How to Use a Brunton Compass
The Brunton pocket transit has survived 130 years of technology because it does three jobs perfectly: bearings, strike, and dip. GPS tells you where you are; the Brunton tells you where the rock is going. Here's the field method, start to finish.
01Set your declination first
Magnetic north and true north disagree by a local angle called declination. Look yours up (NOAA publishes it), then rotate the graduated ring by that amount using the screw on the side. Skip this and every bearing you take all day is systematically wrong.
02Take a bearing
Open the lid ~135°, sight the target through the peep sight and lid window, and center the bubble in the round level. When the needle settles, read the white (north-seeking) end. Remember the Brunton is graduated backwards — E and W are swapped on the dial — precisely so the needle reads the bearing directly. Trust the dial, not your instinct.
03Find the strike of a bedding plane
Strike is the compass direction of a horizontal line on the inclined rock surface. Press the long edge of the open Brunton flat against the bedding plane, then tilt until the round bull's-eye level centers. The edge now lies horizontal on the plane; read the needle. That's your strike.
04Measure the dip
Dip is how steeply the plane tilts, measured perpendicular to strike. Place the Brunton on edge on the rock, 90° to your strike line, and rotate the clinometer arm (the lever on the back) until the long tube level centers. Read the dip angle on the inner scale — and note the dip direction (e.g., 35° SE).
05Record it right
Standard notation: strike/dip, quadrant or azimuth — 'N45E, 35°SE' or '045/35SE.' Use the right-hand rule if your outfit does: strike direction such that dip falls to your right. Consistency beats convention; mixing systems in one mapping campaign creates garbage.
06Check yourself with three measurements
Rock surfaces are irregular. Take strike and dip at three spots on the same exposure and average. If they scatter more than ~5°, you're measuring surface roughness, not structure — find a cleaner face or use a field book as a planar proxy on rubbly surfaces.
Master these four moves and a $500 brass instrument outperforms a dead phone every day of the week. The structural data driving million-dollar pit-slope decisions still starts exactly here.
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