---
title: "Obstacles; field sketch for detail survey"
subject: "Surveying I"
chapter: 3
key: "semester_3/surveying_i:obstacles-and-field-sketch"
source: https://www.bctengineer.com/q/semester_3/surveying_i/obstacles-and-field-sketch
site: BCT Engineering
---

# Obstacles; field sketch for detail survey

Surveying I · Chapter 3: Chain Survey

- Asked in: 2058 Chaitra, 10 marks
- Video: [Obstacles in Chain Surveying! Obstacles to Both Chaining and Ranging](https://www.youtube.com/watch?v=s8k4KHETjGY) — Smart Engineer
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/surveying_i/obstacles-and-field-sketch
- Whole chapter: https://www.bctengineer.com/md/semester_3/surveying_i/chapter-3

## Question

What are the possible obstacles in chain survey? Explain the methods of overcoming them and make a field sketch for detail survey by linear measurements (chain/tape).

## Short answer

Obstacles: chaining free, vision obstructed – reciprocal ranging; vision free, chaining obstructed – equal perpendiculars for pond or right-angle method for river; both obstructed – parallel offset lines around a building. The field sketch shows chain lines, tie lines, and detail locations with offsets and chainages recorded bottom to top.

## Answer

Obstacles in chain surveying are of three types: (1) chaining free but vision obstructed, (2) vision free but chaining obstructed, and (3) both obstructed.

#### 1. Chaining free, vision obstructed
**Rising ground between two stations A and B.** Use **reciprocal ranging**. Two assistants at M1 and N1 (each sees the other and one end station) direct each other alternately onto the lines to the far stations, until M, N, A and B are in one line. The line is then chained over the hill.

```text
        M1 ...... N1
   A ___/  hill     \___ B
```

#### 2. Vision free, chaining obstructed
**(a) Pond.** Erect equal perpendiculars AC and BD at points A and B on the line, both on the **same side** of it. Measure CD; then $AB = CD$.

**(b) River (width AB).** At A set out a perpendicular AC and mark its mid-point D; at C set out CE perpendicular to AC and move along CE until E, D and B are in line. Then triangles ABD and CED are congruent, so $AB = CE$.

```text
          B   (opposite bank)
          :
   river  :
          A ---- D ---- C
                        |
                        E
```

**(c) Distance across by angles.** Set out a perpendicular AC at A and measure the angle ACB; then $AB = AC \tan\angle ACB$.

#### 3. Both chaining and vision obstructed
**A building on the chain line.** At A and B on the line before the building, erect equal perpendiculars AC and BD, long enough to clear it. Join C and D, and extend CD beyond the building to E and F. At E and F erect perpendiculars EG and FH equal to AC. The line G-H is the prolongation of AB, and the hidden distance $BG = DE$.

```text
    C ------------ D ------------------ E ------------ F
    |              |                    |              |
 ---A--------------B----[ building ]----G--------------H--->
```

#### Field sketch for detail survey by linear measurements
The chain lines (framework) and objects are drawn freehand in the field book as a key plan. A detail survey by chain or tape is made by:

1. Running the main chain lines AB and BC and a check line.
2. Taking perpendicular or oblique offsets to buildings, roads, trees and fences, and booking them.
3. Drawing a sketch of each object beside the chain line, with its dimensions.

```text
        C
       /|\    [House] offset 8 m
      / | \ 
     /  |  \     (o) tree
    A---D---B ----> road edge
```

Later, the triangles are plotted by arcs and the details by offsets.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which obstacle needs both chaining and vision to be overcome?
   A. A hill
   B. A river
   C. A building on the line
   D. A pond
   Answer: C. A building blocks both the view and the chain. (Key point: Three classes)

2. In reciprocal ranging, the two assistants at M1 and N1:
   A. Chain from both ends together
   B. Measure offsets for the pond
   C. Mark the station pegs
   D. Direct each other alternately onto the lines to the far stations
   Answer: D. Repeat until M, N, A, B are in one line. (Key point: Reciprocal ranging)

3. If AC is set perpendicular at A and angle ACB is measured, the river width AB is:
   A. AC tan(angle ACB)
   B. AC cos(angle ACB)
   C. AC / tan(angle ACB)
   D. AC sin(angle ACB)
   Answer: A. In right triangle ABC, tan C = AB / AC. (Key point: River by trigonometry)

4. In a detail survey, details are located by:
   A. Contour lines
   B. Offsets from the chain lines
   C. Compass bearings
   D. Levels
   Answer: B. Perpendicular or oblique offsets fix buildings, trees and roads. (Key point: Field sketch)

## Related in Surveying I

- [Field book and obstacles in chain survey](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afield-book-and-obstacles) — Surveying I, chapter 3
- [Obstacles in chain survey and remedies](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Aobstacles-chain-survey) — Surveying I, chapter 3, asked 4×
- [Field sketch and mapping by linear measurement](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Achain-field-sketch-mapping) — Surveying I, chapter 3
- [Principle of chain survey; types of offsets](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Achain-principle-and-offsets) — Surveying I, chapter 3
- [Chain survey principle and operations](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Achain-principle-and-field-procedure) — Surveying I, chapter 3, asked 2×
- [Field procedure of chain surveying](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afield-procedure-chain-survey) — Surveying I, chapter 3, asked 4×

## Related in other subjects

- [Field measurements in traversing, closed/open traverse](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atraverse-field-measurements-closed-open) — Surveying II, chapter 1
- [Field measurements in traversing, Bowditch rule](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atraverse-field-measurements-bowditch-rule) — Surveying II, chapter 1
- [Linear chain polymer; sulphur-based inorganic polymers](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Alinear-polymer-sulphur-inorganic) — Engineering Chemistry, chapter 4