Primary Division of Land Surveying: Modern Tools of Geodetic Surveying

Primary Division of Land Surveying: Plane vs. Geodetic Surveying

Dear readers, when we look at the ground with our own eyes, it appears flat or rugged. But is the Earth actually flat? We all know that the Earth is a spherical (Oblate Spheroid) body. In today’s article, we will understand the significance of the Earth's shape in the field of surveying.

The Shape of the Earth and the Surveying Challenge

The difference between the Earth's equatorial axis (12756.75 km) and polar axis (12713.80 km) is very small. Additionally, heights like Mount Everest (8.848 km) are negligible compared to the vast size of the Earth. This is why the surface appears flat to us.

However, while surveying large areas, this curvature of the Earth can cause significant errors in measurement. To solve this problem, surveying is divided into two primary categories:

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1. Plane Surveying

If the area to be surveyed is small, we can neglect the curvature of the Earth and treat the surface as a flat plane.

• Characteristic: All plumb lines are considered parallel to each other.

• Triangles: All triangles formed in this survey are considered Plane Triangles.

• Usage: Used in most engineering projects such as buildings, roads, and small canals.

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2. Geodetic Surveying

When a survey spans a very large area, it becomes mandatory to account for the Earth's curvature.

• Characteristic: Plumb lines are not parallel but instead converge at the center of the Earth.

• Triangles: The triangles formed are called Spherical Triangles, and Spherical Trigonometry is used for calculations.

• Usage: Used for preparing national maps and establishing high-precision control stations.

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The meaning of primary division is that we choose the correct methodology based on our needs and the size of the area. For small-scale projects, plane surveying is sufficient, but for national-level maps, geodetic surveying is mandatory.

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NTS Study Tips: Remember, for an area larger than 250 $km^2$, geodetic surveying is generally recommended.

💡 Did You Know? (Interesting Facts)

Keep these points in mind to understand the level of precision in surveying:

• The length of a 1.2 km arc on the Earth's surface is only 1 mm longer than the straight chord line connecting its ends.

• For an area of approximately 200 square km, the sum of the interior angles of a spherical triangle differs by only 1 second compared to a plane triangle.

 

Geodetic Surveying: Precision on a Global Scale

Geodetic Surveying is the branch of surveying that takes into account the true shape and size of the Earth. Unlike Plane Surveying, which assumes the Earth is flat, Geodetic Surveying treats the Earth as a curved surface—specifically an oblate spheroid (a sphere slightly flattened at the poles).

Why Do We Need Geodetic Surveying?

If you measure a small area (like a building plot), the Earth's curvature is so slight that it doesn't affect your math. However, as the area increases, the "bulge" of the Earth starts to distort linear and angular measurements. For large-scale projects, failing to account for this curvature would lead to massive errors.

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Core Characteristics

• The Plumb Line: In geodetic surveying, plumb lines (lines pointing toward the center of gravity) are not parallel. They converge toward the Earth's center.

• Spherical Triangles: When three points are connected over a long distance, the lines are curved. The resulting shape is a spherical triangle, and the sum of its angles is always greater than 180°.

• Mathematical Foundation: It utilizes Spherical Trigonometry and advanced calculus rather than simple Euclidean geometry.

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Key Objectives

1. National Control Networks: To establish highly accurate primary control points (Benchmarks) that serve as a reference for all other smaller surveys.

2. Mapping Countries: Used for creating topographical maps of entire states or nations.

3. Scientific Research: Determining the exact size and shape of the Earth and monitoring crustal movements (tectonic shifts).

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Difference at a Glance: Plane vs. Geodetic

Feature	Plane Surveying	Geodetic Surveying
Earth's Curvature	Neglected (Assumed flat)	Considered (Curved surface)
Area Covered	Small (Typically < 250 $km^2$)	Large (National/Global scale)
Plumb Lines	Parallel	Convergent (Meet at center)
Triangle Type	Plane Triangle	Spherical Triangle
Precision	Lower	Very High

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Modern Tools of Geodetic Surveying

Today, geodetic surveys are rarely done with just ground instruments. They rely heavily on:

• GNSS (Global Navigation Satellite Systems): Using GPS, GLONASS, and Galileo to get millimeter-level accuracy.

• VLBI (Very Long Baseline Interferometry): Using signals from distant quasars to measure Earth's rotation.

• Satellite Altimetry: Measuring the sea level and Earth's gravity field from space.

🏗️ 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

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📚 Quick Revision Resources

• Surveying Top 1-100 One-Liners

• Surveying Advanced (101-200) One-Liners

• Surveying Master Class (Part 1)

• Surveying Master Class (Part 2)

• Quick Revision Guide

• Quick Surveying Formulas

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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.

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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.

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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.

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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).

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5. Modern Surveying (Total Station & GPS)

• IS 16481:2016 – Guidelines for Accuracy and Testing of Total Stations.

• IS 14855:2000 – Terminology and Concepts for GIS and Remote Sensing.

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6. Measurement of Building Works

• IS 1200 (Part 27):1992 – Method of Measurement of Building and Civil Engineering Works (Earthwork & Surveying).