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 <jim...@earthlink.net> To: time-nuts@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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
