Home
Surveying
Levelling: The Cornerstone of Surveying

Levelling: The Cornerstone of Surveying

In land surveying, when we measure height (elevation) and depth in addition to linear and angular measurements, the process is called Levelling. In simple terms, levelling is the art of determining the height of a point relative to a fixed Datum Line.

Main Objectives of Levelling

To determine the Reduced Level (RL) or elevation of points relative to a fixed datum.
To establish a desired Gradient (slope) on the ground for the construction of roads, railways, or canals.
To prepare Contour Lines on maps.

Importance of Levelling

Levelling is mandatory for the following engineering tasks:

Building Construction: To determine the Plinth Level and foundation depth.
Transportation Engineering: To fix the Alignment and gradients of roads and railway lines.
Irrigation: To ensure the correct slope for the natural flow of water in canals and sewage lines.
Reservoirs: To measure the height of dams and water storage capacity.

Important Technical Terminology

Understanding these terms is essential for mastering levelling:

level book of surveying for civil engineers

Level Surface: A surface parallel to the Curvature of the Earth. Every point on this surface is equidistant from the center of the Earth.
Horizontal Surface: A surface tangent to the level surface at any point. It is perpendicular to the Plumb line.
Datum: A reference surface (or line) with respect to which elevations of points are measured.
Mean Sea Level (MSL): The average height of the tides over a 19-year period. In India, the MSL is taken from Mumbai High.
Reduced Level (RL): The vertical height (elevation) of a point above or below the datum.

Types of Benchmarks (BM)

A Benchmark is a relatively permanent reference point whose RL is already known.

Type of Benchmark	Description
GTS Benchmark	Established across the country by the Survey of India (Highest accuracy).
Permanent BM	Established by departments like PWD on public buildings or bridges.
Arbitrary BM	A level assumed for small projects (e.g., taking a starting point as 100.00 m).
Temporary BM	A point left at the end of a day's work to resume from the next day.

Equipment Used in Levelling

Levelling primarily requires the coordination of two instruments:

Level: Such as the Dumpy Level, Auto Level, or Tilting Level.
Levelling Staff: A graduated rod used to read vertical distances.

Key Levelling Readings and Procedures

1. Back Sight (BS)

The first reading taken on a point of known RL (Benchmark) after setting up the instrument.

Importance: It connects your survey to the actual elevation (MSL).
Calculating Height of Instrument (HI): You cannot find the HI without a BS.
$HI = \text{RL of Benchmark} + \text{Back Sight (BS)}$

2. Fore Sight (FS)

The last reading taken before shifting the instrument or ending the survey.

Importance: It completes one stage of the survey or acts as a link to the next stage.
Change Point (CP): When the distance is too great or an obstacle occurs, the last reading taken before moving the machine is the FS.

3. Intermediate Sight (IS)

All readings taken between the BS and the FS.

Importance: Used to find the RL of multiple points from a single station, such as during cross-sectioning a road.
Calculation:
$\text{RL of point} = HI - IS$

Calculating Reduced Level (RL): A Practical Example

Suppose your Height of Instrument (HI) is 101.50 m. You sight a staff at a distant point and the reading is 2.10 m. You intend to move the machine there.

Find RL of that point:
$101.50 - 2.10 = 99.40 \text{ m}$
Shift Machine: When you set up at the new position, the first reading you take back on this 99.40 m point will become your new Back Sight (BS).

Comparison: BS vs. IS vs. FS

Feature	Back Sight (BS)	Intermediate Sight (IS)	Fore Sight (FS)
Order	1st reading after setup.	Middle readings.	Last reading before shift.
Point Type	Point of Known RL.	Point of Unknown RL.	Point of Unknown RL.
Calculation	Added to RL to get HI.	Subtracted from HI to get RL.	Subtracted from HI to get RL.

