Accurate navigation, sighting and triangulation with a magnetic compass is a skill that the responsible outdoorsman should never allow to lapse.
The accuracy of a magnetic compass and the information presented depends on land characteristics, the quality of the compass, and the ability and knowledge of the hiker.
The hiker will experience natural factors and conditions in the backcountry that impact accuracy. Some of these factors the hiker will have no control over but may be able to mitigate. One of the most important factors is declination. Declination is the angular difference between true north and magnetic north.
Why is this important? It is because declination’s value varies over time and place. Many outdoorsmen fail to realize how declination impacts the accuracy of arriving at the desired destination. The outdoorsman must understand how to apply the difference of East and West declination –specifically- whether to subtract or add the difference. It can result in either arriving at the desired destination or being way off course.
The Declination value can be found at the map key of most maps. It is important to note that many maps are out of date and the declination value presented has changed. The most current information can be found at the website www.magnetic-declination.com.
Further compounding the declination issue is what is known as local attraction. Local attraction is a result of materials such as iron or nickel deposits located near the earth’s surface at a specific spot. Like declination, local attraction will impact the compass. High voltage power lines, car bodies, and flashlight batteries will impact the compass too.
To lessen the impact on a magnetic compass and improve its accuracy, do these 3 steps:
- use the most current declination data for an area
- use a declination adjustable compass (e.g., the Suunto M3 or Silva Ranger)
- stay clear of iron/steel and electrical sources
When triangulating the hiker’s position with several lines of bearing, the size and shape of an object will make a difference. Ideally, all objects will be distinct and clear like Oregon’s Mt. Washington. Mt. Washington’s tall spire is ideal for accurate sighting. In practice, the hiker will have to accept what is in view and what is pictured on the map.
Another accuracy impact factor is distance. Distance will magnify the errors. Objects that are five or more miles away may have a significantly large cross-range error when lines of bearing are plotted.
The quality of the compass itself will make a difference in accuracy.
The two smaller black compasses (above) have small bearing tick marks that are spaced every 15°. Increments of 15° are of little value for accurate navigation – they provide a rough trend of direction.
The three bottom compasses are good baseplate compasses. Depending on the technical reference, these have an estimated accuracy of roughly +/- 2°; perhaps even more. The three compasses have degree increments of 2° labeled on the rotating dial. While looking at the dial it is difficult to interpolate a bearing (azimuth) more refined than +/-1°. Baseplate compasses have no sighting system and are held at waist level when sighting on an object; not a very precise method to achieve an accurate bearing measurement.
While, a baseplate compass, such as Suunto’s M3, is adequate for general hiking and orientation, I prefer using the Silva Ranger mirrored sighting compass (below) when the view is not obstructed by vegetation.
Because of its sighting system, it is more accurate than the traditional baseplate compass. A quality baseplate compass such as the Suunto M3 will cost about $30. The Silva Ranger will run about $55 or more.
How accurate is this compass?
The degree increments on the rotating dial are spaced at 2° like the other baseplate compasses.
The Silva Ranger offers the hiker the ability to mechanically adjust the compass for declination. A small set screw on the back of the rotating dial can be turned to match easterly or westerly declination.
A significant feature to improve sighting accuracy is with the mirrored black-box component of the compass. When the box is raised a notch on the top edge becomes a convenient sight (below).
This notch allows the compass to sight on a distant object more precisely than just being held at waist level. Other manufacturers making similar models also have a notch cut into the box at the hinge; this is very helpful.
The sight line is scribed into the mirror. When the compass is sighted, the black box is raised and the sight line comes into view.
Ideally, the sight line will intersect the magnetic needle pivot point.
In my navigation classes, I recommend that the map and compass go with the hiker on every outing. Further, two weeks before a backcountry journey the compass goes everywhere; even to the mall. Practice sighting, determine a back bearing, and triangulate your position at every opportunity. Review the fundamentals of the grid system (e.g., Universal Transverse Mercator (UTM) that will support the compass work. This up front effort pays off in the field by the development of a greater sense of confidence and understanding.
