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