Choice of tacheometric method: Subtense tacheometry (using a fixed-length subtense bar or target) is generally preferred under which ground conditions?

Introduction / Context:
Tacheometry estimates distances and elevations without chaining. The subtense method uses a target or bar of known length subtending an angle at the instrument; distance is computed from the measured angle. This approach is especially valuable where physical access to the ground line is difficult.

Given Data / Assumptions:

• A fixed-length subtense bar or distinctive target is available.
• Instrument can measure angles accurately (theodolite/EDM-less setup).
• Lines of sight may be obstructed by relief, vegetation, or rough footing.

Concept / Approach:
Distance by subtense follows D = L / (2 * tan(θ/2)), where L is the known target length and θ is the subtended angle. Because no chaining is required along the ground, this technique shines on steep or rugged slopes and across ravines—typical of mountainous areas. In flat, open terrain, chaining or EDM is faster and more accurate; in deserts, long straight lines favor EDM or tape with supports rather than repeated subtense setups.

Step-by-Step Solution:

Verification / Alternative check:
Comparisons with chained or EDM distances on sample lines typically show good agreement for moderate lengths when angles are observed carefully and targets are levelled/plumbed.

Why Other Options Are Wrong:

• Flat terrain: chaining/EDM is simpler and faster.
• Undulating: workable, but the method is most advantageous in truly rugged terrain.
• Deserts: visibility is good; EDM and tapes usually outperform subtense for long, straight shots.

Common Pitfalls:
Mis-centring of the bar; bar not perpendicular to the line of sight; poor angle reading on shimmering hot days; neglecting vertical angle corrections for height differences.

Final Answer:
Mountainous (hilly) terrain with difficult access