I’ve been blessed to live and work in locations where there is a large difference between true north and magnetic north. On many occasions, I’ve made a living looking through a transit, or pulling a chain to measure distances – though my modern chains were either steel or cloth tapes. Still, as I pulled out my old compass to run a line, I realized that the declination – the difference between magnetic and true north – had changed a lot since I last set it in the early nineties.
This article shows how to set the declination on a Brunton compass. Even in today’s world of GPS, it is a useful skill. This photo, taken from the REI site is shown to convince you to click the link and read their information.
During my working life – say from age five to fifty – the declination was always around 20 degrees in Trego. In 1956, it was 21 degrees East, and by 1999 it had dropped to 17 degrees. Today, it’s 13 degrees 41 minutes East. The magnetic North Pole has been moving, probably forever, but definitely faster over the past few years.
It gets significant when you recall the shooter’s approximation – a minute of angle is an inch at a hundred yards. If I hadn’t corrected my compass declination to show the change from the mid-nineties, my view of the world would have been roughly 3 ½ degrees off. Doesn’t sound like much, but between 40 acre corners (a quarter of a quarter section), 440 yards at 210 minutes of angle gives about 75 feet of error. Following a compass for a mile, and looking for a corner monument that has been there for nearly a century gives an appreciation for reducing your potential errors.
A British Navy officer, James Clark Ross, discovered the magnetic north pole in 1831. I imagine it showed up as some really funky compass readings, and someone made the usual scientific observation – “Now that’s interesting.”
Before that, the difference was determined between a star shot (Polaris) at night, and reading the compass. David Thompson’s night works taking star shots just north of us was part of what made his explorations and mapmaking so significant. By the time I came along, I could look all the calculations up in a book. Now, it’s available online.
At any rate, the magnetic north pole has been moving a lot lately. This National Geographic article provides more, and better information on the topic.