I used a magnet to see if there was any iron nearby, and then a hand bearing
compass to see if it's needle moved as I moved it around the area. I'm about 18 inches above a small
bilge pump, but I don't notice any interference--even when it's running.
I drilled the
pedestal and tapped the holes for the mounting screws. When commissioning the system, the deviation was only 2 degrees (I think
Raymarine says 15 degrees is acceptable).
The
installation process clued me into the fact that my ship's compass is actually off! So I picked up a magnetic compensation card, and I'm about to go out and turn it, and make a deviation card. The
Raymarine agrees with my hand bearing compass, my dive compass, my
iPhone compass, and range marks in my harbor. The "ships" compass is off by 2-3 degrees through most headings. Grrr....