When you are depending on your compass to get you back home, you should be careful to not get your wristwatch too close to the compass. In some cases the watch can throw the compass off as much as 20 degrees.
Most watches have a battery in them. Because of the battery the watch puts out a magnetic field. Hikers should be aware of this magnetic field and keep their compass from making contact with their wristwatch. If the compass is moved inside the magnetic field of the watch, the compass can be as far as 20 degrees off.