TL;DR
Triangulation finds your position on a map by drawing lines from two or more identified landmarks through your position. Take a bearing to each landmark, convert to a back bearing, draw lines from each landmark on the map — you're where the lines cross. Two landmarks give an approximate fix. Three confirm it. Accuracy depends on bearing precision and landmark spacing.
The Principle
When you take a bearing to a known landmark, you're telling yourself: "I am somewhere on the line from that landmark in the direction opposite my bearing." Draw that line on the map. You're on it.
Take a second bearing to a different landmark. Draw a second line. You're on that line too. The intersection of the two lines is your position.
The technique is called resection in military navigation. Triangulation is the popular term. Both mean the same thing: using multiple bearing lines to fix your position.
Prerequisites
What you need:
- A compass with a rotating bezel (baseplate or lensatic)
- A topographic map of the area
- Two or more clearly identified landmarks visible from your position and marked on the map
- A pencil and straight edge
Landmarks that work:
- Named mountain peaks and hilltops (easy to identify on topo maps)
- Towers (radio towers, fire towers, cell towers)
- Building corners of notable structures
- River/road intersections visible and mapped
- Sharp ridgeline points or saddles with distinctive profiles
Landmarks that don't work:
- Features you're not certain of ("I think that's the right hill")
- Features visible in the distance but not distinctly marked on the map
- Features that look similar to several nearby features
If you're uncertain about a landmark, don't use it. An incorrect landmark produces a wrong position fix, which is worse than no fix.
The Procedure
Accounting for Magnetic Declination
If your compass does not have a declination adjustment feature, you must account for declination when transferring bearings to the map.
The rule: Map grid lines point to grid north (very close to true north). Your compass points to magnetic north. The difference is local magnetic declination — printed in your map's margin as a diagram.
When drawing lines on the map from compass bearings:
- If declination is east (positive), subtract the declination value from your magnetic bearing before drawing
- If declination is west (negative), add the declination value
Example: You take a bearing of 070 degrees magnetic. Your local declination is 12 degrees east. Adjust to: 070 - 12 = 058 degrees grid. Draw your line from the landmark at 058 degrees (the back bearing being 058 + 180 = 238 degrees from the landmark toward you).
A compass with a declination adjustment screw eliminates this calculation — set the declination once and the compass reads grid north directly.
Landmark Geometry
The angle between your two landmarks affects the accuracy of your position fix.
Ideal: Landmarks separated by 90 degrees in your field of view. The two bearing lines cross at a clean right angle, and small errors in bearing measurement create only small position errors.
Acceptable: 60-120 degrees of separation.
Poor: Landmarks less than 30 degrees apart or more than 150 degrees apart. The lines nearly parallel each other, and a small bearing error produces a large shift in the intersection point.
Why this matters: If both landmarks are nearly in front of you, both lines run approximately toward you from the same direction. The intersection is an elongated diamond rather than a precise cross. Your accuracy might be within half a mile rather than a few hundred meters.
When choosing landmarks, deliberately pick features spread around your compass horizon for the most reliable fix.
Partial Fixes
In terrain where two landmarks aren't available, partial fixes still help:
One bearing + a line feature: If you know you're on a trail, road, or stream, one bearing to a visible landmark gives you the point where your line of position intersects the linear feature. This is a reliable fix.
One bearing + rough dead reckoning: Combining a single bearing with your estimated travel distance and direction since your last known point narrows your likely position to a small area. Not precise, but much better than a pure guess.
Elevation + one bearing: If you know your elevation (from an altimeter or altimeter watch), you can eliminate all points on your bearing line that don't match that elevation. This sometimes gives a unique fix.
Practice
Triangulation requires practice to do confidently in the field. Do a dry run in familiar terrain: stand in a location you know on the map, take bearings to two identifiable landmarks, triangulate your position, and compare the result to your actual position. The difference is your current skill level's error budget.
Most people are within 100-200 meters of the correct position on their first attempt, improving to within 50 meters with practice. That's navigational accuracy good enough to find water sources, avoid hazards, and reach rendezvous points.
Sources
- U.S. Army FM 3-25.26: Map Reading and Land Navigation
- National Geographic Society - Map Skills
Frequently Asked Questions
How many landmarks do I need to triangulate?
Two landmarks give you a theoretical position fix where the two bearing lines intersect. Three landmarks are better — they create a small triangle of error (the 'cocked hat') that confirms your position or reveals bearing measurement error. If the three lines form a large triangle, you have bearing error to resolve. If they converge tightly, you have a reliable position fix.
What landmarks work best for triangulation?
Landmarks that are: unambiguous (you're certain which feature it is on the map), visible clearly (you can take a precise bearing), identifiable on the map (a marked hilltop, a named peak, a tower), and spread around the compass rather than clustered in one direction. Landmarks 60-120 degrees apart give the best intersection geometry. Two landmarks at nearly the same bearing produce a very elongated intersection — small bearing error creates large position error.
What if I can only identify one landmark?
One bearing line gives you a 'line of position' — you're somewhere along that line. Combined with your last known position and dead reckoning estimate, one bearing can still narrow your location significantly. Alternatively, if you know you're on a trail, road, or ridgeline, one bearing off a visible landmark plus the linear feature gives a position fix at their intersection.