On 11/21/13 11:32 PM, Robert Atkinson wrote:
I'd also go for a compass if you want magnetic north, but then I have a good one, a
"medium landing compass". Mine dates from WWII but they are still made
http://www.sirs.co.uk/ground/landing_compasses/patt2/landing_resource
These are used to align the standbay and remote reading compasses on aircraft.
Good to half a degree. If you need better ther is the Watts Datum Compass.
Robert G8RPI.
That compass is *precise* to 1/2 degree, but not *accurate* to 1/2
degree. It comes with a calibration card, and is presumably used in a
place with a uniform field (e.g. for calibrating an aircraft compass,
which is done in an open area with no known magnetic anomalies). If
you're in a different environment, the card values may be incorrect.
It is essentially as "comparison standard". You put it next to the
aircraft and move both in a systematic pattern and you use it to
"calibrate out" the variations in the plane's internal compass.
However you're going to be subject to the local magnetic field anomalies
(and they're surprisingly large).
http://minerals.usgs.gov/news/newsletter/v1n2/3aeromag.html
On the 1km scale maps in the USGS reports, you can see magnetic
anomalies of 500 nT. Earth's field is about 30-60 microTesla, so these
anomalies are in the "one part in 100" kind of range. It is true that
the gradient is fairly small: It is unlikely you have an anomaly of 500
nT and your neighbor has -200 nT. But it's obvious that the magnetic
variation (angle between true and indicated magnetic north) isn't the
nice smooth surface implied by the map of variation you get with the
compass.
http://dspace.sunyconnect.suny.edu/bitstream/handle/1951/47859/Winslow_MS.pdf;jsessionid=7D7A116045A54816C9DCF963AF3D2580?sequence=1
is a short paper that talks about gradients in a small scale anomaly of
0.22 nT/meter (and I get the impression that that is big).
There's also other locally produced magnetic fields you'd have to worry
about.
I gave an E&M class a problem to figure out if you could tell whether
you could use a hand compass to tell if the Pacific Intertie 1MV HVDC
link (3000 A) was operating bipolar or unipolar. (at 50 meters, the
field from one wire is about 6 microTesla, so yes, you can detect it)
http://msi.nga.mil/MSISiteContent/StaticFiles/HoMCA.pdf
is all about calibrating a ship's compass
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