Clinometer: How to Measure Slope and Height
A clinometer, or inclinometer, measures an angle of incline or decline relative to gravity. Surveyors, foresters, conservation planners, builders, and geologists use it to estimate slope, elevation, or object height.
Degrees and percent slope are different
Many instruments show two scales. Degrees describe the angle from horizontal. Percent slope describes vertical change per 100 units of horizontal distance:
percent slope = 100 × rise ÷ horizontal run.
A 45° angle equals a 100% slope, not 45%. Check the scale label before recording a value.
How to measure ground slope
- Choose two points that represent the section of slope you need to measure.
- Mark the same height at both points, using equal stakes or a partner's eye-level mark.
- Stand at one point and sight the equal-height mark at the other.
- Allow the indicator to settle, then read either the degree or percent scale.
- Repeat from the opposite direction. Large disagreement suggests poor alignment, an uneven surface, or instrument error.
Do not confuse distance along the ground with horizontal run when calculating percent slope.
Estimating object height
If you know the horizontal distance d to a vertical object, measure the angle θ to its top. When the base is level with your position, the height above eye level is approximately:
height above eye level = d × tan(θ).
Add your eye height to estimate total height. If the base is above or below you, measure a second angle to the base and account for both vertical components. Use horizontal distance, not sloped distance.
Example: at a horizontal distance of 20 m, a 30° sight angle gives 20 × tan(30°) ≈ 11.55 m above eye level. With an eye height of 1.6 m, the estimate is about 13.15 m.
Instrument types
- Mechanical: a pendulum, weighted disk, or bubble indicates inclination.
- Optical: a sighting window combines the target view with a scale.
- Digital: electronic sensors show angle or percent slope directly.
- Phone sensors: useful for rough checks, but the case, sensor calibration, and placement can introduce error.
Common error sources
- Reading the degree scale when the formula expects percent slope, or vice versa.
- Using line-of-sight distance instead of horizontal distance.
- Tilting the instrument sideways or sighting unequal reference heights.
- Measuring a poorly defined tree top or base.
- Relying on a phone case edge that is not parallel to the sensor axis.
For engineering layout, property boundaries, safety decisions, or regulatory work, use an appropriate surveyed method and calibrated instrument.