[time-nuts] Crude Survey Technique

[Robert Atkinson]

Hi Jim,
I disagree, the Medium Landing Compass IS accurate to better than 30 minutes (0.5deg). This is also the smallest graduation so it can be read to better than 15 minutes. The calibration chart is given to 10 minutes every 15 degrees. It does of course indicate the orientation of the local magnetic field. To ensure there is no local distortions you need to do a site survey. This can be acheived by comparing bearings to a distant object along a baseline or taking reciprical bearings. CAA CAP562 part 8 leaflet 1 http://www.helitavia.com/docs/CAP562_2007_08.pdf  
has lots of good info.

(full version here http://www.caa.co.uk/docs/33/CAP562RFS.pdf  large file over 10MB)

Robert G8RPI (CEng, licenced aircraft engineer for 30 years, I've done many aircraft swings)  



________________________________
 From: Jim Lux <jimlux at earthlink.net>
To: time-nuts at febo.com 
Sent: Friday, 22 November 2013, 14:25
Subject: Re: [time-nuts] Crude Survey Technique
 

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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