📚 CTEVT | Control Survey – Chapter 1: Levelling
(Full Notes + Practice Questions + Free PDF)
Levelling is one of the most important and fundamental techniques in Control Survey (EG 2102 GE).
Chapter 1 introduces students to the principles, methods, field procedures, and error adjustments of levelling — which is essential for establishing vertical control points in geodetic and engineering projects.
From constructing roads and canals to preparing contour maps and benchmarks, levelling plays a vital role in surveying practice.
🧩 Chapter Overview: Levelling
Levelling = The branch of surveying that deals with determining the relative heights (elevations) of points on the Earth’s surface.
Chapter 1 helps students understand:
✔️ Basic principles of levelling
✔️ Important levelling terms and definitions
✔️ Types of levelling methods (direct & indirect)
✔️ Field procedures and booking methods
✔️ Errors in levelling and their adjustment
✔️ Benchmarks and Nepal Survey Department specifications
This chapter forms the foundation for vertical control establishment before moving into traversing and triangulation.
📥 Download the Full Chapter 1 Notes (Free PDF)
🎥 Watch the Chapter 1 Video on YouTube
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👉 [Click Here to Watch Chapter 1 on YouTube]
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🔍 1.1 Introduction & Principle of Levelling
Levelling is used to determine the difference in elevation between two or more points.
⭐ Principle of Levelling
The basic principle is:
📌 A horizontal line of sight is established using a levelling instrument, and vertical distances are measured using a staff.
Levelling is mainly used for:
✔️ Establishing Benchmarks (BM)
✔️ Road, railway and canal construction
✔️ Contour mapping
✔️ Setting out engineering projects
✔️ Height transfer in control networks
🔍 1.1.2 Definitions of Important Terms
Understanding levelling terminology is essential for exams and fieldwork.
Key Terms:
-
Level: Instrument used to establish a horizontal line of sight
-
Levelling: Process of determining elevations of points
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Level Surface: Curved surface parallel to mean sea level
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Level Line: Line lying on a level surface
-
Datum: Reference surface for elevation measurement
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MSL (Mean Sea Level): Standard datum used in Nepal
Reduced Level (RL)
Height of a point above or below the datum.
Benchmark (BM)
A fixed reference point of known RL.
-
PBM: Permanent Benchmark
-
TBM: Temporary Benchmark
Height of Instrument (HI)
Elevation of the line of sight of the instrument.
Staff Readings:
-
BS (Back Sight): First reading taken from known point
-
FS (Fore Sight): Last reading taken before shifting instrument
-
IS (Intermediate Sight): Reading taken on intermediate points
Turning Point (TP)
Point used to transfer elevation when shifting instrument.
🔍 1.1.3 Levelling Instruments and Accessories
Levelling requires instruments for accurate height measurement.
Common Instruments:
✔️ Dumpy Level
✔️ Tilting Level
✔️ Automatic Level
✔️ Digital Level
Accessories:
✔️ Levelling Staff
✔️ Tripod Stand
✔️ Staff Bubble
✔️ Pegs and Chains
✔️ Field Book
Modern levelling instruments improve accuracy and efficiency in control surveys.
🔍 1.2 Methods of Levelling
Levelling methods are broadly classified into:
✅ 1.2.1 Direct Levelling (Spirit Levelling)
Direct levelling is the most accurate and widely used method.
Types:
✔️ Simple Levelling
Used to find height difference between two nearby points.
✔️ Differential Levelling
Used when points are far apart and multiple setups are needed.
✔️ Check Levelling
Performed to verify accuracy of levelling work.
✔️ Fly Levelling
Quick method to transfer benchmark RLs.
✔️ Reciprocal Levelling
Used when obstacles like rivers prevent direct measurement.
✔️ Precise Levelling
High accuracy levelling for geodetic benchmarks.
✔️ Profile Levelling
Used in road, canal and pipeline alignment surveys.
✔️ Cross-Sectioning
Used to take elevations perpendicular to a survey line.
✅ 1.2.2 Indirect Levelling
Indirect levelling determines elevation without spirit level.
