Wireless Joint

If you don't know, maps use true north, and your compass uses magnetic.
They are off generally about 12-13 degrees for most of us.
Useful if you are trying to aim an antenna.

Pop in your zip code and check yours:

https://www.ngdc.noaa.gov/geomag/calcul ... eclination

Thanks, a friend of mine was wondering about this, as I came across this post!