You can’t even trust your compass anymore. (You actually never could!) Most people incorrectly believe that a compass needle points directly to the north magnetic pole. There are many complicating factors involving the molten iron/nickel core of the earth and multiple magnetic poles that average each other out as well as local effects. The net effect is that if you followed your compass straight north until you hit the magnetic pole, you would not even be at the geographic north pole. They are not one and the same spot. In fact, they are 590 miles apart. Therefore each position on the earth has a particular error unique to its location called the magnetic declination. Long distance travelers such as airplane pilots have to compensate as they travel which is a complex task. Navigators need to regularly update their value of declination in their calculations to stay on course.
Even worse, some locations around the globe and even around the US have anomalies that may significantly effect a simple compass. For instance, an area 45 miles west of Boulder Colorado has an anomalous declination amounting to almost 46 degrees west of geographic north. Even more interesting the magnetic pole moves around pretty quickly. The north magnetic pole has wandered over 1000 kilometers (600 miles) since Sir John Ross first reached it in 1831. We actually have an 11 degree easterly declination where Moline Illinois currently sits on the 0 declination line. When I started mapping in Wyoming, the declination was 14 degrees so it changes pretty fast.
The whole concept set you up for failure to begin with. The end of the compass needle that points to the north pole is actually the south pole of the magnetic needle. How confusing! It is actually the north seeking end of the compass needle. The north and south pole were defined long ago before some curious fellow placed a lode stone on mercury or placed one on wood floating freely on water and observed the lodestone consistently aligned to the same direction. Backwards thinking I suppose.
So what does this mean to you here in Wyoming? Simple compasses are simply not accurate. If you are navigating by compass, try to buy one that has the ability to adjust for declination. The Brunton company make many compasses that do this in many price ranges. Remember that maps are drawn to true north not magnetic north so do your best to compensate if you are out in the wilds. Or the alternative is to do what I do and buy yourself a really good GPS and not worry a bit about it since the GPS uses geographic location and is not influenced by magnetic declinations anywhere on the planet. Just keep some spare batteries about just in case!