A compass is one of the two most essential pieces of navigation gear that you should carry, the other being a map. GPS may break or fall into disrepair during a PAW, the sun isn't always shining, and it takes a great deal of time and effort to learn stellar navigation, but a map and compass will continue to work until you break or lose them. Few things are more important than knowing where you are and where you're going and the compass helps you to do both. In selecting a compass there many types with many different features to consider, some of them are novelties while others are tremendous conveniences. There are several basic types of compass, and some compasses fall under more than one type.
Types of Compass
Button compasses are very small offering a decent idea of which was is north and not much more. They are generally part of one's EDC, being attached to a keychain or are of a very small survival kit.
Baseplate compass are generally clear with various markings on them, they are designed to be easier to navigate with a map. They are often very feature rich and come at a variety of price points.
Sighting Compasses generally have either a mirror or sighting whole on perpendicular folding part of the compass. They allow you to look at both the compass and a landmark and line them up for better navigation.
The Transit is the Granddaddy of compasses, usually including more features than you can shake a stick at these are more of a miner or engineer's tool. They are extremely expensive, but offer many abilities other compasses do not.
Magnetic North is not in the same location as true North and depending on where you are in the world the difference between a magnetic North reading and where truth North is could vary by a couple of degrees or 30. A Declination scale allows you to calculate or adjust for this difference by knowing approximately where you are in the world and what the deviation is. If you have a fixed declination scale you will have to do the math, but if you have an adjustable deviation scale you can set it and forget it until you are in a location far away.
A compass that floats could be a life saver in some situations.
A lanyard attachment is a common and highly recommended feature, this will help you to avoid losing your compass.
If you have to navigate at night, glow in the dark compass point will be extremely helpful.