When most people think of a compass, the image that comes to mind is a simple device with a magnetized metal needle floating in fluid. With a history dating back at least a thousand years, a magnetic compass is a staple for most travelers, and compass construction is still one of the first skills taught in outdoor survival. In fact, a functional magnetic compass is easily improvised with a setup as simple as a sewing needle and a puddle of still water.
While an arrow pointing to magnetic north may be all that’s needed for a hiker to find his or her way back to camp, on longer trips or in more advanced applications greater precision is required.
Magnetic vs. True North
As the name implies, a magnetic compass points to magnetic north, which is different than true north. The magnetic field of the Earth moves over time, and a magnetic compass will reflect this change, aligning to a different direction than a meridian line from that point to the geographic North Pole (also known as “true north”). This deviation is known as declination.
While average declination for most parts of the world is small, it can be quite large closer to the North or South Pole, and varies with localized irregularities in the Earth’s magnetic field. Even a small declination is worth noting, as it can lead to a significant deviation over the course of a long trip. To compensate for this, maps indicate the declination when the map was made, and many magnetic compasses allow for adjustment accordingly. Of course, this makes it critical to have current maps and to be familiar with the simple correction needed to correlate magnetic and true north. On the simplest compasses the user needs to perform this calculation himself. More nicely designed models make it easier to account for declination right on the compass, and electronic models perform any needed calculations directly.
An Indispensable Guide to Navigation
Today, when even phones come with turn-by-turn GPS guidance, a magnetic compass continues to be a useful addition to any navigation system in which dependability and accuracy is paramount. While compasses are not complete navigation systems on their own, they still serve a vital role in determining heading. After all, a traditional magnetic compasses doesn’t even require batteries.
For the foreseeable future no navigation system will be without a magnetic compass, the technology behind an instrument that many of us learned to use as a child and that guided our ancestors home for centuries.