I need to establish either a true north or a magnetic north line on the ground at the airport to calibrate my magnetometer. There is no compass rose here or at any other airport around this area.

I have several compass units but they don't seem what I can use...I want pretty great accuracy. If if had (and I do not) a theodolite I could 'shoot' Polaris, but can't find one...

What suggestions can you give me. I really need some way to get a very accurate line.