Compass — Dr. Norrie's

—

point the top of the phone at what you are reading

True heading—

Magnetic heading—

Declination applied—

Back-bearing—

Tilt (hold level)—

Steadiness—

Lay the phone flat on the surface — back against the rock

Strike

—

Dip

—

start the sensors, then place the phone on the plane

Dip direction—

Quadrant—

Dip from gravity alone—

Samples averaged—

Fisher κ—

α₉₅ confidence cone—

Readings

Dip is computed from gravity and is unaffected by iron. Only the azimuth uses the magnetometer, so a compass disturbance corrupts the strike but never the dip.

Declination

East +, west −

iPhone applies declination itself and reports true heading. Most Android browsers report magnetic, so set it here — or press the button above and the app derives it by comparing your compass with the sun's true azimuth. That is a genuine field calibration, not a lookup.

If the heading jitters by more than a few degrees while you stand still, something ferrous is near — a hammer on your belt, a vehicle, a fence, or the rock itself. Move ten metres and read again.