Surveyors have long relied on portable instruments to measure slopes and vertical angles in the field. Among these, the De Lisle’s clinometer stands out as a specialized tool for rapid gradient measurement. Developed as a variant of the traditional clinometer, this instrument shares similarities with the burel hand level used in leveling but incorporates a unique mirror-and-arc mechanism for direct gradient readings. Whether setting out road gradients, checking drainage slopes, or verifying earthwork inclinations, this instrument offers a practical solution for civil engineers and surveyors.
What Is a De Lisle’s Clinometer?
A De Lisle’s clinometer is a hand-held surveying instrument designed to measure vertical angles, determine ground slopes, and set out specified gradients. It belongs to the broader family of clinometers used to measure angles of inclination relative to gravity. The De Lisle design uses a reflective mirror and a graduated semicircular arc to provide direct gradient readings without requiring trigonometric conversion. The instrument is suspended from the observer’s thumb and held at arm’s length, making it one of the most portable devices available for field surveying operations.
The device proves especially useful in topographic surveying, road construction, and drainage works where quick slope assessments are needed. Unlike complex instruments such as theodolites or total stations, the De Lisle’s clinometer requires no tripod, no leveling screws, and no power source. Its simplicity makes it ideal for preliminary surveys where speed matters more than absolute precision. The instrument uses the reflection of the observer’s eye as a reference for alignment, with the gradient read directly from a scale, eliminating angle-to-slope calculations.
Components and Construction Details
The De Lisle’s clinometer consists of three main assemblies that work together to measure gradients. Understanding each component is essential for proper field operation.
1. The Frame and Mirror Assembly
The main body is a simple frame similar in construction to a burel hand level. It carries a mirror that extends across half the frame width, while the other half remains open for direct sighting. The edge of the mirror functions as a vertical reference line for alignment. The entire frame is designed to be suspended in gimbals, allowing it to hang freely under gravity and maintain a consistent orientation regardless of how the observer holds the instrument.
2. The Semicircular Arc
Attached to the lower end of the frame is a heavy semicircular arc that carries the gradient scale. The arc is graduated in gradient ratios rather than degrees, with markings typically ranging from 1 in 5 for steep slopes to 1 in 50 for gentle slopes. It is mounted on a vertical axis that can be rotated toward the observer for rising gradients or away from the observer for falling gradients, which allows the operator to distinguish between uphill and downhill slopes.
3. The Radial Arm and Sliding Weight
A radial arm is fitted to the center of the arc with a beveled edge that acts as an index pointer. Moving the arm along the arc inclines the mirror relative to the vertical plane. The arm carries a sliding weight with a dual purpose. When moved to the outer stop at the end of the arm, it counterbalances the arc and brings the mirror vertical, corresponding to a horizontal line of sight. The optical principle behind the instrument is that the inclination of the line of sight from horizontal equals the mirror’s inclination from vertical, allowing direct gradient readings from the scale.
Procedure for Measuring an Existing Gradient
Using a De Lisle’s clinometer to measure an existing gradient requires a systematic procedure based on optical alignment. The following steps outline the correct field method:
- Prepare the instrument Slide the weight to the inner stop of the radial arm. For rising gradients, rotate the arc forward toward the observer; for falling gradients, rotate it away.
- Suspend and sight Hold the instrument by the thumb using the suspension ring at arm’s length. Position it so the observer sees the reflected image of their own eye at the edge of the mirror.
- Align the target Move the radial arm along the arc until the object sighted through the open half of the frame coincides with the reflection of the eye in the mirror. This indicates the line of sight matches the angle set by the arm.
- Read the gradient Note the reading on the arc against the beveled edge of the radial arm. The value is expressed as a gradient ratio such as 1 in 20 and can be converted to degrees if needed.
For improved accuracy, a vane or target equal to the height of the observer’s eye should be placed at the target point. This eliminates errors caused by differences between eye height and the observed point. Related instruments such as the theodolite used in land surveying follow similar alignment principles but offer greater precision for angular measurements.
Technique for Setting Out a Specified Gradient
Setting out a point on a predetermined gradient requires a different procedure. Here the surveyor knows the target gradient and must find the corresponding ground point rather than measuring an unknown slope. This is common in road construction, drainage installation, and earthwork grading.
