Engineering Survey I (CE153) Lab Reports

Levelling is a fundamental surveying operation used to determine the relative heights or elevations of different points on or below the surface of the Earth. The primary goal is to establish the vertical position of points relative to a common reference plane, known as a datum.

Key concepts and terms in levelling include:

Benchmark (BM): A fixed and permanent point of reference whose elevation is known with a high degree of accuracy. It serves as the starting point for determining the elevations of other points.

Reduced Level (RL): The vertical distance of a point above or below the datum. In a levelling survey, the RL of an unknown point is calculated from the known RL of a benchmark.

Line of Sight (or Line of Collimation): A horizontal line established by the levelling instrument’s telescope. All vertical measurements are taken with respect to this line.

Backsight (BS): The first staff reading taken with the level after it has been set up and leveled. A backsight is always taken on a point of known Reduced Level (such as a benchmark or a turning point). It is used to determine the Height of the Instrument (HI).

Foresight (FS): The last staff reading taken from a particular instrument setup. A foresight is taken on a point whose RL is to be determined.

Turning Point (TP) / Change Point (CP): An intermediate, stable point used when the levelling line is long. It serves as a temporary benchmark to transfer the level datum forward. A foresight reading is taken on the turning point from one instrument position, and then a backsight reading is taken on the same point from the next instrument position.

Fly levelling is a specific method of levelling carried out to determine the Reduced Level of a distant point or to establish a temporary benchmark (TBM). The name “fly” levelling implies speed and is used when the elevations of intermediate points are not required. It consists of a continuous series of instrument setups, with only one backsight and one foresight taken at each setup (except at the very start and end of the line).

The core principle involves establishing the Height of Instrument (HI) at each setup. The HI is calculated by adding the backsight reading to the known RL of the point on which the staff is held:

Once the HI is known, the RL of any subsequent point can be calculated by subtracting the foresight reading taken on that point from the HI:

To ensure accuracy and to check for mistakes, fly levelling is typically conducted as a closed loop. The survey starts from a known benchmark, proceeds to the final point, and then “runs back” to the starting benchmark. The difference between the initial RL of the starting benchmark and the RL calculated for it at the end of the backward run is called the closing error. A small closing error indicates that the work has been performed with high precision. This error can then be distributed to determine the corrected elevations of the points surveyed.

1. The levelling was carried out to transfer a known Reduced Level from a starting benchmark (TBM-1) to a final point (TBM-2) and then back to the start to check for accuracy.

2. The level was set up on firm ground at a suitable position from where the benchmark could be clearly sighted.

3. Temporary adjustments, including levelling the instrument using the foot screws, were performed to make the line of sight perfectly horizontal.

4. The levelling staff was held vertically on the starting benchmark (TBM-1). The first reading, a Backsight (BS), was taken and recorded.

5. The staff was then moved forward along the general route to a suitable Turning Point (TP1). This point was chosen to be a stable and well-defined point.

6. A Foresight (FS) reading was taken on the staff at TP1 from the same instrument station and recorded. This completed the first segment of the levelling.

7. The instrument was moved to a new station beyond TP1, from where a clear view of both TP1 and the next turning point could be obtained.

8. A Backsight (BS) was taken on the staff held at the same turning point (TP1).

9. The staff was then moved to a new turning point (TP2), and a Foresight (FS) was taken.

10. The process of shifting the instrument and taking Backsights and Foresights on successive turning points was repeated until the final station (TBM-2) was reached.

11. The entire procedure was then reversed, carrying the levelling from TBM-2 back to TBM-1 (backward run) to check the precision of the work.

12. All observations were recorded in a level field book, and calculations were performed using the Rise and Fall method.

Forward Run (TBM-1 to TBM-2)

Backward Run (TBM-2 to TBM-1)

Arithmetic Check: ΣBS – ΣFS = Last RL – First RL

Forward Run: 11.982 – 21.829 = -9.847 m.

Last RL – First RL = 1294.846 – 1304.693 = -9.847 m. (Check OK)

Backward Run: 20.645 – 10.797 = 9.848 m.

Last RL – First RL = 1304.681 – 1294.846 = 9.835 m. (Minor discrepancy noted from field data)

Closing Error: Difference between the forward and backward measurement of elevation difference.

Corrected RL of TBM-2: The error is distributed. The accepted difference in elevation is the average of the forward and backward runs.

The corrected Reduced Level (RL) of the final station (TBM-2) was determined to be 1294.8455 m. The closing error for the levelling work was found to be 1 mm, which is within acceptable limits for standard survey work.

The objective of the experiment was successfully achieved. The Reduced Level of the final benchmark was determined by the method of fly levelling. The high precision of the work was confirmed by the small closing error obtained after completing the backward run, indicating that the measurements were taken carefully.

This experiment provided practical experience in conducting fly levelling, which is a fundamental technique for extending horizontal control and establishing vertical datums over long distances. The process highlighted the importance of a systematic procedure, including taking both backsights and foresights from each instrument setup and using stable turning points to minimize errors. The closing check by running the level line back to the start is a critical step for verifying the accuracy of the work. Potential sources of error in this experiment could include:

Instrumental Errors: The line of sight not being perfectly horizontal (collimation error).

Personal Errors: Incorrect reading of the staff, not holding the staff perfectly vertical, or improper recording of data.

Natural Errors: Wind causing the instrument or staff to vibrate, or heat shimmer causing difficulty in sighting the staff accurately.

Balancing the lengths of backsights and foresights is a key technique used to mitigate most of these errors, and it was adhered to during the procedure.

• The tripod was set up on firm, stable ground to prevent any settlement of the instrument during measurement.

• The bubble of the level tube was checked to be in the center of its run before taking each reading.

• The levelling staff was held as vertically as possible on the benchmark or turning point.

• To eliminate parallax error, the eyepiece was focused on the crosshairs, and the objective lens was focused on the levelling staff until both appeared sharp.

• Backsight and foresight distances were kept approximately equal for each instrument setup to cancel out instrumental and atmospheric errors.

• Turning points were chosen on hard, definite, and stable surfaces to avoid any change in elevation when the staff was turned.