Determine your location using a compass bearing

Here's a picture of the compass I sometimes use on hikes:

It's a model-27 military-grade lensatic compass that I bought before I got out of the Army about <mumble mumble> years ago. The Army had two kinds of lensatic compasses when I was in: one with radioactive tritium paint on the dial that would permanently glow for use at night and another with phosphorescent paint on the dial. The tritium dial compass was not available at the PX so I ended up with the one with phosphorescent paint. It's a rugged design and makes for a very cool (if a little heavy) piece of kit.

Unlike the clear plastic Silva navigation compasses you normally see people using, the lensatic compass is designed for taking bearings quickly and easily. The folding rear site allows you to look down through the lens and see the dial without needing to move the compass away from your face. This increases accuracy, which is a good thing if you're going to be calling in an air strike based off your reading.

If you're like me and mostly stay on established trails then a compass is most useful for estimating your current location using a single bearing measurement. Off-trail bushwhackers can also use a compass to follow a bearing toward some destination. Both skills are pretty easy to master but for this post I'm just going to show how to find your location on a trail or road using a compass bearing.

Okay, so let's say you've been hiking up the Weatherford Trail on the San Francisco Peaks for a couple of hours and while you know you're still in Weatherford Canyon you want to know exactly where you are. To do this you look across the canyon and take a compass bearing to a prominent location, which in this case would be Schultz Peak:

The compass tells you that Schultz Peak is at bearing 95° from your current location on the trail. If someone standing atop Schultz Peak were to shoot a compass bearing back to you the angle would be 275°(95°+ 180°). This is called the "back-bearing". With the back-bearing and a map you can determine the location on the Weatherford Trail that intersects with that angle, i.e. your current location!

Now, there's a slight complication with that 275° angle. Because you used a magnetic compass to obtain it the angle is in relation to magnetic north, which is not the same as the true north your map uses. Magnetic north drifts around slowly and depending where you are on the Earth the difference between magnetic and true north can be quite large. So you must correct for this difference, otherwise your calculations will be off. Fortunately, all good maps will tell you what the difference is, called magnetic declination, somewhere on them, usually in the map's legend. My map's legend tells me that magnetic declination for Flagstaff is 13° east, which we then add to the 275° magnetic back-bearing to get 288° true north bearing (275°MN + 13° = 288°TN).

So now that you've converted your magnetic back-bearing to map bearing it's time to find the location on the Weatherford Trail that intersects with that angle. To do this get out your map and orient it to magnetic north. This allows you to make angle measurements directly on the map with the compass, negating the need to carry a protractor along with you on the hike. Lay the map on the ground, align the compass with a north-south line on the map, and then turn the entire thing (both compass and map) until the compass needle reads zero degrees:

In the above example I aligned the compass to three UTM markers on the map but any north-south line will do, even the edge of the map.

Once you've oriented the map to magnetic north all that remains is to find Schultz Peak on the map and then shoot a 288° back-bearing from it to find your location on the Weatherford Trail. Because you know you're on the Weatherford Trail you know your location has to be where the bearing line intersects with the trail on the map:

 

In the above picture I placed the lower edge of the compass's straight edge on top of Schultz Peak and then rotated the compass until the dial read 288° below the hairline. This places the intersection of the green line (the 288°back-bearing) with the Weatherford Trail about here:

 

The black circle denotes my estimated location, which happened to be where I stopped for lunch on this particular hike. Checking with my GPS and Topo! show that the location determined via compass and back-bearing from Schultz Peak was about 200 feet off. My true location as given by the GPS was actually a little further down the trail near the southern edge of the black circle. Not perfect but also not too bad for someone without a lot of experience doing this sort of thing.

So what did we learn from this exercise?

1. How to use the lensatic compass to take a magnetic bearing reading to a distant location (Schultz Peak was 95°).
2. How to convert the bearing to a back-bearing (added 180° to bearing to get the back-bearing of 275°).
3. How to convert the back-bearing from magnetic north to true north as used on the topographic map. Flagstaff's magnetic declination is 13° so we added that to 275° and got 288°.
4. We then oriented the map to magnetic north so we could use the compass to measure angles instead of a protractor.
5. And finally, we placed the edge of the compass straight edge on Schultz Peak and rotated the compass to 288° to find our location on the Weatherford Trail.

As I said, this didn't give perfect results but it was close enough for me to know that I was only a half mile or so from the Doyle Saddle. I'm pretty sure the error was due to inexperience shooting bearings and not with the map or the compass.

Comments

[Carrie]

Del,

Thanks for pointing the way to the info and I will miss your charts. I have to agree, though....after looking at the Zillow chart for the past 10 years, it was awful to see that in just 9 years, houses in Flag more than doubled in price. That's just egregious in the extreme. The prices haven't fallen far enough to reflect what a more normal rise in that 9 years should have been. Thanks for the insights you have shared; I still like coming here to read things you say even without the housing chart.

Carrie