Posted June 26, 2026

A manual inclinometer probe is one of the most widely used instruments in geotechnical monitoring — yet the path from raw readings to meaningful displacement data is often misunderstood.

This guide walks through the fundamental concepts of what an inclinometer measures, how those measurements are processed, and how to read the two most common output plots.

The lowering of a manual inclinometer probe into grooved casing installed in a borehole.

What does an inclinometer read?

A manual inclinometer probe works by lowering a biaxial sensor probe through a grooved casing installed in a borehole. As the probe is pulled upward at fixed intervals, it records the tilt angle of the casing at each depth. These angles, taken together, build a picture of the casing's shape along its entire length.

The probe measures tilt in two orthogonal directions, referred to as the A-axis and the B-axis. The inclinometer wheels ride in one pair of grooves to measure the A-axis, then in the perpendicular grooves to measure the B-axis.

Each angle reading monitors the inclination of the casing at a single depth. To convert those angles into horizontal displacement, a simple trigonometric relationship is applied:

Where:

• D = Horizontal displacement over the sensor segment
• L = Gauge length of the sensor segment
• θ = Tilt angle measured by the sensor at each depth

This calculation is performed at each depth interval along the casing, giving a displacement value for each segment.

Sensor Orientation and the Meaning of A+ and B+

Before any meaningful interpretation can take place, it is essential to understand how the inclinometer casing was oriented during installation. One pair of grooves is designated as the A-direction, and the opposite pair as the B-direction. By convention:

• A+: points toward one of the A-axis grooves
• A−: points in the opposite direction
• B+ and B− follow the same logic for the orthogonal axis

Best practice is to orient the casing so that the A+ direction faces the anticipated direction of movement. For example, the downslope direction in a slope stability application or the excavation side in a retaining wall monitoring program. When this convention is followed, positive A readings directly indicate movement toward the area of concern, making data interpretation more intuitive and reducing the chance of a sign error.

Note: If the casing orientation is not documented at installation, subsequent data interpretation becomes significantly more difficult.

Establishing a Baseline

Because no borehole is perfectly vertical and no casing is installed without some initial inclination, raw readings from any single survey campaign are not useful by themselves. All displacement calculations are relative to an initial reference measurement known as the baseline.

The baseline represents the casing's initial geometry at the start of the monitoring program, before any ground movement of interest has occurred. All subsequent surveys are compared against the baseline. The difference between the later reading and the baseline reading at each depth yields the displacement value reported in graphs and tables.

Selecting a good baseline is critical. A baseline taken during or immediately after installation may still reflect drilling disturbance, casing settlement, or grout curing effects. For this reason, it is strongly recommended to collect at least three independent survey readings before selecting a baseline. Reviewing these early readings for consistency — and discarding any that appear anomalous — gives confidence that the chosen baseline reflects the true undisturbed geometry of the casing.

The corrected tilt angle at each depth for any given survey is therefore:

This corrected value is what feeds into the displacement calculations described above.

Graph visualization

Once displacement values have been calculated at each depth interval, the data is typically presented in one or both of the following standard plot types.

Incremental Displacement

The incremental displacement plot displays the displacement contribution of each individual segment at each monitoring date, compared to the baseline.

On a typical incremental displacement graph:

• Horizontal axis: displacement (mm or inches)
• Vertical axis: depth
• Each line: one monitoring date
• Each point: one measurement depth

The incremental displacement plot is particularly useful for identifying where the casing movement is concentrated. A segment that is tilting progressively more with each survey will appear as a cluster of lines that grow wider at that depth. Sudden spikes or anomalous offsets at a particular depth often signal the location of a shear zone or discontinuity.

An incremental displacement plot, useful for identifying where casing movement is concentrated.

Cumulative Displacement

The cumulative displacement plot shows the total horizontal displacement at each depth, calculated by summing the incremental displacements from the bottom of the casing upward. The bottom of the casing is assumed to be anchored in stable material below any zone of movement and is treated as a fixed reference point with zero displacement.

Summing upward from this fixed base, each successive depth interval adds its increment, so the cumulative displacement at the top of the casing reflects the total relative movement of the casing head with respect to the bottom.

The cumulative displacement plot is most effective for visualizing the overall shape of the deforming casing and for identifying the depth of a shear zone.

The cumulative displacement at each depth z is:

For all segments (i) from base to depth (z).

A cumulative displacement plot shows the total horizontal displacement at each depth.

Practical Notes for Data Review

A few additional points are worth keeping in mind when reviewing inclinometer data:

Check Both Axes

Movement is rarely perfectly aligned with the A-axis. Reviewing the B-axis data alongside the A-axis can reveal oblique movement directions.

Look for Repeatability, Not Just Magnitude

A single survey with an unexpected displacement spike may reflect a data quality issue rather than actual movement. Consistent trends across multiple survey dates are much more meaningful than any single reading.

Watch the Base

If the base of the casing is moving because the borehole was not drilled deep enough to anchor below the zone of movement, all cumulative displacement calculations will be underestimated.

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