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Difficulty: HardElementary Surveying Instruments and Fieldwork

A geography student carrying out a chain survey along a designated baseline encounters a wide pond that obstructs direct distance measurement but allows clear line-of-sight vision across it. To determine the exact chainage distance across this obstacle without crossing the water, which of the following fieldwork procedures using elementary surveying instruments is correct?

  1. Erect perpendicular offset lines at both edges of the obstacle using an optical square, chain a parallel line clear of the pond, and record the parallel segment length as equal to the obstructed distance.Answer
  2. B
    Measure the angle of inclination between ranging poles on opposite banks using a clinometer and convert the vertical angle directly to horizontal distance.
  3. C
    Take forward and back bearings across the pond with a prismatic compass and multiply the angular difference by the total chain length.
  4. D
    Set up an Abney level on the near bank to read the atmospheric pressure difference across the pond and apply the gradient elevation ratio.

Answer

Erect perpendicular offset lines at both edges of the obstacle using an optical square, chain a parallel line clear of the pond, and record the parallel segment length as equal to the obstructed distance.
When chaining is obstructed by a body of water but vision remains clear, setting up right-angle offsets using an optical square allows the surveyor to construct a rectangular parallel line clear of the obstacle. Because opposite sides of a rectangle are equal, measuring the parallel line gives the exact distance across the pond.

Step-by-Step Solution

1
Identify the type of obstacle in chain surveying.
The pond obstructs chaining (linear measurement) but does not obstruct vision (ranging poles are visible across it).
Choosing the correct geometric method depends on whether vision, chaining, or both are obstructed.
2
Apply geometric principles of rectangular offsets using appropriate surveying tools.
By setting off 90∘90^\circ angles at two points on the baseline using an optical square or cross-staff, equal perpendicular lines are measured to clear the pond.
An optical square ensures exact right angles (90∘90^\circ) to form a parallel line equal in length to the missing baseline segment.
3
Evaluate instrument functions to rule out incorrect procedures.
Clinometers, Abney levels, and prismatic compasses measure angles or bearings rather than linear baseline offsets across flat water obstacles.
Linear distance across obstacles in chain surveying requires geometric construction using offset instruments.

Key Concept

Obstacle Handling and Offset Procedures in Chain Surveying
Estimated Time:2m 0s
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Topics

Elementary Surveying Instruments and Fieldwork

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