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-- Richard Langley

On Thu, 12 Feb 2004, Brooke Clarke wrote:

>Hi George:
>
>There are a number of magnetic observatories located all over the
>world.  They have permanetly mounted magnetometers recording the X, Y
>and Z components of the Earth's magnetic field.  From this data a
>mathematical model of the Earth's field is made once every 5 years.  You
>can use this model on line given a Lon, Lat and date.  This is the model
>that's built into some GPS receivers, allowing the receiver to tell you
>the true bearing to a way point.  Since GPS receivers have o compass
>functionality, the receiver says something like waypoint # 57 is 37
>Degrees (true) from here, and it's up to you make use of that
>information.  You can see more about the Earth's magnetic filed on my
>web page at:
>http://www.pacificsites.com/~brooke/Sensors.shtml#Earth%27s%20Magnetic
>
>Have Fun,
>
>Brooke Clarke, N6GCE
>http://www.PRC68.com
>
>George Huxtable wrote:
>
>>Doug shows an interest in magnetic variation; perhaps it interests others too.
>>
>>Variation is the difference between the way a compass-needle points and the
>>North-South direction. If there's any iron around, then the needle is also
>>affected by the local deviation, which varies with the course of the
>>vessel. But in the absence of any such local deflection (or if it's been
>>well-compensated out), the variation is what remains, due to the fact that
>>the Earth acts as an immense magnet with poles that are misaligned with
>>respect to the Earth's axis. What's more, that misalignment changes with
>>time, and there are also local fluctuations: so that the variation doesn't
>>follow a simple pattern over the Earth's surface that a bar magnet would
>>produce. Worse still, infrequently the direction of the compass will even
>>reverse, but as this occurs at intervals of hundreds of thousands of years,
>>it's not going to bother us.
>>
>>Even now, I doubt whether the Earth's magnetism is completely understood,
>>but it's attributed to the swirling motion of electrically-conducting
>>liquid rock deep within the Earth. (I'm not a geophysicist, so stand to be
>>corrected about that.)
>>
>>It's an important matter for the Earth's magnetism to be well mapped, and
>>its variation with time predicted as far as possible, because all our
>>compass courses depend on the variations marked on our charts.
>>Unfortunately, the British team who contributed to this magnetic survey
>>work was disbanded a few years ago. I wonder who does it now: is there a US
>>survey team at work? On our charts, there's an entry by the compass rose,
>>that usually states something like- "4deg 50'W 1985 (10'E)", where the term
>>in brackets is the predicted annual change from the 1985 value. Over time,
>>however, the variation will start to diverge from that prediction. Where
>>will the updating information come from, I wonder?
>>
>>When mariners were exploring unknown oceans, they needed to measure their
>>local variation:  for one reason, to make sense of their own compass
>>bearings as they travelled; for another, to bring back as information to go
>>with the charts they would produce.
>>
>>There was another reason, too. In the early 1700s, knowledge of local
>>magnetic variation around the Earth was proposed as a way of "discovering"
>>the longitude, by Halley (of Halley's comet), the Astronomer Royal of the
>>time. This was before the days of chronometers and lunar-distances. The
>>proposal was rather doomed to fail, because it was hard to measure
>>variation to sufficient accuracy, because there was so much local
>>fluctuation, because of the variation with time, and because there were
>>large areas of sea over which the variation didn't change much with
>>longitude. Halley's proposal stimulated mariners into measuring and
>>reporting variation, so that Halley was enabled (with a lot of
>>interpolation, exrapolation, and intuition) to compile a map of variation
>>over the then-known world. It was useful, but not for the purpose Halley
>>intended.
>>
>>How do you measure variation? In theory, just take a compass-bearing on the
>>Pole Star. Make a small adjustment depending on GHA Polaris, for the
>>displacement of Polaris from the Pole itself. The trouble is, it's hard to
>>take an accurate compass-bearing on an object that's high up in the sky.
>>It's easier when the object is on or near the horizon, as in the case of
>>the rising (or setting) Sun. Amplitude tables exist, which show the
>>difference of the Sun's azimuth from true East or West, when rising and
>>setting, usually for the moment when the horizon bisects the Sun..
>>
>>In the tropics, when the Sun is rising and setting from the horizon almost
>>vertically, this is accurate enough. In higher latitudes, the Sun is
>>arriving or leaving at a shallow angle. Because refraction near the horizon
>>is highly uncertain, this can then affect the azimuth somewhat. But it's
>>hard to take a compass-bearing to better than a degree or so at the best of
>>times, so measuring variation at sea is at best an inexact science.
>>
>>In fact, you can measure variation from the bearing of any object at all in
>>the sky, if you have a good idea of your own geographical position. Choose
>>a convenient low-altitude object, get its dec and GHA from the almanac, and
>>calculate its azimuth just as if you were obtaining a celestial
>>position-line. Any difference with the compass-bearing of that object is
>>the variation. Indeed, if you take that compass-bearing at the same moment
>>as you measure a sextant-altitude, you have killed two birds with one
>>stone.
>>
>>George.
>>
>>================================================================
>>contact George Huxtable by email at george@huxtable.u-net.com, by phone at
>>01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
>>Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>>================================================================
>>
>>
>>
>>
>


===============================================================================
 Richard B. Langley                            E-mail: lang@unb.ca
 Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
 Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
 University of New Brunswick                   Fax:      +1 506 453-4943
 Fredericton, N.B., Canada  E3B 5A3
     Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
===============================================================================

   

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