It's exactly what it looks like, a field. A neutral place generally free of magnetic sources. I say this because there is no place on earth free of magnetic fields. We all live on a giant roundish rotating magnet. But in this empty field there isn't much. On our planet ambient background magnetic fields typically vary around 25 to 65 microTeslas. The extra microTeslas on you see on the meter are largely coming from power lines and other things around me. On the whole this isn't much and the local field is reading zero. However, a large green field isn't boat by any measure, and this is where we start.
Here is a similar pic on an unrigged boat in a marina. The motors are hung. There is no power on. Like our empty field the magnetic fields are very low.
Below is where everything starts to fall apart. This is a very similar boat except it's now fully rigged and the power is not yet turned on. I'm standing at the same location as the picture above and already there is a local field of 21. Not good at all if autopilot and its associated compass have to be installed on the boat, and it does... hopefully.
On this center console boat it's the speakers that are the primary problem and the console interior is the only pragmatic mounting location for the compass. It's actually worse. Inside the console are the batteries, power steering assist pump, assorted water pumps, two stereo amplifiers, two MFDs, an assortment of black boxes all jammed in a small space.
There are rules of thumb for your autopilot compass location, although they vary somewhat by manufacturer. You should keep your high tech compass about two feet away from the old school compass you have mounted in the vicinity of your helm. You do have one, don't you? They also want you to have it in the center of the boat at the waterline. And lastly you should keep it six feet away from ferrous metals and magnetic fields. On occasion, I have been able to follow the rules, but in most cases not so much. Something almost always has to give, but you can't give too much.
Magnetic fields can't be shielded in the sense most of us think of the term. They pass right through lead, 3M 5200, fiberglass walls and everything else. Their fields however can be managed and redirected. When shipping magnets they are packed with their polarities opposing each other.
Keeper bars help as well. Materials like Mu-Metal provide a path for the magnetic fields to travel through mostly constraining them. But in all cases some leaks out, it's just a matter of how much. Magnetic fields, like antennas create lobe patterns that have directional characteristics. Take a meter and rotate a bar magnet in front of it and note the strength changes.
This was my first pass at installing the Simrad Precision 9 compass on this boat. I've installed dozens of them on others and to a unit all worked perfectly out of the box, except this one. It was my fault. The location looked good, and it was on the opposite side of the console that had the big sub woofer installed. It was also out-of-the-way meaning it wouldn't get stepped on.
My mistake was I didn't turn the power on and start the engines (power steering pump in the hood) when I used my Gemeco meter to scout the location. I ended up with a local field of 22, and the Precision 9 compass wanted 15 or less.
Sometimes if it's edgy you can put in an offset to correct but my sea trial test and setup clearly showed me it wasn't nearly good enough for gummit work and would be unsafe.
It's an oxymoron thing. When engines and power were turned on, and the stereo was thumping the final location was on the opposite side of the console even closer to the sub woofer, and I ended up with a local field of 9. I can live with that. I wish I had camera like NASA has to see magnetic fields. It would save me a lot of time, and would make my job much cooler.
Credit where credit is due:
Wikipedia ring magnet field diagram by user Geek3.
Gemeco's iNstall app was used for measurements and screen captures.