What it does
A geologist’s compass-clinometer measures how rock surfaces and lines are oriented. Your phone has the same sensors: an accelerometer that knows which way is down and a magnetometer that knows magnetic north. This page uses them to measure:
- Planes (bedding, joints, foliation, faults): lay the phone flat on the surface. The page gives the strike and dip by the right-hand rule (the plane dips to your right when you face along the strike) or the dip direction and dip.
- Lines (lineations, fold axes, slickenlines): lay the phone’s long edge along the line. The page gives the plunge and trend, pointing down the line.
Each reading is averaged over one second of sensor data and shows its spread (how much the phone moved). Directions are corrected by the magnetic declination you type, so they are from true north. Readings go into a field log with your note and, if you press the button, your position. The log stays in this browser, works offline, and downloads as CSV, including files ready for the Stereonet plotter.
How to use it
- Type the declination for your area (east positive). Your map’s margin or a magnetic-model calculator gives it; it changes slowly over the years.
- Press Turn on the sensor. On iPhones, allow Motion & Orientation access.
- For a plane, lay the phone’s back flat on the surface (screen up; screen down also works on an overhang). For a line, lay the long edge along it.
- Type a note if you like, press Take a reading and hold still for a second.
- Repeat. Download the log or the stereonet files when you are done.
Worked example (demo)
On a computer, open the demo and tick Use these angles. With alpha 300°, beta 40° and gamma 0°, the phone’s top points to 060° and is raised 40°, so the surface dips 40° towards 240°: the page shows Strike/dip 150/40.0 (dip direction/dip 240/40.0). Type a declination of 5 and it becomes 155/40.0. With the declination back at 0 and Measure a line chosen, the same position reads plunge → trend 40.0 → 240 along the long edge. To check a plane, type dip direction 120 and dip 35, press Set the angles for this plane, and the page reads 030/35.0.
Accuracy
- Phone compasses are easily thrown off by magnets, steel (hammers, cars, phone cases with magnets) and power lines. Keep metal away and compare with a real compass now and then. Errors of 5° or more in direction are common; dips are usually better than 1°.
- A phone case or a rough surface tilts the phone: use a notebook or clipboard laid on the surface for an average plane.
- On a near-horizontal plane the dip direction is poorly defined; on a near-vertical line the trend is.
FAQ
Does it need a signal? No. Once the page has loaded it works offline. Position comes from the phone’s GPS when you ask.
Where is my log kept? Only in this browser on this device. Nothing is sent anywhere. Delete readings one by one, or all of them.
Which north does it use? Phones report magnetic north (Android and iPhone compass readings); adding your declination gives true north.
Where the numbers come from
- Device orientation angles and the rotation R = Rz(alpha)·Rx(beta)·Ry(gamma) from the W3C DeviceOrientation Event specification; checked against SciPy's Rotation (intrinsic Z-X'-Y'' Euler angles) for 48 angle sets.
- Strike and dip (right-hand rule), dip direction, trend and plunge from the plane's normal and the line's direction vector: standard structural geology, written by fibub.
- Magnetic declination is typed by the user; no magnetic-field model or table is included. The demo angles are made up.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.