Chain survey is mainly done using the triangulation technique because:
📝 Detailed Explanation: The Principle of Triangulation
The core principle behind chain surveying, also known as chain triangulation, is based on a fundamental geometric property that makes the entire method possible. The area to be surveyed is divided into a number of small triangles because the triangle is a simple figure that can be plotted from the length of its sides alone, without any angular measurements.
Why Triangles? The Concept of Rigidity
Chain surveying relies exclusively on linear measurements (distances measured with a chain or tape) and does not involve measuring angles. The reason for dividing the entire survey area into a network of triangles is that a triangle is the only simple polygon that is a rigid figure.
This means that if you know the lengths of the three sides of a triangle, its shape and size are uniquely and unchangeably fixed. You can draw it to scale on paper with no ambiguity. Any other polygon, like a quadrilateral, is not rigid. If you only know the lengths of its four sides, it can be flexed into different shapes. To fix a quadrilateral, you would need an additional measurement, such as a diagonal, which effectively divides it into two triangles.
Plotting from Field Data
Because of this unique property, surveyors can measure a series of connected triangles in the field and then accurately plot them on a map later, using only the recorded distances. This is the essence of why triangulation is the chosen method for a survey technique that omits angular measurements.
Types of Chains Used in Surveying
While the principle remains the same, different types of chains have been developed over time for specific purposes and measurement systems. The choice of chain often depended on the project's requirements and the standard units of the region.
| Chain Type | Length | Number of Links |
|---|---|---|
| Metric Chain | 20 m or 30 m | 100 or 150 links |
| Gunter's Chain | 66 ft | 100 links |
| Engineer's Chain | 100 ft | 100 links |
| Revenue Chain | 33 ft | 16 links |
- Metric Chains: These are the standard in countries that use the metric system. Their lengths of 20m and 30m are convenient for modern construction and land surveying projects, making calculations straightforward.
- Gunter's Chain: Invented by Edmund Gunter in the 17th century, this chain was revolutionary. Its length of 66 feet is directly related to traditional British land measurement units: 10 square chains equal 1 acre, and 80 chains equal 1 mile.
- Engineer's Chain: With 100 links of 1 foot each, this chain simplifies engineering calculations. Distances are easily recorded in feet, and any measurement is a direct decimal representation (e.g., 52 links = 52 feet).
- Revenue Chain: This shorter chain was commonly used in cadastral surveys for land revenue and property assessment, particularly in the Indian subcontinent.
The Role of Well-Conditioned Triangles
While option (a) is an important concept in triangulation, it's a rule for ensuring accuracy, not the fundamental reason for using triangles. For the best results, surveyors aim to form well-conditioned triangles, where no angle is less than 30° or greater than 120°. This minimizes the errors that can occur during plotting. However, the primary reason for using triangles at all is their unique geometric rigidity.