MINOR INSTRUMENTS IN SURVEYING
(Civil Engineering)
1. Introduction
In surveying, apart from major instruments like theodolite, total station, and level, many small tools are used to support fieldwork.
These are called minor instruments.
They help in:
-
Marking points
-
Ranging lines
-
Measuring short distances
-
Setting right angles
-
Taking offsets
-
Recording data
2. List of Common Minor Instruments
-
Ranging Rod
-
Chains and Tapes
-
Arrows (Chain Pins)
-
Pegs
-
Plumb Bob
-
Cross-Staff
-
Optical Square
-
Line Ranger
-
Offset Rod
-
Field Book
3. Description of Each Instrument
3.1 Ranging Rod
Description
A ranging rod is a straight rod of about 2 to 3 m length, painted in alternate red and white bands.
Uses
-
Marking survey stations
-
Ranging straight lines
-
Indicating points at a distance
3.2 Chains and Tapes
Description
Used for measuring distances.
Types
-
Metric chain (20 m, 30 m)
-
Steel tape
-
Invar tape (high accuracy)
-
Fiberglass tape
Uses
-
Measuring lengths of survey lines
-
Setting out dimensions on site
3.3 Arrows (Chain Pins)
Description
Thin steel wires about 40 cm long, bent at one end.
Uses
-
Marking each chain length
-
Counting number of chain lengths measured
3.4 Pegs
Description
Small wooden or steel stakes driven into the ground.
Uses
-
Marking stations and reference points
-
Fixing alignment
-
Showing boundaries
3.5 Plumb Bob
Description
A pointed metal weight suspended by a string.
Uses
-
Transferring points from ground to underground
-
Centering instruments over a station
-
Keeping vertical alignment
3.6 Cross-Staff
Description
A simple instrument used to set right angles in chain surveying.
Types
-
Open cross-staff
-
French cross-staff
-
Adjustable cross-staff
Uses
-
Taking perpendicular offsets
-
Setting out rectangular plots
3.7 Optical Square
Description
A small hand-held instrument using mirrors or prisms to form a 90° angle.
Uses
-
Setting out right angles more accurately than cross-staff
-
Taking offsets in road and building surveys
3.8 Line Ranger
Description
An optical instrument used to locate intermediate points on a straight line.
Uses
-
Ranging long lines quickly
-
Reduces time in alignment work
3.9 Offset Rod
Description
A light rod about 3 m long with a hook or ring at top.
Uses
-
Measuring short offsets
-
Pulling chain through hedges
-
Marking distances
3.10 Field Book
Description
A notebook specially designed for recording survey data.
Uses
-
Entering chainages
-
Writing offsets
-
Sketching field details
-
Permanent survey record
4. Classification of Minor Instruments
| Purpose | Instruments |
|---|---|
| Marking points | Ranging rods, pegs |
| Measuring | Chains, tapes, offset rods |
| Right angles | Cross-staff, optical square |
| Alignment | Line ranger |
| Vertical control | Plumb bob |
| Recording | Field book |
5. Advantages of Minor Instruments
-
Simple to use
-
Low cost
-
Easy to carry
-
Saves time in fieldwork
-
Essential support to major instruments
6. Limitations
-
Limited accuracy
-
Not suitable for precise surveying alone
-
Depends heavily on user skill
7. Practical Examples
Example 1 – Chain Survey of a Plot
A team measures a rectangular field using:
-
Chain → to measure sides
-
Ranging rods → to mark corners
-
Cross-staff → to set right angles
-
Pegs → to fix stations
Example 2 – Road Alignment
While marking a village road:
-
Line ranger helps find intermediate points
-
Offset rod is used to take side measurements
-
Plumb bob ensures proper centering of instruments
Example 3 – Building Layout
For setting out a small house:
-
Optical square sets right angles
-
Tape measures room sizes
-
Pegs mark foundation corners
8. Difference Between Major and Minor Instruments
| Feature | Major Instruments | Minor Instruments |
|---|---|---|
| Example | Theodolite, Level | Ranging rod, cross-staff |
| Accuracy | High | Moderate |
| Cost | High | Low |
| Purpose | Main measurements | Support work |
9. Viva / Exam Important Points
-
Minor instruments support surveying work
-
Used mainly in chain surveying
-
Improve speed and efficiency
-
Essential for setting out and marking