Curvature & Refraction Effect and Sensitivity of Bubble Tube
In levelling, certain natural factors affect the accuracy of measurements. Two important effects are curvature of the earth and atmospheric refraction. Along with this, the sensitivity of the bubble tube plays a vital role in the precision of levelling instruments. For ITI Surveyor trainees, understanding these concepts is essential for accurate surveying.
Curvature of the Earth
The earth is not flat but curved. When levelling is carried out over long distances, the line of sight remains straight while the earth’s surface curves away from it. This causes an apparent increase in staff readings.
Effect of Curvature
- Staff reading appears higher than actual
- Error increases with distance
Correction for Curvature
Curvature correction is always negative:
Cc = -0.0785 d²
Where d = distance in kilometers
Atmospheric Refraction
Refraction is the bending of light rays as they pass through different layers of the atmosphere. This causes the line of sight to bend slightly downward.
Effect of Refraction
- Staff reading appears lower than actual
- Partially compensates curvature effect
Correction for Refraction
Refraction correction is positive:
Cr = +0.0112 d²
Combined Correction
The combined effect of curvature and refraction is:
C = -0.0673 d²
This correction is applied to long-distance levelling work.
Sensitivity of Bubble Tube
The bubble tube (spirit level) is used to ensure that the levelling instrument is perfectly horizontal. Its sensitivity determines how accurately the instrument can be levelled.
Definition
Sensitivity of a bubble tube is defined as the angular value of one division movement of the bubble.
Factors Affecting Sensitivity
- Radius of curvature of the tube
- Length of the bubble
- Quality of liquid inside the tube
Higher Sensitivity Means
- Greater accuracy in levelling
- Small tilt causes noticeable bubble movement
Formula
Sensitivity ∝ 1 / Radius of curvature
Importance in Surveying
- Curvature and refraction affect long-distance measurements
- Corrections improve accuracy of levelling
- Bubble sensitivity ensures proper instrument setup
Precautions
- Apply curvature and refraction correction for long distances
- Ensure proper leveling of instrument
- Use sensitive bubble tubes for precise work
- Avoid observations in extreme weather conditions
Conclusion
Curvature and refraction are natural factors that affect levelling accuracy. Proper corrections must be applied to obtain true results. Sensitivity of the bubble tube is equally important for precise leveling of instruments.
For ITI Surveyor trainees, understanding these concepts is essential for achieving accurate and reliable survey measurements.