart Well-Known Member

    Repetitive tests carried out in my local area of Milton Keynes, have been fine agreeing with my Garmin GPS62s, and with my OS maps. My problem has been when shooting from what I now assume, were worked out ironstone quarry workings.
     
  3. Petrochemist

    Petrochemist Well-Known Member

    Not nearly as difficult as you're making it. Pick a visible object on a known bearing, set the sextant to show the required angle to North then make the sight horizontally, to see where north is. Yes it's a bit awkward but Malcolm's usage case is very different than the deck of a yacht at sea. If he's in an iron ore quarry he's rather likely to be onshore.

    I've had a few occasions on yachts where managing a simple bearing to within 10 degrees would have been near impossible. I know I couldn't manage to get an ordinary fix on a landmark when on a 25' yacht in the north sea in a force 7. Movements on that occasion where somewhat severe & rather unpredictable (not just a regular pitch/roll). Fortunately we were never more than 100 miles from shore & simply heading vaguely west was pretty much bound to get us in sight of recognisable bouys or coastal landmarks. Erring slightly to the north was enough to ensure we didn't end up on the Gabbard sands if our lookout's failed to spot the bouys...

    The crossing in which we were becalmed in fog would have been impossible to get a stellar fix as well, but at least we had RDF, so didn't have to rely totally on dead reckoning. The crossing time on that occasion was more than twice that of the trip above despite the engine running continuously.
     
  4. SqueamishOssifrage

    SqueamishOssifrage Well-Known Member

    My point was that a sextant is just not necessary. My first solution required only the 'phone (for GPS position, false bearing, precise time and web access) plus the sight of any astronomical object, the sun being the simplest, and then a couple of minutes on the web somewhere like Time and Date (https://www.timeanddate.com). My second solution required a map, ruler and protractor, but this was just to establish a 'known bearing' to which you refer, but with no suggestion of how it should become 'known'. If you already have a 'known bearing', just point the compass at it and calculate the error.
     

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