Not sure if this is on-topic but I've found compass north horribly variable as I move around a site - every site or location I've tried. I always thought a decent compass, away from lumps of iron (cars etc) would be pretty pin-accurate. 5 degree swing ea way seems routine - and that's uncomfortable when designing to maximise capture of mid winter insolation coming thro a 'window' in the southern horizon. Then I really do need to know where solar south is, and that's calculated from magnetic south which in turn I'd want to establish by compass.
Is it just me/my compass, or is this soimething I must live with? Needless to say, mobile-phone based GPS is far far worse, making nonsense of apparently wonderful aps like Theodolite. Other than in wide-open countryside, I don't think 'proper' GPS is that much better at finding north.
If a five degree error on a magnetic compass were routine, there would be far more wrecked ships around the world than there have been over the past two thousand years! And that's notwithstanding being on a moving deck.
There are a few factors to confound the innocent though. An 'ordinary' or magnetic compass points to 'magnetic north', which is not the same as 'true north'; the difference between the two is called the 'variation' and is known for [pretty much] every point on the earth and printed on the right type of maps. It does vary significantly from place to place and changes slowly over time. It shouldn't change within a plot of land, though, unless there are large lumps of iron or magnetic rocks.
Compasses also have something called 'deviation'. That is the error built in to the compass or into the way it is installed. Again, large lumps of metal or magnets near the compass are the usual culprits. A favourite trick of sailing instructors is to place a portable radio, which has a big magnet in its loudspeaker, near the compass.
So if you're regularly getting an error of 5 degrees each way, go see an expert. You either need a better compass or you need a lesson in how to read it more consistently. Or get a professional-quality gyrocompass.
Oh, and solar south is calculated from true south rather than magnetic south.