Principle of Chain Surveying

The Principle of Chain Surveying is based on triangulation, dividing the area into well-conditioned triangles with sides measured directly using chains or tapes, without angular measurements. It is suitable for small, flat areas, with components like main stations, check lines, baselines, and offsets ensuring precision. This method avoids ill-conditioned triangles to maintain accuracy in plotting.

The concept of chain surveying is based on triangulation. This implies that the area to be surveyed is split into several small triangles, which must be well-conditioned. In chain surveying, the lengths of the triangles’ sides are directly measured on-site using a chain or tape, with no angular measurements involved. In this method, the tie lines and check lines ensure the precision of the work Principle of Chain Surveying.

It is important to note that plotting triangles does not require angular measurements when the three sides are known, Principle of Chain Surveying.

Chain surveying is recommended when

1. The terrain is relatively flat, 
2. The area to be surveyed is small, 
3. A map of a smaller scale is to be created, and 
4. Forming well-conditioned triangles is straightforward.

Chain surveying is unsuitable when

1. The area is densely packed with numerous details, 
2. The terrain has excessive undulations, 
3. The area is very large, and
4. Creating well-conditioned triangles becomes challenging because of obstacles.

A. Large-Scale and Small-Scale Maps

When 1 cm on a map represents a small distance, it is referred to as a large-scale map. For example,

1 cm = 1 m, i.e., RF = 1 / 100

When 1 cm on the map represents a larger distance, it is known as a small-scale map. For example, 1 cm = 100 m,

i.e., RF = 1 / 10,000

A map with an RF of less than 1/500 is considered a large-scale map. A map with an RF greater than 1/500 is considered a small-scale map, Principle of Chain Surveying.

Well-Conditioned And Ill-Conditioned Triangles

A triangle is considered well-conditioned when no angle is smaller than 30° or larger than 120°. An equilateral triangle is regarded as the ideal or best-conditioned triangle [Figs 2.1 (a) and (b)] Principle of Chain Surveying.

Well-conditioned triangles are preferred because their apex points are sharply defined and can be pinpointed with a single ‘dot’. In this case, there is no risk of relative displacement of the plotted point, Principle of Chain Surveying.

A triangle with an angle smaller than 30° or larger than 120° is considered ill-conditioned [Fig. 2.1(c)].

Ill-conditioned triangles are avoided in chain surveying because their apex points are not well-defined, making even slight displacements of these points result in significant errors in plotting.2.3

Reconnaissance Survey and Index Sketch

Before the initiation of any survey work, the area to be surveyed is carefully inspected by the surveyor, who then considers the possible configuration of the framework of the survey. This initial assessment of the area is termed as preliminary survey or reconnoitre, Principle of Chain Surveying.

During the preliminary survey, the surveyor should traverse the area and note the various obstacles and whether or not the selected stations are visible to each other. The main stations should be so selected that they cover the whole area. The surveyor should also ensure that the triangles formed are properly formed. He should note the various features that are to be located.

The clear manual drawing of the area which is prepared during the preliminary survey is known as the reference sketch or key plan. The reference sketch shows the outline of the survey work, Principle of Chain Surveying.

It shows the main survey stations, secondary stations, the stations’ reference line, the structure for the framework of triangles, and the approximate positions of different features. This sketch is an important document for the surveyor and for the person who will plot the map. It should be included in the starting page of the field book (Fig. 2.2) Principle of Chain Surveying.

Definitions and Illustrations

A. Survey Stations

Here is the change with some words modified: Survey stations are the points at the start and the termination of a chain line. They may also occur at any appropriate points on the chain line. Such stations may be Principle of Chain Surveying

1. Main stations,
2. Subsidiary stations, and
3. Tie stations.

1. Main Stations

Here is the sentence with some words modified:

Stations taken along the perimeter of an area as reference points are known as primary stations. The lines joining the primary stations are called main survey lines. The main survey lines should span the whole area to be surveyed. The primary stations are denoted by ‘A’ with letters A, B, C, D, etc. The chain lines are denoted by Principle of Chain Surveying.

2. Subsidiary Stations

Stations which are on the primary survey lines or any other survey lines are known as secondary stations. These stations are taken to run secondary lines for dividing the area into triangles, for verifying the accuracy of triangles, and for marking interior details. These stations are denoted by letters S₁, S₂, S₃, etc.

