How to Use a Compass with a Topographic Map: Triangulation & Route Planning

Before you do anything else with a topographic map, you must orient it so that the map's depicted terrain matches the physical landscape around you:

1. Lay the topographic map flat on the ground or a flat rock.
2. Place your compass on top of the map.
3. Rotate the entire map sheet until the North-South grid lines on the map run parallel to your compass needle pointing North (accounting for local declination).
4. Look up: the valleys, ridges, and peaks in front of you will now perfectly align with the contour lines printed on the paper.

When you want to hike from your current campsite (Point A) to a backcountry lake (Point B) shown on the map:

• Lay the straight edge of your baseplate connecting Point A and Point B, with the direction-of-travel arrow pointing toward the lake.
• Rotate the compass bezel until the orienting lines match the North-South grid lines of the map.
• Read the degree azimuth from the dial.
• Adjust for Declination: If your map has 12° West declination, add 12° to convert the grid bearing to a magnetic field bearing.
• Hold the compass in hand, turn until the needle aligns with the dial, and follow your bearing into the field.

If you are lost on a hillside and don't know where you are along a trail, you can find your exact grid coordinates using resection (triangulation):

1. Identify two prominent, known landmarks in the real world that also appear on your map (e.g., Mount Baldy and Eagle Rock).
2. Take an accurate magnetic bearing to the first landmark (e.g., Mount Baldy at 040°).
3. Calculate the back-bearing (040° + 180° = 220°). Convert to grid bearing.
4. Place your compass on Mount Baldy on the map and draw a line along 220° passing through the mountain peak.
5. Now take a bearing to the second landmark (e.g., Eagle Rock at 310°). Compute its back-bearing (310° - 180° = 130°). Convert to grid and draw a line passing through Eagle Rock.
6. Your Location: The exact spot where the two lines cross on the map is where you are standing. Adding a third landmark creates a small triangle ("cocked hat") for maximum precision.

Topographic lines represent constant elevation above sea level. Reading their density allows you to anticipate terrain difficulty before taking a single step:

• Closely spaced contours: Indicate steep cliffs, bluffs, or canyon walls. Walking straight along a compass bearing through these lines may lead into sheer vertical drops.
• Widely spaced contours: Indicate gentle slopes, meadows, or valley floors.
• V-shaped contours pointing uphill: Indicate stream beds or drainage gullies. V-shapes pointing downhill indicate mountain spurs or ridgelines.