In addition to Adam's reply, the following numbers are a rough guide to some minimum working distances to be aware of when using compasses:
Transmission High Tension Lines.....80m
Vehicles.....75m
Railway Lines.....20m
Fencing Wire.....10m
Tools, Shovels (
Steel Telescope Mounts 
).....3m
Every compass, no matter how well made will have an inherant error. Once this error is known, it can be allowed for.