Which of the following is/are the correct reason(s) for closing errors in compass surveying at the time of traversing?

🧭 Understanding Closing Errors

In a closed traverse, the survey starts at a known point and ends at the same point, forming a closed loop. Ideally, when the survey is plotted, the final point should coincide exactly with the starting point. However, due to unavoidable errors in measurement, there is usually a gap between the start and end points. This gap is known as the closing error.

The closing error is a direct indication of the accumulated errors in both the angular (bearing) and linear (distance) measurements made during the survey.

🔬 Detailed Analysis of the Statements

The question asks for the correct reasons for closing errors among the given statements.

• Statement I: "The end station of a traverse generally coincides exactly with its starting station."
  This statement is incorrect. It describes the ideal, error-free outcome of a survey. The very existence of a closing error is because the end station does not generally coincide with the starting station in practice.
• Statement II: "There is no error in the magnetic bearing observation."
  This statement is also incorrect. It describes a perfect, unrealistic scenario. Errors in measuring magnetic bearings (due to local attraction, instrumental faults, or observer mistakes) are a primary cause of closing errors.
• Statement III: "There is an error in the linear distance measurement."
  This statement is correct. Inaccuracies in measuring the lengths of the traverse lines (using a chain or tape) are a fundamental reason why a traverse fails to close perfectly.

Therefore, of the three statements provided, only Statement III correctly identifies a real cause of closing errors in a traverse.

📚 Major Sources of Error in Compass Surveying

Closing errors arise from a combination of different types of errors. These can be broadly categorized as follows:

1. Angular Measurement Errors

• Instrumental Errors: Caused by faults in the compass, such as the pivot not being centered, the needle not being straight, or the line of sight not passing through the center of the instrument.
• Observational Errors: Mistakes made by the surveyor, like inaccurate centering or leveling of the compass, or careless reading of the graduated circle (parallax).
• External Influences (Local Attraction): The presence of nearby magnetic objects (e.g., power lines, iron pipes, vehicles) can deflect the magnetic needle, leading to incorrect bearing readings.

2. Linear Measurement Errors

• Instrumental Errors: Using a chain or tape that is not of the standard length (i.e., too long or too short).
• Natural Errors: Variations caused by temperature, wind, or uneven ground that affect the tape's length or alignment.
• Personal Errors: Mistakes in reading the tape, incorrect alignment (ranging), or not applying corrections for slope and sag.