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Re: Aligning a Theodolite

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Fri, 15 Feb 2008 17:57:44 +0000

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 ```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! Mark 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 > object. > 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. > > Regards > > 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, >> >> Mike >> >> -- >> This email has been verified as Virus free. >> Virus Protection and more available at http://www.plus.net```