Conducting accurate and reliable mapping is a fundamental responsibility of a land surveyor engaged in mine survey activities. The process involves detailed measurement and recording of the terrain and features within a mining area to support planning, development, and safety management. Among the core surveying techniques, levelling plays a critical role in determining height differences and establishing vertical control, which is essential for preparing precise mine maps. In the Kenyan mining sector, where terrain variability and safety considerations are paramount, mastering levelling techniques ensures that survey data supports efficient resource extraction and regulatory compliance.

3.1 Levelling

Levelling in mine surveying is the process of determining the relative heights of points on or near the earth’s surface. This technique is vital for creating contour maps, designing drainage systems, and planning excavation works in mines. In Kenya, where mining activities often occur in rugged and varied topography, accurate levelling helps surveyors produce reliable vertical data that informs safe and effective mine operations. The technique requires precise instruments and systematic procedures to minimize errors and achieve the desired accuracy.

3.1.1 Principles and Objectives of Levelling

Understanding the fundamental principles and objectives of levelling is crucial for mine surveyors to apply the technique effectively in the field. Levelling relies on establishing a horizontal line of sight from a known reference point to determine the elevation of other points. The primary objectives focus on obtaining accurate height differences and ensuring vertical control across the mining site.

Principles of Levelling

• Horizontal Line of Sight: Levelling instruments are designed to provide a perfectly horizontal line of sight, which is the baseline for measuring height differences. This ensures that any vertical displacement is accurately detected relative to this line.
• Datum Reference: All height measurements relate to a fixed reference surface called a datum, often mean sea level or a locally established benchmark, to maintain consistency across surveys.
• Height Differences Measurement: Levelling determines the difference in elevation between two or more points rather than absolute heights, enabling relative positioning within the mine.
• Systematic Observation: Levelling requires systematic observations with backsights and foresights to calculate the height differences stepwise along a traverse or grid.
• Error Minimization: Accurate levelling involves procedures to reduce instrumental, observational, and natural errors, including repeated readings and instrument calibration.

Objectives of Levelling

• Establish Vertical Control: Provide a network of points with known elevations that serve as references for all subsequent mine surveying activities.
• Create Contour Maps: Generate detailed representations of the mine’s topography to assist in planning excavation and drainage.
• Support Design and Construction: Inform the design of mine infrastructure such as ramps, benches, and drainage channels by providing reliable height data.
• Monitor Ground Movement: Detect subsidence or deformation within the mine by comparing levelling data over time.
• Ensure Safety Compliance: Facilitate the design of safe working slopes and drainage systems, reducing risks associated with unstable ground conditions.

3.1.2 Types of Levelling Used in Mine Surveying

Mine surveyors employ various levelling methods, each suited to different requirements and site conditions. Selecting the appropriate levelling type depends on the accuracy needed, terrain complexity, and available equipment.

Spirit Levelling

Spirit levelling is the most common and precise method used in mine surveying. It involves a dumpy or automatic level instrument and a graduated levelling staff. The surveyor reads the staff at different points to calculate height differences relative to a benchmark. This method is preferred for establishing primary vertical control and detailed contouring because of its high accuracy.

Trigonometric Levelling

Trigonometric levelling uses angular measurements and distance to calculate height differences. By measuring vertical angles to a target point and the horizontal distance from the instrument, the height difference is computed using trigonometric formulas. This method is useful in inaccessible or steep areas where direct levelling is difficult. However, it is less accurate than spirit levelling and is often used to complement it.

Barometric Levelling

Barometric levelling estimates elevation differences based on atmospheric pressure readings from a barometer. Since atmospheric pressure decreases with height, changes in pressure readings can approximate height differences. Although quick and useful for reconnaissance surveys, barometric levelling lacks the precision required for detailed mine mapping and is rarely used for final data.

Differential Levelling

Differential levelling involves measuring height differences between points over a long distance, often combining several levelling setups. This method is essential for establishing a vertical control network that spans the entire mining area. It requires careful planning to ensure continuity and accuracy across the traverse.

3.1.3 Equipment for Levelling in Mine Surveying

Successful levelling depends on the proper selection, handling, and maintenance of surveying instruments. The key equipment used in levelling includes the levelling instrument, levelling staff, tripods, and accessories.

Levelling instruments such as the automatic level are preferred in mine surveying for their ease of use and reliable horizontal line of sight. The levelling staff must be held perfectly vertical to avoid errors, which is aided by using a plumb bob. Tripods provide a stable platform for the instrument, minimizing vibrations and movement during readings.

Routine maintenance includes cleaning optical parts with lens tissue, checking and adjusting the instrument’s internal compensator, and ensuring the levelling staff’s graduations are clearly visible and undamaged. Proper storage in protective cases prevents damage during transport between mining sites.

3.1.4 Procedure for Conducting Spirit Levelling in Mine Surveying

Spirit levelling requires a systematic approach to obtain accurate height data. The procedure involves setting up the instrument, taking readings, recording data, and calculating reduced levels.

1. Set Up the Instrument: Position the levelling instrument on a tripod over stable ground, ensuring it is roughly level using the foot screws.
2. Level the Instrument Precisely: Use the circular bubble and foot screws to bring the instrument to exact horizontal level.
3. Take the Backsight Reading: Have an assistant hold the levelling staff vertically on a known benchmark or starting point. Read the measurement through the telescope and record it as the backsight (BS).
4. Take the Foresight Reading: Move the staff to the next point of interest. Read the staff through the instrument and record the foresight (FS) reading.
5. Calculate Height Difference: Determine the change in elevation between the instrument and the staff positions by subtracting FS from BS.
6. Determine Reduced Level (RL): Add or subtract the height difference from the known RL of the benchmark to compute the RL of the new point.

When conducting levelling over long distances, the instrument must be moved forward along the survey line, repeating the process to maintain accuracy. Careful recording in field books and cross-checking calculations minimize errors.

Example: At a county government quarry site, surveyors used spirit levelling to establish vertical control points to guide excavation depths. Accurate RLs ensured that the pit benches were cut to safe heights, minimizing collapse risk and facilitating drainage design.

Chapter Summary

This chapter on conducting mapping in mine surveying begins with levelling, which involves determining height differences between points to create accurate ground profiles essential for mine planning. The process of traversing follows, where a series of connected survey lines are measured to establish control networks that guide detailed mapping and ensure positional accuracy. Calculating resource volumes is then explored, focusing on how survey data is used to estimate the quantity of mineral deposits or excavated material, a critical factor for operational and financial decision-making. The chapter concludes with field measurements, emphasizing the practical techniques and instruments used to collect reliable data in the challenging mining environment. Together, these topics provide a comprehensive understanding of how precise mapping supports efficient mine development and resource management.