It was about 10 years back, and I was driving some of the boondock roads of Central Oregon.
I was on a dirt road that seem to be running directly north south, but I was feeling a little turned around. So I stopped the car, reached for the glove box and got out my trusty Suunto M3 baseplate compass, to make sure I was driving in the direction I thought I was.
Sitting behind the wheel, I pointed my compass straight ahead, spun the dial to put red in the shed and . . . The compass indicated that the road was heading pretty much east/west! Very puzzling.
Then I had a little idea. I got out of the car, walked to the other side of the road, and once again took a bearing on the road in front of me. The road now ran exactly north south, just like I thought. Keeping a close eye on my compass needle, I walked back toward my car. When I got within about 1 meter, sure enough, the needle started slowly moving randomly. As I slowly walked around the car with compass in hand, that large block of metal continued twitching my compass needle.
So, here’s the Takeaway. Don’t use a baseplate compass intended for backcountry navigation inside your car or any other large metal object. (Compasses that come build into the car, such is the direction indicator that might be on the rearview mirror of your Subaru, are already calibrated by the clever car engineering gnomes to be correct.)
And, somewhat related to this, avoid using the hood of your car at the trailhead is a place to lay out your maps, take bearings, and in general do any kind of work with your compass. It usually is the largest flattest and most convenient object to use to look a map. But as you now know, trying to do anything that involves the magnetic needle of your compass on the hood of your car is a Bad Idea.
What are some other metal objects that can interfere with your compass? Electrical power lines, large belt buckles, zippers, in most any sort of electronics, especially cell phones. Try this: Put your mobile phone flat on the table, and move your compass slowly toward it and waive the compass of the top of the phone. You should see a few places where the needle swings dramatically.