Angular Observations – Why Keep the Line of Sight High Above Ground? In theodolite work, the line of sight is kept as high above the ground surface as practicable to reduce errors in observed angles primarily due to which effects?

Introduction / Context:
Atmospheric effects near the ground can degrade angular observations. Temperature gradients and air turbulence are most severe within the lowest metre above the surface, causing apparent target movement (shimmering) and bending of the light path (refraction). Raising the line of sight mitigates these errors, improving precision in angle reading and pointing.

Given Data / Assumptions:

• Observations made over sun-heated ground or roads are prone to turbulence.
• Targets may be at low height unless elevated on tripods or poles.
• Theodolite and staff/target heights can be adjusted in the field.

Concept / Approach:

Shimmering arises from turbulent cells with varying refractive indices, making the image dance and blurring the cross-hair coincidence. Horizontal refraction results from lateral gradients in refractive index close to the ground, displacing the apparent horizontal direction. Elevating the line of sight moves the observation path into more stable air with weaker gradients, reducing both effects simultaneously. Vertical refraction is a significant concern in vertical angle/levelling over long sights, but the question emphasizes minimization by higher line of sight for angle observations, especially horizontal components.

Step-by-Step Solution:

Verification / Alternative check:

Field experience and optical surveying texts consistently advise higher sight lines to minimize scintillation and near-ground refraction on warm days.

Why Other Options Are Wrong:

Focusing on only one effect ignores the compound benefit; instrument collimation error is an instrument adjustment issue not solved by raising the line of sight.

Common Pitfalls:

Observing at low target heights over hot surfaces; attributing all image motion to instrument faults rather than atmospheric causes.

Final Answer:

Both shimmering and horizontal refraction