The 'levelling staff' reading is corrected for 'curvature and refraction' by the formula:
Choose the correct answer
Curvature correction alone 0.0785 D² without refraction
Combined C−R = 0.0673 D² (metres, D in km) — curvature raises apparent staff reading, refraction partially offsets it
No correction needed for any distance
0.0673 D (linear, not squared)
Correct Answer
B. Combined C−R = 0.0673 D² (metres, D in km) — curvature raises apparent staff reading, refraction partially offsets it
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Combined correction for curvature (C) and refraction (R): C−R = 0.0673 D² (metres) where D = distance in km (combined, net correction is positive — staff reading too high due to curved Earth minus refraction bending ray down). Curvature alone: 0.0785 D². Refraction correction: −0.0112 D². Net: 0.0673 D². Important for precise levelling over long sights (> 200 m). Eliminated by equal backsight and foresight lengths.
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