LEVELLING IN CIVIL ENGINEERING
1. Definition
Levelling is the branch of surveying that deals with:
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Determining the relative heights of different points on the ground
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Finding the difference in elevation between two or more points
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Establishing points at a required height for construction works
2. Purpose of Levelling
Levelling is done to:
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Design roads, railways, canals, and drains
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Fix foundation levels of buildings
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Calculate earthwork (cut and fill)
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Prepare contour maps
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Set proper slopes for water flow
3. Instruments Used
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Dumpy Level / Auto Level / Digital Level – for taking readings
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Levelling Staff – graduated rod for reading heights
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Tripod – to support the level
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Field Book – to record observations
4. Principle of Levelling
Levelling works on the principle that:
A horizontal line of sight is established using a level instrument, and vertical distances are measured from this line to different points using a staff.
By comparing these staff readings, the difference in elevation is calculated.
5. Important Terms
| Term | Meaning |
|---|---|
| Benchmark (BM) | Fixed reference point of known elevation |
| Back Sight (BS) | First reading taken on BM |
| Fore Sight (FS) | Last reading before shifting instrument |
| Intermediate Sight (IS) | Any reading between BS and FS |
| Height of Instrument (HI) | Elevation of line of sight |
| Reduced Level (RL) | Elevation of any point |
6. Types of Levelling
(A) Simple Levelling
Used to find the difference in level between two points.
(B) Differential Levelling
Used when points are far apart and many setups are required.
(C) Fly Levelling
Used to transfer a benchmark from one place to another.
(D) Profile Levelling
Used to find levels along a road, canal, or pipeline route.
(E) Cross-Section Levelling
Used to calculate earthwork quantity.
(F) Reciprocal Levelling
Used when obstacles like rivers prevent direct measurement.
7. Methods of Booking Levels
Method 1: Height of Instrument (HI) Method
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HI = RL of BM + BS
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RL of any point = HI – staff reading
Method 2: Rise and Fall Method
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Compare consecutive readings
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If next reading is smaller → Rise
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If next reading is larger → Fall
8. Worked Examples
Example 1: Simple Levelling
Given:
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RL of BM = 100.000 m
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BS on BM = 1.250 m
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FS on point A = 2.000 m
Step 1: Find HI
HI = 100.000 + 1.250 = 101.250 m
Step 2: Find RL of A
RL(A) = 101.250 − 2.000 = 99.250 m
✅ Point A is 0.75 m lower than BM.
Example 2: Differential Levelling (HI Method)
| Station | BS (m) | IS (m) | FS (m) | RL (m) | Remarks |
|---|---|---|---|---|---|
| BM | 1.500 | — | — | 50.000 | BM |
| P1 | — | 2.000 | — | 49.500 | |
| P2 | — | — | 1.200 | 50.300 | CP |
Calculation:
HI = 50.000 + 1.500 = 51.500 m
RL(P1) = 51.500 − 2.000 = 49.500 m
RL(P2) = 51.500 − 1.200 = 50.300 m
Example 3: Rise and Fall Method
| Point | Staff Reading (m) | Rise (m) | Fall (m) |
|---|---|---|---|
| A | 1.200 | — | — |
| B | 0.900 | 0.300 | — |
| C | 1.400 | — | 0.500 |
From A → B = Rise 0.30 m
From B → C = Fall 0.50 m
Example 4: Profile Levelling (Road Construction)
A road is proposed over 200 m. Levels are taken every 20 m.
| Chainage (m) | RL (m) |
|---|---|
| 0 | 102.00 |
| 20 | 101.60 |
| 40 | 101.20 |
| 60 | 100.90 |
These values are plotted to form a longitudinal section, which helps engineers decide:
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Where cutting is needed
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Where filling is required
9. Errors in Levelling
Instrumental Errors
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Collimation error
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Bubble not centered
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Faulty staff
Natural Errors
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Earth’s curvature
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Atmospheric refraction
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Wind affecting staff
Personal Errors
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Wrong staff reading
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Improper focusing
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Poor booking
10. Precautions in Levelling
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Always center the bubble
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Take equal back sight and fore sight distances
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Keep staff vertical
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Check readings carefully
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Use firm ground for tripod
11. Applications of Levelling
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Building foundations
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Road and railway design
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Canals, drains, and sewers
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Dams and bridges
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Contour mapping
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Earthwork calculation
12. Difference Between Levelling and Trigonometric Levelling
| Feature | Ordinary Levelling | Trigonometric Levelling |
|---|---|---|
| Instrument | Level + staff | Theodolite/Total station |
| Accuracy | High | Moderate |
| Distance | Short & medium | Long distances |
| Method | Direct reading | Uses angles & distance |
📏 1. Basic Diagram of a Levelling Instrument (Dumpy Level)
This shows the key components of a dumpy or engineer’s level:
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Telescope – for sighting the staff
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Level vial/bubble – ensures horizontal line of sight
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Focusing knob – to focus the telescope
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Tribrach & foot screws – to level the instrument precisely
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Tripod stand – stable support for the instrument
📌 2. Close-up Diagram with Labels
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Telescope and eyepiece
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Crosshair/diaphragm inside telescope
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Bubble tubes (longitudinal & cross)
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Tribrach and foot screws group
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Tripod stand
This is a good schematic to learn the names of parts for exams.
🧰 3. Simple Sketch of Levelling Instrument + Tripod
This helps visualize how the level sits on the tripod and is oriented in the field.
📏 4. Levelling Staff (Rod)
While I can’t show the exact image in this text, imagine a long graduated rod (up to ~5 m), marked in meters and centimeters, held vertically on the point being measured. This rod is read through the level’s telescope