December 29, 2015 at 1:22 am #46297
Ernst Stavro BlofeldVeteran
I do not see a thread that has anything to do with Map Reading, land Navigation, Star tracking etc… I would be happy to help with this thread. I am a competent land Nav person and have many pdf books that I can contribute, if you would like?
"It is not that I can and others can't. It is that I will and others won't."December 29, 2015 at 4:24 am #46298
Sure, I have a question. How do you do declination in the Southern Hemisphere with a compass.December 29, 2015 at 6:47 pm #46308
Ernst Stavro BlofeldVeteran
That is a very good question. I have not considered that one before. So I looked it up to get your answer.
It depends where you are on your hemisphere, that is the distance from your point to the equator. If you’re standing five degrees south the equator a “northern Hemisphere compass” should pose no problems. In NZ it’s a different story. Even if you’re able to “unhang” the needle, you’re not pointing at your waypoint or route, but in the sky, which is a bit pointless …
Maybe this site can be helpful (paragraph 5c), or google for “Magnetic Inclination (dip angle)”
Close5. How does a magnetic compass work?.
Close5a. Does the compass needle point toward the magnetic pole?.
No. The compass points in the directions of the horizontal component of the magnetic field where the compass is located, and not to any single point. Knowing the magnetic declination (the angle between true north and the horizontal trace of the magnetic field) for your location allows you to correct your compass for the magnetic field in your area. A mile or two away the magnetic declination may be considerably different, requiring a different correction. NGDC has an on-line magnetic declination calculator where you can enter your location (or zip code for the USA) and get the Declination value. Remember: east declination is positive, west negative.
Close5b. What happens to my compass at the magnetic pole?.
A magnetic compass needle tries to align itself with the magnetic field lines. However, at (and near) the magnetic poles, the fields of force are vertically converging on the region (the inclination (I) is near 90 degrees and the horizontal intensity (H) is weak). The strength and direction tend to “tilt” the compass needle up or down into the Earth. This causes the needle to “point” in the direction where the compass is tilted regardless of the compass direction, rendering the compass useless.
There are established zones around the north and south magnetic poles where compass behavior is deemed to be “erratic” and “unusable”. These zones are defined where H (the horizontal intensity) is between 3000 nT – 6000 nT (erratic zone) and H is less than 3000 nT (unusable zone). Experts in the field claim that if you have a good compass and are careful, you can get decent results through the “erratic” zone. However, when H is small (H < 2000nT), the daily variation in D can easily be greater than 10 degrees. The Canadian Geological Survey has excellent information on their web site concerning magnetism and the north magnetic pole.
Close5c. What happens to my compass in the southern hemisphere?.
For a compass to work properly, the compass needle must be free to rotate and align with the magnetic field. The difference between compasses designed to work in the northern and southern hemispheres is simply the location of the “balance”, a weight placed on the needle to ensure it remains in a horizontal plane and hence free to rotate. In the northern hemisphere, the magnetic field dips down into the Earth so the compass needle has a weight on the south end of the needle to keep the needle in the horizontal plane. In the southern hemisphere, the weight needs to be on the north end of the needle. If you did not change the weight, the needle would not rotate freely, and hence would not work properly.
The best solution for your problem might be to use a compass with a so-called global needle, like the Recta dp-6G http://www.recta.ch/en/dp-6g
"It is not that I can and others can't. It is that I will and others won't."
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