In my last post on orienteering I explained how to use a single compass bearing to find your location on a trail or road. With this post I'll show how to use multiple bearings to triangulate your location anywhere, whether you're on a trail or not. This is a useful technique if you're hiking off-trail or have become lost.
Here is a panoramic photo I took from somewhere up on the San Francisco Peaks with three compass bearings in red:
From left to right the bearings are to Freemont Peak, Humphreys Peak, and Aubineau Peak. To determine your location using the compass bearings you first have to convert them to back-bearings and then adjust for magnetic declination. Remember, to convert to a back-bearing you add/subtract 180° to obtain the bearing as it would be for someone standing on Freemont/Humphreys/Aubineau back to your current location. Finally, you add the magnetic declination as determined from your map's legend. This gives you the True North back-bearing which can be used on a topographic map:
Freemont Peak = 223-180+13 = 56°TN
Humphreys Peak = 298-180+13 = 131°TN
Aubineau Peak = 334-180+13 = 167°TN
With the True North back-bearings you can then plot course lines on the map as I've done in the Topo! screenshot below (click for full size version):
The intersection of the three back-bearings points to my location, which turns out to be the Doyle Saddle (no surprise there). The topographic maps all have Doyle Saddle and Freemont Saddle swapped so ignore the label next to the location. When I made the panoramic photo I walked downslope a little to avoid getting too much of the ground in the shot so that's why my location is not precisely in the center of the saddle.
I made the above plot using my copy of National Geographic Topo!, which is very nice software if you're an outdoorsy sort of geek, but not exactly practical in the field. Instead, you'd use a protractor or else orient the map to magnetic north and then use your compass to plot the course lines as I discussed in my previous post. With some practice you can get very close to what Topo! will give you.
This example used three compass bearings but it's perfectly acceptable to use only two bearings. Your accuracy won't be quite as precise but you should be close enough. The third bearing acts as a check on the other two: if instead of converging to a tight point on the map you get a big triangle then you know that one (or more) of the bearings is off.
Comments