| By :
Roger Brown
When it comes to watches with altimeters, you need to be careful about which functions you trust the most. Just because your altimeter watch has a compass function does not mean that compass will read exactly where you are heading. You need to understand some details about disturbances that might impact the reading of your watch before you totally trust it. Having access to a compass is hugely valuable in the woods. You can take a map and draw a straight line to use a compass rose until what direction you're travel in. Then it is simply a matter of following that path in using your compass to direct you alone the proper course. Many people however disregard the impact of magnetic interference on the course that they are trying to follow. The compass device is heavily influenced by sources of deviation along the path. There are certain aspects of the physical nature of the Earth that will impact the accuracy of your compass on your watch. The first thing to keep in mind is the shape of the Earth. It is not really round like we normally think of it, instead it is somewhat oval-shaped. The axis runs through the center of it , going from north to south of course. That is pretty much the definition of True North and what you need to understand is the waty that effects the compass. Since the planet is largely one big magnet, the Earth puts off a field of magnetism that converges at the top of the planet or at the north end. Herein lies the difficulty - the closer you get to either the North Pole and South Pole the greater the deviation on your compass due to strength of those magnetic fields. Those deviations are different depending on where in the country you actually are. In some places near the equator, the deviation is almost 0. Whereas, in the northern part of the United States that deviation might be as high as 4 or 5°. Normally, on a map all this is laid out quite clearly. You just need to make sure you make those corrections in your mind when you are traveling via the compass with your altimeter watch. The simplest thing is just to subtract or add based on how much magnetic deviation is in the area you are traveling in. So, using watches with altimeters can be the very best way to travel through the woods because the number of functions that are available. With a compass in particular though you need to make sure that you constantly "mentally" recalibrate based on magnetic deviation in the area that you happen to be traveling in.
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