As the compass points to "Magnetic North" with the pole position at approximately 77 degrees North, 122 degrees West (in northern Canada) there is a correctable offset to "True North". The biggest problem with a magnetic compass overland is knowing the magnetic variation for your locale. You can buy or download a "Pilot Chart" which give the lines of magnetic variation on the earth for your locale. If you are in an area where there is no variation, then the magnetic compass and "true" are closely the same. Some areas of the world have magnetic variations in the range of 25 degrees(east or west) which must be applied to the compass. That means if you are traveling southeast (135 degrees), and the local variation is 25 east, then the compass heading you must maintain is 110 degrees! Your GPS will track 135 degrees (based on true polar north). The other influence on the magnetic compass is magnetic deviation which is based on the magnetic effect of your bike influencing the compass. It is minor and would be difficult to compute. Lastly, if you are in an area of "Magnetic Anomanolies" (lots of Iron in the ground) (they are listed) then your compass can go haywire and point to the anomanalie. Here is an easy way to correct the variation (if known) and go in the correct direction with a magnetic compass. TVMDC-AW.
That stands for True Virgins Make Dull Companions--- At Weddings. Under the T put the "True" direction you want to go... 135 degrees. Under the V put your magnetic variation and label it east or west. D is for Deviation which we will disregard. Under C will be your magnetic compass heading in relation to true. AW means "Add West". So if you have 23 degrees west variation, you add it to 135 and get 158 degrees. If it is easterly, you subtract and and you C would be 110 degrees. C is the magnetic compass heading you will follow.Regarding the dip, better compasses are corrected with with flinders bars and correcting magnets. It's the variation that can really throw you off track.... Good luck and don't get lost. Best regards, Smitty

Way to go Smitty! UK maps and I guess most maps also show the variation information for Grid North which is related to the differences caused by a flat map trying to represent a curved, more or less round, globe.
So, we end up being able to deal with True, Magnetic and Gird North in order to travel.

__________________
Dave

Given that the earths magnetic poles flip polarity on average every 300,000 years, then we're approximately 500,000 years overdue for a flip!

At sunrise or sunset, just as the sun is breaking the horizon, line up the compass as best you can and check the heading. As the sun "rises in the east and sets in the west, the compass heading should be near 90 degrees in the morning and near 180 degrees at sunset. Time of year and latitude will effect the results as the sun travels 23.5 degrees north of the equator in summer and 23.5 degrees south in the winter. You can tell which side of the sun you are on (bearing north or south of you just by looking). At "noon" when it is overhead, Is it directly overhead or more to the north or south of you. That is the suns bearing in relation to you (at your lattitude). The more directly overhead it is, the more accurate you morning or evening compass check will be. If you find that your morning check (you want to see near 90 degrees) is say 115 degrees, then, using the formula TVMDC--AW you can backtrack and get a rough idea of the variation in the area. People have sucessfully traveled the planet for hundreds of years by magnetic compasses. They do work. I'll bet alexpezzi's key chain compass was better than what Magellan used and he didn't do to bad. If all else fails a good pocket compass is a good investment if you know how to use it. Best, Smitty

If you are using a mounted magnetic compass and use a magnetic tank bag, then you are going to have issues with deviation. The tank bag will really screw up the compass and you will you will never achieve accuracy with the compass. Even by walking away from the bike with the compass, the sphere of influence could not be determined. Use a "Strap On". Thank's Phoenix. Never considered the bag. Smitty

good tip, thanks mate

It's worse than that.. I don't put any electronics in the tank bag... strong magnetic fields (those little magnets are realy strong) can play havoc with electronics..

I found that my compass was re-magnetised by the tank bag, so I couldn't even use it once I'd walked away from the bike.. I had to throw it away.


Next time, I'll go for a strap secured tank bag.. definitely not magnet secured!