Survey and control for tunnelling: identify the correct composite statement covering the use of Survey of India topo maps in mountains, reciprocal ranging for ridge height points, planimetric control accuracy, and the angular precision target with a 1-second theodolite.

Introduction / Context:Tunnel alignment demands robust surface control and careful transfer underground. Understanding how topographic resources, ranging methods, linear-control accuracy, and angular precision targets fit together helps avoid breakthrough errors and costly corrections. This item checks several foundational survey concepts used in tunnel projects.

Given Data / Assumptions:

• Mountainous terrain requires careful reconnaissance; national topo maps guide feasibility and preliminary alignment.
• Reciprocal ranging helps intervisibility and height checks across ridges/valleys.
• Planimetric control accuracy on the order of 1 in 10,000 is a reasonable target for tunnel works.
• Using a 1-second theodolite, an often-quoted practical closure target is about 15 seconds times square root of the number of angles (15″√N).

Concept / Approach:Topo maps enable route reconnaissance and initial control planning. Reciprocal ranging is a classic technique to mitigate line-of-sight curvature/refraction errors across obstacles. Planimetric accuracy of 1:10,000 aligns with the precision typically required for long drives. Angular precision guidelines translate instrument capability into expected closure error, giving surveyors a yardstick for quality control in triangulation/traverses.

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