Types:
✔️ Barometric Levelling
✔️ Hydrostatic Levelling
✔️ Trigonometric Levelling
✔️ Hypsometric Levelling
✔️ GPS Levelling
Indirect methods are useful in rough terrain and large-scale surveys.
🔍 1.3 Field Procedure in Levelling
Levelling must follow systematic field procedures.
🔍 1.3.1 Testing Levels and Collimation Error
Before starting fieldwork, instruments must be tested.
✔️ Two-peg test is commonly used
✔️ Collimation error occurs when line of sight is not truly horizontal
🔍 1.3.2 Steps in Field Procedure
Levelling operations include:
1. Reconnaissance
Planning route and selecting benchmark locations.
2. Monumentation
Fixing permanent points for control.
3. Observation
Taking staff readings carefully.
4. Recording
Booking readings in levelling field book.
5. Computation Methods:
✔️ Rise and Fall Method
Most accurate method.
✔️ Height of Instrument (HI) Method
Faster method for routine work.
🔍 1.3.3 Field Precautions
✔️ Instrument must be properly levelled
✔️ Staff should be held vertical
✔️ Avoid long sights
✔️ Take readings in correct sequence
✔️ Perform check levelling regularly
🔍 1.4 Errors and Adjustment in Levelling
Errors are unavoidable in levelling and must be minimized.
🔍 1.4.1 Nepal Survey Department Specifications
Survey Department of Nepal has defined benchmark orders and permissible limits for vertical control establishment.
🔍 1.4.2 Types of Errors
✔️ Systematic Errors
✔️ Random Errors
✔️ Gross Errors (Mistakes)
🔍 1.4.3 Sources of Errors
Instrumental Errors:
-
Collimation error
-
Faulty bubble tube
Personal Errors:
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Staff not vertical
-
Wrong reading/booking
Natural Errors:
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Curvature and refraction
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Wind and temperature effects
🔍 1.4.4 Permissible Error
Permissible error depends on the order of levelling:
📌 Higher precision surveys require smaller error limits.
🔍 1.4.5 Adjustment of Levelling Errors
Levelling misclosure is adjusted by:
✔️ Arithmetic correction
✔️ Distribution proportional to distance
✔️ Closing error balancing
Adjustment ensures reliable benchmark elevations.
🧪 Practical Insights for Students (Chapter 1)
By studying levelling, students learn:
✔️ How vertical control points are established
✔️ How benchmarks are transferred
✔️ How errors affect elevation accuracy
✔️ Why levelling is essential in engineering projects
This prepares them for later chapters such as:
📌 Traverse Survey
📌 Triangulation
📌 Trilateration
📌 Resection and Intersection
📝 Short Questions
1️⃣ Define levelling.
2️⃣ What is a benchmark?
3️⃣ Differentiate PBM and TBM.
4️⃣ What is HI?
5️⃣ Define BS and FS.
6️⃣ What is reciprocal levelling used for?
7️⃣ Name two indirect levelling methods.
8️⃣ What is collimation error?
📝 Long Questions
1️⃣ Explain the principle and importance of levelling in control surveys.
2️⃣ Describe different methods of direct levelling.
3️⃣ Discuss rise and fall method and HI method of computation.
4️⃣ Explain types, sources, and adjustment of errors in levelling.
5️⃣ Write detailed field procedures for establishing benchmarks.
🎯 Key Takeaways
✔️ Levelling establishes vertical control points.
✔️ Benchmarks are reference points of known elevation.
✔️ Spirit levelling is the most accurate method.
✔️ Errors must be adjusted for reliable results.
✔️ Levelling is essential in all engineering and geodetic surveys.
📂 Download Free PDF Notes – Chapter 1: Levelling
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🎥 Watch the Full Chapter 1 Video
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📂 Download Control Survey PDF
📘 SYLLABUS – Download PDF
📘 Unit 1: Levelling – Download PDF
📘 Unit 2: Traverse – Download PDF
📘 Unit 3: Triangulation – Download PDF
📘 Unit 4: Trilateration – Download PDF
📘 Unit 5: Resection & Intersection – Download PDF
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