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Pooh-Bah
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Pooh-Bah
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Looking through some old books last night I came across A Dictionary of Civil Engineering published by Penguin in 1958. Some good words in there, but most probably of little interest to most people. One entry caught my attention:
Agonic line – a line on a map of the earth along which the magnetic declination is zero. From the Greek; agonios, without angle.
To find this line I guess you draw a line between the magnetic North Pole and the geographic North Pole and then project that line both ways around the globe. The idea is new to me and I cannot see that it has any use, although the dictionary says that it is a surveying term. I’ve done my share of surveying for roads and railways in years past, but not once did I think ‘Damn, I wish I knew where the line of zero declination was’. Does anyone know how surveyors use it, or why they might need to know it?
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Joined: Jan 2001
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Carpal Tunnel
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Carpal Tunnel
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Dear dxb: If you have done surveying you know more about it than the rest of us, I should think.
I know that the compass points to magnetic north, and a correction has to be made to get
true north, but I don't know how surveyors handle that critically important task. It will give me
something to look up, though I expect a headache.
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Joined: Dec 2000
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Carpal Tunnel
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Carpal Tunnel
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draw a line between the magnetic North Pole and the geographic North Pole
Due to local variations in the magnetic field it isn't quite that easy. One could google something and come up with specifics.
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Joined: Jan 2001
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Carpal Tunnel
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Carpal Tunnel
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Here is a URL that explains it, but I ;haven't read it yet, and expect difficulty making useful post about it. http://www.geoshare.org/new_web_site/technical/12/html/1.htmP.S. Two hours later, I went back and tried to understand that URL site. As I feared my smarts have deteriorated to point I cannot master it. My respects to those of you who can.
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Joined: Dec 2000
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Carpal Tunnel
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Carpal Tunnel
Joined: Dec 2000
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Here's a link to a computer generated model of the Earth's magnetic field. Note most of the spaghetti is probably in the core, but it gives you an idea. http://antwrp.gsfc.nasa.gov/apod/ap021125.html
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Pooh-Bah
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Pooh-Bah
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Gentlemen,
Thank you for those links. The model of the magnetic field shows amazing complexity - it resembles the head of a rather tatty corn dolly. I was aware that the strength and direction of the field varies with time and place and the Geoshare site confirms that. In fact what that site says leads me to doubt whether an agonic line, other than a very short local one, could in fact be drawn. Perhaps the technology has developed since the space age and a more simplistic approach was taken in the '50s. I guess that it is of more significance in mapping, for example, than in surveying for the construction industry. Because the link is to a Geoshare site the information one can reach from it is connected to the oil industry. I'll look wider.
Thanks again. dxb.
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Pooh-Bah
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Pooh-Bah
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I've just consulted Mrs Rhuby, who was a cartographic draughtsperson for many years, but she disclaims any knowledge of the term at all. Not that this proves anything, other than that it was not a term in common use amongst cartographers up to around 20 years ago. (Indeed, when asked the meaning of the word, she suggested that it described the tenor of letters written to the advice columns of women's magazines!) 
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Pooh-Bah
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Pooh-Bah
Joined: Mar 2002
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Nice one - what's her take on alidade?
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Joined: Dec 2000
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Carpal Tunnel
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Carpal Tunnel
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alidade
Isn't that a city in Australia?
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Carpal Tunnel
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Carpal Tunnel
Joined: Apr 2000
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agonic advice columns... yes, derived from "Agony Aunts" (the originals of whom are both gone from our papers now.)
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