About the author
Blake Miller received significant hands-on navigational training during his 20 years of service in the US Navy. He has taught map and compass, GPS, and wilderness survival classes in Central Oregon since 1998. As a part-time faculty member at Central Oregon Community College, he currently teaches land navigation classes to Natural Resource students. He has been an active member of the Deschutes County’s Search and Rescue (SAR) team since 2009. Blake is also a featured speaker at regional SAR conferences, Sportsman Shows, and Cabelas. His articles have been published in several national magazines.
Interesting how few know the box type Compasses like Recta make. Suunto has one model too. I think these offer far and away the best sighting.
Thanks Chris…Never heard of them and I’ll have to check them out. Website? Blake
Hi Blake – check out Suunto MB-6, or recta.ch, e.g. the DP-6
I just checked the Recta website and they say their compasses are manufactured by Suunto in Finland these days. The DP-6 sure looks like the MB-6. I’m going to check them out.
Chris, Turns out that the Recta was my first orienteering compass back in the mid ’70s……lost in safe keeping somewhere. Today I really like that Silva Ranger style of compass.
What are the situations where a degree of accuracy better than 2 degrees are required? Unless you are in a landscape devoid of topographic features shown on a map, and you need to know your position precisely, it seems a bit much to go with a sighted compass. Otherwise the contours of the land or other features should fill any small precision gap.
Good question. 2 degrees is probably the best many will get with a standard baseplate compass. It’s a matter of personal choice and what is important. For me, a Silva Ranger fits my needs nicely.
I also carry a Garmin 62S too.
I was taught Orienteering in the Boy Scouts by some US Army soldiers. They were sticklers for accurate triangulation. Of course, accuracy matters when calling in artillery strikes, perhaps not something a civilian generally needs. Maybe if you are trying to discern which of two closely spaced features you are near? Or if you’re doing some informal surveying of property? I always thought it would be fun to make my on maps and compare them for accuracy with professional maps, but admittedly that’s not typical.
An important aspect too is that you can’t necessarily use a northern hemisphere compass south of the equator and vice versa.
Why are the needles for the bottom three compasses all pointing very differently – far greater than +/- 2·? Proximity?
Why do needles point in different directions? The short answer is, not all compasses point in the same direction and not all individuals will point their compass in the same direction. Yes there are compass manufacturing issues and yes individual’s dominate eye and the lack of reading glasses all play a part. The fix is easy if you have a known bearing, established with an expensive compass or surveyor’s transit, or a typical leg of an existing compass course.
Load the known bearing into the compass and sight it to the next point. If the compass points to the right, increase the declination, that will force the compass to point more left, etc. This compass calibration process is just like adjusting the sights on a rifle. As a retired surveyor that teaches land navigation, this process works good.
Perhaps you can help me with this.. I bought a Suunota MC2-G compass. the declination is 10 degrees westward (Toronto Canada). however when i compare to my iPhone 11X compass and another compass app called Commander all are pointing in different directions. I’ve looked at all the settings such as true north, magnetic declination etc. Any advice suggestions? Which of the 3 would you trust hiking in the wilderness? How would you deal with this?
Try this. Set the electronic devices to magnetic declination.
Now hold the compass level (very important) in your hand. (this assumes you have NOT set the magnetic declination on the compass).
All three should be pointing in the correct direction.
(Note the compass app uses the iPhone compass, so it’s hard to believe that they’d be different.)
When in doubt, always trust your magnetic compass over a device. I also often carry a second compass, in case I lose my primary or a comrade needs one.
In the picture? Because the magnetic fields from the needles are interacting with each other because they sitting so close together.
I often leave my GPS at home because I love to practice using my compass. I also taught navigation; for a college mountaineering class. However, sometimes in the woods the GPS is the only way to be certain where you are. One of the fun games to play from a summit is “what mountain is that in the distance?” I have taught a few novices quite a bit just doing that with a map and compass. Thanks for your article.
The initial step is to look for the declination value of the place you are currently located. The declination value varies depending on your current location on the globe. Once you plan to hike in a particular area, you need to check first on the declination value before using the compass.