Article by Capt. Mark Sampson
A friend called and asked if I might be able to help him with a problem he was having while installing a compass on his new boat. At first, he thought the process would be as simple as screwing the compass to the console and then he’d be good to go. But after reading the instructions and realizing how important it was to get the compass perfectly square on the boat so that it would show the vessel’s proper heading, he wisely took the time and made his best effort to get it right the first time.
It took half a day to get it properly mounted and then he couldn’t wait to get out on the water and check the accuracy of his work. To accomplish this, he started by taking a chart of our local bay-waters and finding a couple buoys that were in an almost perfect north-south relationship. He figured that by running directly from the southern buoy to the northern buoy his compass should read 0-degrees (north) and then going the exact opposite way his compass should read exactly 180-degrees or due south.
Unfortunately, things didn’t quite work out for my friend. On his first run, he noticed that instead of the compass showing 0-degrees it was holding steady at about 350-degrees. When he turned and ran south, instead of 180, his compass was reading 172-degrees. Figuring he had mounted the compass about 10-degrees off, he went back to the dock and drilled some new holes in his console that allowed him to mount the compass at a slightly different angle. Out he went again, but this time he realized that the boat wasn’t following the track line that his compass was pointed to so he concluded that he had adjusted the compass 10-degrees in the wrong direction. So again, out came the drill and this time he moved the compass 20-degrees in the other direction. The next sea trial had him 20-degrees off from his desired course and that’s when he called me. “I’m obviously doing something wrong here.” he squawked. “And before I make any more Swiss cheese out of my new console I thought I’d get a second opinion.” By his description I knew exactly what he had done, but figured I’d wait to explain in person when I could show him on a chart where he had made his error. I told him to put the drill away and I’d be over in a couple minutes.
Where my friend went astray was in thinking that his compass would point to the same north that the chart did. The 0-degree heading between the buoys he pulled off the chart is known as a “true heading” because it was taken from a chart, and when charts are made they are always oriented such that a perfectly vertical line from bottom to top is 0-degrees - or heading straight up towards the north pole. So if buoy “A” is due north (0-degrees) of buoy “B,” then if you ran from “B” to “A” and kept precisely on that course long enough you would eventually wind up exactly at the North Pole. That’s what “true north” is all about.
Even though it is the northern axis of the earth’s rotation, compasses do not point to true north. Instead, they all point to “magnetic north” which is a location on earth that has the strongest magnetic field. Magnetic north is located a bit south of true north up in the Arctic Circle in Canada. To make matters even more confusing for navigators, magnetic north is really not a “place”, it’s a “magnetic field” that shifts a little each year.
The difference in a compass between true north and magnetic north is called “variation” and must be taken into account anytime a compass is used for navigation. Variation affects all compasses in the same area equally, but because the magnetic north is off to one side of true north as you travel east-to-west or west-to-east, the difference between true and magnetic north varies. If you’re exactly due south of magnetic north the variation would be zero since magnetic and true would be in the same line. Around Ocean City our variation is a little better than 12-degrees west, which means that our compasses are pointing 12-degrees west of true north or 348-degrees, which is mostly why my buddy’s compass was reading about 350 instead of the 0-degrees the chart showed for that course.
Since most of us just steer our compasses on whatever course our GPS units tell us to and we don’t worry about variation, some might wonder how we ever get to where we want to go if our compasses are out by 12-degrees. The simple answer is that most GPS units are preprogrammed to know what the variation is for the area we’re in, and by default, give us a “magnetic” course to steer. This also means that if you asked your GPS “what’s the course from Buoy “B” to Buoy “A”, it would say “348-degrees”, but if you plotted a 348-degree line on a chart from buoy “B” it would miss buoy “A” by 12-degrees. Got it?
“Deviation” is something else that pulls a compass away from true north by a few degrees, but it has to do with the unique magnetic forces aboard each boat. Deviation actually changes with the heading of the boat and is adjusted for differently than variation. "Compass error" is the term used to describe how much a compass is off by both variation and deviation.
To the beginning boater all this might sound very complicated and unnecessary to learn. After all, we can choose to forget about the compass altogether and just let the GPS tell us to steer “port” or steer “starboard” and get there and back just fine. Just fine that is as long as the good-old GPS keeps working. But then again, just like any piece of electronic equipment, a GPS can fail just when you need it most. The last I heard the earth’s magnetic field has never let us down – ever. Learn how to navigate using a GPS and you’ll “usually” find your way home. Learn how to navigate with a chart and a compass and you’ll always find your way home!
Capt. Mark Sampson is an outdoor writer and captain of the charter boat, “Fish Finder”, docked at the Ocean City Fishing Center. During the winter months, Capt. Mark runs charters in the Lower Keys.