Of course compasses are too inaccurate, and in many parts of the
world maps are at such a small scale that this doesnt really work.
The only reliable way is to use sun sights, backed up with a nautical
almanac. I am amazed this doesnt get taught any more, and I wonder
how many surveyors know how to do it. The basic method is set on in
JC Pugh Surveying for Field Scientists, a true vade mecum for anyone
working in obscure parts of the world, and I have used his method
extensively in recording several lost cities in the jungle.......
There are probably a few copies still around secondhand!
On 15 Feb 2008, at 09:12, David Bowler wrote:
> If you know where you are (roughly), and can see a distant object
> (eg a hilltop, a transmitter mast, a tall building):-
> Find where you are on the map.
> Find the distant object on the map.
> On the map, with a protractor, measure the angular heading from
> where you are to the distant object (ie set Grid North as 0
> degrees, and measure round clockwise to the distant object).
> On the ground, set up your theodolite, and point it at the distant
> Adjust the horizontal degree circle on your theodolite until the
> reading agrees with the heading you calculated above.
> Your degree circle is now zeroed to Grid North.
> Use a compass and common sense to check that you have not made a
> gross (eg 180 degree) error. If it is about lunchtime, the Sun
> should be roughly to the south. If you gave a mobile phone or a
> pocket GPS system, put them down in a safe place and stand
> somewhere else before using a magnetic compass; they are both
> strongly magnetic, and will cause wildly inaccurate compass readings.
> For greatest accuracy:-
> You need to know exactly where you are, ie set up your theodolite
> in a place you can find on the map, and
> Choose a very distant object, so long as you can see it, and it is
> actually on your map.
> David Bowler
> ----- Original Message ----- From: "Michael Haseler"
> <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Thursday, February 14, 2008 8:11 PM
> Subject: [BRITARCH] Aligning a Theodolite
>> Can anyone tell me how you are supposed to accurately align a
>> theodolite with map North? I've tried searching and I've found
>> absolutely nothing apart from a suggesting of using some kind of
>> gyroscopic device "if you can't use astronomical observations"
>> both of which struck me as bit extreme.
>> Can I just use a Silva compass or is that far too inaccurate?
>> And can I do theodolites have ferrous material in them?
>> thanking in advance,
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