Do Magnetic Anomalies Affect Your Compass in the Boundary Waters?

Paddlers often worry about magnetic anomalies throwing off their compass in the Boundary Waters. The truth? It almost never happens, and the things most likely to mess up your compass are already in your pack.

What Is Magnetic Deviation?

When localized magnetic forces influence your compass, they cause what’s called magnetic deviation. These forces pull the needle away from its correct bearing and create an inaccurate reading, i.e. deviate from the reading.

What Causes Compass Deviation in the Boundary Waters?

Most cases of magnetic deviation come from your own gear. Anything with metal or a magnetic component, especially when kept close to your compass, can throw off the needle.

Where Magnetic Anomalies Actually Occur

There are a few places in and near the Boundary Waters that can cause compass deviation, but you have to be very close to them to notice any effect. The most well-known is Magnetic Rock on the Gunflint Trail. Hold a compass right next to the rock and you’ll see the needle drift. Step back a few yards, and the effect disappears.

Other localized spots exist, such as a few rocks on an island on Iron Lake or cliffs on Snowbank on the hiking trail, but again, the range is limited. If you step away, the effect disappears. If you’re on the water and away from these rocky areas, you likely won’t see the effects at all.

More commonly, your gear will cause problems. For example, a camera or GPS unit stored in your life vest can cause deviation. I’ve also seen non-stainless screws used in a canoe cause minor deviation. It’s rare, but I’ve seen it.

How We Actually Navigate in the BWCA

For everyday travel in the Boundary Waters, magnetic anomalies won’t affect your compass or navigation. Most of the time, we navigate using something called piloting. That is navigating by visible landmarks like campsites, portages, islands, or other prominent features, and referencing them to the map. A compass often sits unused for this style of navigation.

Paddlers also use dead reckoning: estimating your location from a last know location by using your speed, time traveled, and direction. While a compass can help, most of the time canoeists follow a shoreline or clear path down a lake, so a compass isn’t always essential.

On larger lakes with complex island groups or when in the fog, compasses become more handy. Even then, the localized magnetic anomalies in the Boundary Waters aren’t strong enough or widespread enough to create significant navigation errors.

Should You Worry About Compass Errors?

In practical terms, no. It’s extremely unlikely that you’ll experience a naturally occurring magnetic anomaly large enough to impact your navigation on any Boundary Waters trip. Many maps or guidebooks mark any significant spots, and even those require very close proximity to matter. For fun, you might wander around a campsite looking to see if you can find rocks that cause a compass needle to move.

Magnetic Deviation vs. Magnetic Declination in the BWCA

Magnetic deviation comes from localized forces, like gear or magnetic rocks, but magnetic declination is different. Magnetic declination is the angle between true north and magnetic north. In the Boundary Waters, it ranges from about 1°W at Crane Lake to around 3° W at South Fowl. It’s small enough to ignore for many trips, but I personally set my compass to account for it. Even a simple compass, such as Bruton’s TruArc 3, has an adjustment for declination.

Practical Tips to Keep Your Compass Accurate

• Keep electronics away from your compass.
• Avoid storing your compass near metal objects.
• Set your declination (optional but good practice).
• Trust piloting first. Landmarks are your best friend.

Author’s Takeaway

In nearly thirty years of paddling and guiding in and around the Boundary Waters, and more than two decades living in Grand Marais, I’ve never experienced a magnetic anomaly that affected real-world navigation. If your compass suddenly points somewhere strange, the culprit is almost always the gear sitting too close to it.