I want to add a compass to my truck. Do the third-party units you can pick up at JC Whitney or camping supply places work? Anybody have one?
I ask because I put one in one of my cars about 20 years ago and it was worthless. The indicator swung around wildly, and even if I stopped, it was often wrong by as much as 90 degrees! I get offroad with this truck fairly often, and travel in remote areas with unmarked roads. A compass would be a great navigation aid, but only if I can actually depend on it. Plus or minus five degrees wouldn’t bother me a bit, but 45 degrees would.
I know the built-in compasses on many modern cars work well, but I’m specifically asking about the after-market units you mount on the dashboard.
You said the one you had before would swing around wildly, so I assume it wasn’t oil damped. Having the “card” float in oil will keep it from jiggling madly. Many compasses can be adjusted to compensate for nearby metal. Just install it, and, aligning the truck with a known north-south line, tweak the compass until it reads north. That’ll be close enough for your purposes.
If you want a really good compass, go to a boat equipment store (such as West Marine.) They’ll have many models of varying prices, including some that cost as much as a handheld GPS.
Thanks. Actually, I’m hoping to go with a digital unit, so oil damping won’t be required. I see digital compasses for vehicles in the $30 range now, which looks reasonable if they’re accurate.
I had a digital one a few years back. When first placed on the dashboard, had to calibrate it by having the car face in one direction, mash a button, and then turn 180 degrees and hit the buton again.
From then on, unless I removed it, it was quite accurate.
I’ve always wanted a compass and wondered why most cars don’t come with one. It seems absurd to have to wonder what direction I’m facing. I tried a cheap analog one, and simply aligning it with true north didn’t work at all. I couldn’t get any sort of regular behavior out of it. I may try a digital one. Radio Shack has one for $20:
Normal compasses are distorted by the presence of large amounts of metal nearby. Like a car body all around them, duh!
So you’re never going to get a cheap compass to work reliably inside a car. Better ones can be calibrated to compensate for this somewhat, but still not very accurate.
Take that same cheap compass 25 yards/meters away from the car, and it’ll be reasonably accurate. There are many stories of WWII desert soldiers who forgot to get away from their vehicle before checking the compass, with bad consequences.
And most cars don’t come with one because most buyers won’t pay the extra money it would cost.
Compasses can read inaccurately due to metal near them - as already mentioned.
Compasses can also be thrown off by electrical fields. So it may read accurately with the radio on but not when off, or vice versa. If you have a lot of electrical stuff in your vehicle this can seriously complicate using a compass.
Compasses have error during acceleration. That means speeding up or slowing down. Once your speed steadies this will go away, but it can’t be eliminated during acceleration.
Compasses also show error when turning. When you stop turning they correct, but during the turn itself they are off. Whether they under or overshoot depends on which direction you’re turning.
In other words, navigation by compass takes some skill, awareness of your surrondings, and a firm grip on the limiitations of the device.