Why are back-sight and fore-sight distances kept approximately equal in ordinary differential levelling?
Correct answer: C. To reduce the effects of collimation, curvature and refraction
- A. To eliminate the need for a benchmark
- B. To make every staff reading numerically identical
- C. To reduce the effects of collimation, curvature and refraction
- D. To ensure that the instrument remains permanently adjusted
Explanation
Equal sight lengths cause several systematic effects to be nearly equal for the two readings and therefore largely cancel in their difference. This includes collimation error and the combined effects of curvature and refraction.
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Surveying covers measurement of distances, angles and elevations, traversing, levelling, contouring, plane table work, theodolites, total stations and basic photogrammetry. It includes error adjustment, coordinate calculations, area and volume determination, and the distinction between accuracy and precision, as well as between temporary and permanent adjustments.
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