25.5 : Common Leveling Mistakes and Errors

A survey team is tasked with determining the elevation difference between points Point A and Point B, separated by uneven terrain. They use a leveling instrument and a leveling rod.

Common Mistakes

1. Misreading the Rod: During a backsight reading at Point A, the instrumentman observes the rod partially obscured by tall grass. Instead of reading 1.135 m, they mistakenly record 1.735 m due to the misalignment of the crosshair with the wrong graduation. This error adds 0.600 m to all subsequent measurements, causing systematic errors in elevation calculations.
2. Incorrect Turning Point Handling: At the intermediate turning point, the rodman moves the rod to a different location after the instrument is relocated. This new position has a slightly different elevation, leading to an incorrect foresight reading. As a result, the difference in elevation between Points A and B will be miscalculated.
3. Field Note Errors: When recording the readings, the rodman writes 0.785 m instead of 0.857 m. This transcription error further skews the elevation difference.
4. Misaligned Rod Extensions: The rodman uses an extended rod at Point B but fails to secure its upper section properly. The rod collapses slightly during the reading, introducing an additional error in the recorded foresight.

Common Errors

1. Rod Not Vertical: At the turning point, the rod leans slightly due to uneven ground. The smallest rod reading, which would indicate verticality, is not identified. This tilting increases the measured height and distorts elevation differences.
2. Unequal Backsight and Foresight Distances: The instrument is placed much closer to the backsight (Point A) than the foresight (Point B). For example, the backsight distance is 5.0 m, while the foresight distance is 25.0 m. This imbalance can result in inaccuracy in the calculated elevation due to increased effects of Earth’s curvature and refraction.
3. Bubble Tube Not Centered: The bubble on the leveling instrument is not centered correctly during setup. This results in the line of sight being slightly inclined, causing consistent errors in backsight and foresight readings.
4. Environmental Factors: Strong winds shake the leveling instrument, making maintaining a steady line of sight difficult. Heat waves from the sun can also distort the telescope’s view, particularly over long-sight distances, leading to fluctuations in rod readings.

If these mistakes and errors occur simultaneously, the cumulative effect can result in a significantly inaccurate elevation difference between Points A and B. For example, the actual elevation difference might be 3.000 m, but the survey team calculates 3.450 m due to the compounded errors, leading to a +0.450 m error.

Mitigation Steps:

1. Clear the line of sight to avoid obstructions and use targets for verification.
2. Mark turning points clearly to ensure consistent placement.
3. Double-check all readings and field notes to avoid transcription errors.
4. Ensure the rod is vertical and properly extended using bubble levels.
5. Balance backsight and foresight distances to minimize curvature and refraction errors.
6. Secure the instrument against wind and limit sight lengths during extreme weather conditions.

The survey team can eliminate or reduce errors and achieve more accurate elevation measurements by being mindful of these potential problems and following the mitigation practices.