Errors in Levelling

Levelling, like any survey method, is subject to errors. These errors can be categorized into three types:

a. Personal Errors

These are mistakes made by the surveyor, often due to human error or carelessness:

I. Error in Sighting: Incorrect aiming of the instrument.

II. Errors in Manipulation:

• Careless setup of the level.
• Imperfect focusing of the instrument.
• The bubble not being centered in the level tube.
• Non-vertical alignment of the staff.

III. Error in Reading Staff: Misreading the staff due to parallax or incorrect interpretation of the scale.

IV. Error in Recording and Computation: Mistakes made during data entry or mathematical calculations.

b. Instrumental Errors

These errors arise from the instrument itself:

• Imperfect Adjustment of the Level: If the instrument is not calibrated properly.
• Defective Level Tube: Damage or issues with the level tube leading to incorrect readings.
• Shaky Tripod: Instability in the instrument’s support.
• Incorrect Graduations on Staff: Errors in the markings on the staff.

c. Natural Errors

Natural factors can also introduce errors:

• Error Due to Curvature: The Earth’s curvature can affect long-distance measurements.
• Error Due to Refraction: Changes in air density (temperature or pressure) can bend light rays and affect accuracy.
• Error Due to Wind and Sun: External environmental conditions, like wind shaking the instrument or sunlight affecting readings.