Magnetic forces contained within your kayak can cause your compass to read an incorrect bearing. This type of error is known as magnetic deviation. With 1 degree of compass error, over a mile, you’ll end up about 92 feet away from your destination. If your deviation is extreme like shown in the image, you could completely miss your target by over 1.7 miles on a 10 mile crossing. Worse still, deviation varies as you turn your kayak. For example, you might have a negative 10 degree error when pointing northwest, but that might change to a positive three when pointing southwest. Making a chart showing the deviation error at each point of the compass rose is the only way to know. Those charts are a pain to use when on-the-water. It’s best to correct the error before paddling.
To calculate your ground error, you need to know your deviation and the distance to your destination. Run those numbers through this formula:
Ground error = tan(degrees off) * distance to destination
This formula is also handy for calculating distance from destination when “aiming off” — a technique where you purposefully set your bearing a few degrees off of your destination, because then you’ll know which side of your destination you’re on if you miss it. For example, if your destination is due east at 90 degrees, you’d paddle on a bearing of 91 or 92 degrees. Then once you hit the shore, you’d turn left or north and paddle to the destination. Using the formula allows you to estimate how far off you’ll be from the destination when you hit the shore. If that destination is 3 miles away and you aim-off 2 degrees, you’ll be about 550 feet south of your target destination.
Avoiding and Fixing Deviation
Unlike magnetic declination (variation), you can often eliminate magnetic deviation from your kayak. Eliminating it removes the headache of trying to correct for it. When you first install your compass, check for its accuracy at all compass points by comparing it to a compass that you know is correct. Don’t hold the compasses too close to each other, because you’ll experience deviation caused by the magnetic forces in the compasses.
If you notice any deviation, try to figure out what is causing it. Usually, it’s the screws you used. Sometimes, it’s a nearby fitting. If you run into these problems, change the screws. If you can’t figure out what’s causing the problem, see if your compass has any adjustment mechanism. The Brunton 70P has an optional adjustment part, but it’s unlikely to be useful in a kayak. If the variation is under 5 degrees, making a chart is probably your best option.
After you correct for initial deviation, you’ll only need to check for deviation when you load your front hatch. Pack any gear that creates magnetic fields, like knives, tent poles, electronics, camera gear or other metal items, away from the compass. After you pack the kayak, check for deviation using a separate hand-held compass. If you notice any, figure out what’s causing it and pack it far away from the compass. For example, in the picture, a knife caused a negative 10-degree deviation. Once moved, even to the bilge just below, the deviation disappeared.