3. Tie Stations

These are also secondary stations taken on the primary survey lines. Lines joining the tie stations are known as connecting lines. Connecting lines are mainly taken to determine the directions of adjacent sides of the chain survey map. These are also taken to form chain angles in chain traversing, when triangulation is not possible (chain angles are described in Chapter 3). Sometimes, connecting lines are taken to mark interior details. Tie stations are denoted by ” with letters T₁, T₂, Tₓ, etc, Principle of Chain Surveying.

B. Base Line

The line on which the structure of the survey is built is known as the reference line. It is the most crucial line of the survey. Generally, the longest of the primary survey lines is considered the reference line. This line should be taken through relatively flat ground, and should be measured very precisely and accurately. The magnetic directions of the reference line are taken to fix the north direction of the map, Principle of Chain Surveying.

C. Check Line

The line joining the apex point of a triangle to some fixed point on its base is known as the check line. It is taken to check the accuracy of the triangle. Sometimes this line helps to locate interior details.

D. Offset

The lateral measurement taken from an object to the chain line is known as an offset. Offsets are taken to locate objects with reference to the chain line. They may be of two kinds—perpendicular and oblique.

1. Perpendicular Offsets

When the lateral measurements are taken perpendicular to the chain line, they are known as perpendicular offsets

Perpendicular offsets may be taken in the following ways:

a. By setting a right angle by swinging a tape from the object to the chain line. The point of minimum reading on the tape will be the base of the right angle.

b. By setting a right angle using the ratio 3:4:5

c. By setting a right angle with the help of a woodworker’s square or triangular square.

d. By setting a right angle with the aid of a cross-staff or optical square.

2. Oblique Offsets

Any offset not perpendicular to the chain line is said to be oblique. Oblique offsets are taken when the objects are at a significant distance from the chain line or when it is not possible to set up a right angle due to some barriers. Such offsets are taken in the following manner, Principle of Chain Surveying.

Suppose AB is a chain line and p is the corner of a building. Two points a and b are taken on the chain line. The chainages of a and b are noted. The distances ap and bp are measured and noted in the field book. Then ap and bp are the oblique offsets. When the triangle abp is plotted, the vertex point p will represent the position of the corner of the building.

Perpendicular offsets are preferred for the following reasons:

1. They can be taken very quickly.
2. The progress of the survey is not delayed.
3. The entry in the field book becomes simpler.
4. The plotting of the offsets also becomes simplified.

3. Number of Offsets

The offsets should be taken based on the characteristics of the object. Thus, there is no strict rule regarding the number of offsets. It should be remembered that the objects must be precisely represented, and hence the number of offsets should be decided on-site.

Some recommendations are given below:

(a) When the boundary of the object is approximately parallel to the chain line, perpendicular offsets are taken at regular intervals (Fig. 2.8), Principle of Chain Surveying.

(b) When the boundary is straight, perpendicular offsets are taken at both ends of it (Fig. 2.9).

(c) When the boundary line is zigzag, perpendicular offsets are taken at every point of turn to represent the shape of the boundary accurately. In such a case, the interval of the offsets may be uneven (Fig. 2.10).

(d) When a road crosses the chain line perpendicularly, the chainage of the intersection point is to be noted (Fig. 2.11).

(e) When a road crosses a chain line obliquely, the measurements of intersection points a and b are noted. Then at least one measurement is taken on both sides of the intersection points. Additional measurements may be taken depending on the nature of the road. Here, right-angle offsets are taken at c and d (Fig. 2.12), Principle of Chain Surveying.

(f) When the building is small, its edges are determined by perpendicular or oblique offsets, and the other measurements are taken directly on-site and recorded in the field book (Fig. 2.13).

(g) When the building is large, serpentine in shape and angled to the chain line, then the corners are fixed by right-angle or angled offsets. Then the full plan of the building is drawn on a separate page along with all the dimensions. This page should be included in the field book at the proper place (Fig. 2.14), Principle of Chain Surveying.

Limiting Length of Offset

The maximum length of the offset should not surpass the length of the tape used in the survey. Typically, the maximum length of an offset is restricted to 15 m Principle of Chain Surveying.

However, this length also varies based on the following factors:

• The desired accuracy of the map,
• The scale of the map,
• The maximum permissible deviation of the offset from its actual direction, and
• The nature of the ground.

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