Step-by-Step Practical Scenario

Imagine you are surveying a road:

Setup: You set the machine and take a reading on a Benchmark ( $RL = 100.00$ ): 1.20 m (BS).
- $HI = 100.00 + 1.20 = 101.20 \text{ m}$ .
Detailing: You take three readings at 10m intervals: 1.50 m, 1.80 m, and 2.00 m (IS).
- Point 1 RL: $101.20 - 1.50 = 99.70 \text{ m}$ .
- Point 2 RL: $101.20 - 1.80 = 99.40 \text{ m}$ .
- Point 3 RL: $101.20 - 2.00 = 99.20 \text{ m}$ .
Completion: At the end of the road, you take the final reading: 2.50 m (FS).
- Final Point RL: $101.20 - 2.50 = 98.70 \text{ m}$ .

NTS Study Summary:
BS: First reading (Known point).
IS: Middle readings (Detailing).
FS: Last reading (Final point/Shift point).

🏗️ Surveying: Complete Study Guide & Index

📔 Part 1: Fundamentals of Surveying

Surveying: A Bird's Eye View – Meaning and significance of land surveying.
Fundamental Principles – Classification and types of surveying.
Primary Division – Understanding Plane vs. Geodetic Surveying.
Representative Fraction (RF) – Utilization of scales and reduction factors.

📏 Part 2: Linear Measurement & Chain Survey

Chain Surveying – Basic procedures and workflow.
Errors & Adjustments in Chaining – Deficiencies in measurement and their remedies.
Distance Measurement Methods – Detailed discussion on linear surveying tools.
Tape Corrections – Adjustments for Sag, Temperature, and Pull.

🧭 Part 3: Angular & Instrumental Survey

Compass Surveying – Magnetic bearing survey and its applications.
Plane Table Surveying – Equipment used and graphical methods.
Theodolite Surveying – Horizontal and vertical angle measurement.
Total Station – Components, features, and modern digital use.

🏔️ Part 4: Levelling & Elevation

Need for Levelling – Why vertical measurement is vital in civil engineering.
Key Concepts: RL & Datum – Definitions of Reduced Level, Datum, and Benchmarks.
Operating Levelling Instruments – Handling Auto Level and Tilting Level.

🛰️ Part 5: Modern Technologies

Remote Sensing – Information on INSAT and IRS Series satellites.
GIS & LIS Systems – Geographic data management and functionality.
Laser Scanning – Advanced application and control.
Geoid & Ellipsoid – Understanding the mathematical shape of the Earth.

📝 Part 6: Practice & Quizzes (MCQs)

Surveying Quiz 1 (01-25) – GPS, Remote Sensing, and Photogrammetry.
Surveying Quiz 2 (26-50) – Ranging, EDM, and Tacheometry.
Surveying Quiz 3 (51-75) – Contouring and HI Method Levelling.
Surveying Quiz 4 (76-100) – Transition Curves and Bowditch Rule.
Surveying Quiz 5 (101-125) – Plane table and Compass
Surveying Quiz 6 (125-150) – Theodolite Surveying and Levelling
Surveying Quiz 7 (151-175) – Tacheometry, Curves, Modern Surveying Instruments (EDM/GPS)
Surveying Quiz 8 (175-200) – Area & Volume Calculation, Minor Instruments

📚 Quick Revision Resources

Surveying IS Codes with Latest Revision Years

1. General Surveying & Instruments

IS 1491:1959 – Specification for Prismatic Compass (Liquid and Non-liquid).
IS 1963:1981 – Specification for Bubbles for Surveying Instruments.
IS 2988:1995 – Glossary of Terms Relating to Surveying Instruments.
IS 1634:1992 – Code of Practice for Design and Construction of Storage for Surveying Instruments.

2. Chain and Tape Surveying

IS 1492:1970 – Specification for Metric Surveying Chains.
IS 1269 (Part 1):1997 – Material and Construction of Steel Tapes.
IS 1269 (Part 2):1997 – Woven Metallic and Glass Fibre Tapes.
IS 1659:2006 – Specification for Invar Tapes for High Precision Measurement.

3. Theodolite and Tacheometry

IS 8002:1976 – Specification for Surveying Chain Vertical Vernier Theodolite.
IS 8330:1976 – Specification for Tilting Levels (Optical).
IS 8636:1977 – Specification for Tacheometers.

4. Leveling and Contouring

IS 9128:1992 – Specification for Tilting Levels.
IS 9573:1980 – Specification for Automatic Levels.
IS 1779:1961 – Specification for 4-metre Leveling Staff (Folding Type).