Dean, we had problems at my mates place with polar alignment with compas's and we thing the main influence was possibly the High voltage power lines. The only reason we found out is that my mate decided to use the Sun to draw a True North South Line based on the shadow of a vertical stick at exactly solar noon for his location. There is a website you can use with your GPS cordinates to find out to the minute the solar noon for your area. At first we went with the compas for rough alignment and after drifting we realised that we were way off with the compas and the the shadow line was spot on. We should have trusted the age old method first.
Found this via Google. They use a plumbob to ensure the shadow is cast from a true perpendicular line (to the tangent of the earth at that point).
http://www.wsanford.com/~wsanford/su...True-North.pdf