Levelling methods – eliminating curvature and refraction effects Which levelling method is specifically used to determine the difference in elevation between two points while eliminating the effects of Earth’s curvature and atmospheric refraction?

Introduction / Context:
When two points are separated by obstacles such as water bodies or deep valleys, standard levelling suffers from systematic errors due to Earth’s curvature and atmospheric refraction. Surveyors use a special method to cancel these effects and obtain an accurate difference in elevation. This question asks you to identify that method by name.

Given Data / Assumptions:

• Two stations A and B are far apart with an obstacle in between.
• Simultaneous or near-simultaneous readings are possible from setups near both ends.
• Atmospheric conditions affect line of sight similarly over the same path.

Concept / Approach:

Reciprocal levelling places the instrument near A to observe readings on a staff at B and near B to observe readings on a staff at A, along essentially the same line of sight. By combining the two sets of readings, the equal and opposite effects of curvature and refraction cancel, leaving the true difference in elevation. The computation uses averaged or corrected readings to remove systematic bias from long sights over obstacles.

Step-by-Step Solution:

Verification / Alternative check:

Worked examples show that the algebraic combination of the two instrument setups eliminates curvature/refraction terms, achieving far better accuracy than a single long shot across the obstacle.

Why Other Options Are Wrong:

Precise levelling: increases precision by short sights and refined procedures but does not by itself cancel curvature/refraction on long across-obstacle shots.

Differential levelling: general method for transfer of RLs; not specifically designed to cancel these effects.

Flying levelling: rapid approximate levelling for reconnaissance, not for eliminating systematic long-sight errors.

Common Pitfalls:

Trying a single setup with very long sights; neglecting equal sight lengths; ignoring atmospheric variability when scheduling reciprocal readings.

Final Answer:

Reciprocal levelling