Well I wouldn’t hold it too close when taking a reading. At Shark River in NJ the magnets in the Ocean Ave. bridge motor will throw the compass off, I’ve been on wrecks where the iron makes the compass useless but if you can see the bridge or wreck you probably don’t need the compass anyhow.
I use one on a retractor, there is one in my SPG console but I rarely use that one because if its not absolutely level it sticks and gives you a wrong heading.