Chain surveying practice: In indirect (reciprocal) ranging between two end stations that are not intervisible, how many ranging rods are required as a minimum to carry out the process effectively?

Introduction / Context:
Ranging ensures that intermediate points lie on a straight line between two end stations. When the end stations are not intervisible due to intervening ground, indirect (reciprocal) ranging is used to keep the line straight. Knowing the minimum equipment—especially the number of ranging rods—helps organize the field party efficiently.

Given Data / Assumptions:

• Two fixed end points A and B define the line.
• Direct line of sight between A and B is blocked.
• Standard reciprocal ranging procedure is followed.

Concept / Approach:

Reciprocal ranging involves placing an intermediate rod so that it appears in line from each end alternately. Typically, one rod is held at A, another at B, and a third is moved back and forth in between until both end observers agree that it is aligned. This iterative process usually requires at least three rods to maintain references at both ends while adjusting the intermediate point.

Step-by-Step Solution:

Verification / Alternative check:

Textbook procedures and field manuals consistently specify three rods as the practical minimum for reciprocal ranging; more rods can be used for longer lines to maintain intermediate reference points.

Why Other Options Are Wrong:

One or two rods cannot maintain simultaneous end references; four or five rods may help on long lines but exceed the minimum required.

Common Pitfalls:

Attempting reciprocal ranging with only two rods, leading to repeated re-sighting and loss of end references; poor signaling causing the intermediate rod to wander off line.

Final Answer:

3