- Set the radial arm to the desired gradient reading, for example 1 in 20 for a drainage slope.
- Rotate the arc forward for rising gradients or backward for falling slopes.
- Drive a peg at the starting point where the gradient begins.
- Place a vane equal to the observer’s eye height at the other end where the target point will be established.
- The operator sights through the clinometer and signals an assistant to raise or lower the vane until it aligns with the reflected eye image in the mirror.
- Drive the peg at the aligned location so the top is level with the bottom of the vane.
This procedure can be repeated at regular intervals along a line to establish continuous gradients for road subgrades, drainage channels, or earthwork platforms. The De Lisle’s clinometer is especially valuable for gradient control in leveling operations where quick verification is needed across variable terrain.
Comparison with Other Gradient-Measuring Instruments
Surveyors have several options for measuring slopes and vertical angles. The following table compares the De Lisle’s clinometer with other common surveying instruments:
| Instrument | Precision | Setup Time | Gradient Reading | Best Application |
|---|---|---|---|---|
| De Lisle’s Clinometer | Moderate (1 in 5 to 1 in 50) | Instant (hand-held) | Direct gradient ratio | Preliminary surveys, drainage slopes |
| Burel Hand Level | Low to moderate | Instant (hand-held) | Requires conversion | Rough leveling, site reconnaissance |
| Dumpy Level | High | 10-15 minutes | Staff reading conversion | Precise leveling, construction control |
| Theodolite | Very high | 15-20 minutes | Degree-minute-second | Precision angle measurement, traversing |
| Total Station | Highest | 15-20 minutes | Digital readout | Large-scale surveys, earthwork volumes |
The De Lisle’s clinometer occupies a niche between basic hand levels and precision optical instruments. It offers better accuracy than a simple hand level for gradient work while requiring none of the setup time associated with a theodolite used for bearing measurements. The direct gradient readout eliminates field calculations, reducing conversion errors on site.
For work requiring high precision such as foundation leveling or road pavement profiling, the instrument’s moderate accuracy makes it best suited for preliminary rather than final setting out. Modern electronic clinometers offer significantly higher precision at greater cost and complexity.
Field Applications and Practical Considerations
The De Lisle’s clinometer finds application in several civil engineering tasks where rapid gradient assessment is needed. Common uses include:
- Road construction Setting out and verifying longitudinal gradients on earthworks, subgrades, and pavement layers before final surfacing.
- Drainage works Checking slopes of drain pipes, open channels, and culverts to ensure adequate flow velocities and self-cleansing gradients.
- Landscape grading Establishing surface slopes for stormwater runoff, swales, and retention basins during site development.
- Pipeline installation Verifying trench gradients for gravity sewer lines where minimum slope requirements must be maintained.
- Preliminary surveying Conducting quick reconnaissance surveys to assess site topography before detailed surveys are commissioned.
Several factors affect accuracy in the field. Wind can cause the suspended instrument to swing, making alignment difficult. Bright sunlight can reduce the visibility of the reflected eye image. The observer must hold the instrument steady at arm’s length, which is fatiguing over extended periods. Despite these limitations, the instrument remains valuable where speed and simplicity are prioritized over maximum precision.
The instrument also holds educational value for teaching optical alignment and gradient measurement principles. Civil engineering students who learn to use the De Lisle’s clinometer develop stronger intuition for gradient calculations and the optical principles used in field measurement. For surveyors working in remote locations with limited access to power, it offers the reliability of a purely mechanical instrument requiring no electricity, calibration, or software updates.
When selecting a gradient-measuring instrument for a project, surveyors should consider the required precision, distance to be covered, terrain conditions, and available setup time. For rapid assessments, the De Lisle’s clinometer provides an effective balance of speed and adequate accuracy. For final works requiring certification, conventional leveling instruments remain the standard. In summary, the De Lisle’s clinometer is an effective tool whose straightforward design based on a mirror, arc, and radial arm allows direct gradient readings with minimal setup, keeping it relevant for preliminary surveys and educational demonstrations even in the age of digital surveying.