Lesson 3. On Chain Surveying Introduction

3.1 PRINCIPLE OF CHAIN SURVEYING

The principle of chain surveying is triangulation. This means that the area to be surveyed is divided into a number of small triangles which should be well conditioned. In chain surveying the sides of the triangles which should be well conditioned. In chain surveying the sides of the triangles are measured directly on the field by chain or tape, and no angular measurements are taken. Here, the tie lines and check lines control the accuracy of work.

It should be noted that plotting triangles requires no angular measurements to be made, if the three sides are known.

Chain surveying is recommended when:

1.     The ground surface is more or less level

2.     A small area is to be surveyed

3.     A small-scale map is to be prepared and

4.     The formation of well-conditioned triangles is easy

Chain surveying is unsuitable when:

1.     The area is crowded with many details

2.     The area consists of too many undulations

3.     The area is very large and

4.     The formation of well-conditioned triangles becomes difficult due to obstacles

A. Large-Scale and Small-Scale Maps

When 1 cm of a map represents a small distance, it is said to be a large-scale map.

For example,

         

When 1 cm of the map represents a large distance, it is called a small-scale map.

For example,

A map having an RF of less than 1/500 is considered to be large-scale. A map of RF more than 1/500 is said to be small-scale.

3.2 WELL-CONDITIONED AND ILL-CONDITIONED TRIANGLES

A triangle is said to be well-conditioned when no angle in it is less than 30⁰ or greater than 120⁰ . An equilateral triangle is considered to be the best-condition or ideal triangle

Well-conditioned triangles are preferred because their apex points are very sharp and can be located by a single ‘dot’. In such a case, there is no possibility of relative displacement of the plotted point.

A triangle in which an angle is less than 30⁰ or more than 120⁰ is said to be ill-conditioned

Well - conditioned triangles are not used in chain surveying. This is because their apex points are not sharp and well defined, which is why a slight displacement of these points may cause considerable error in plotting.

3.3 RECONNAISSANCE SURVEY AND INDEX SKETCH

Before the commencement of any survey work, the area to be surveyed is thoroughly examined by the surveyor, who then thinks about the possible arrangement of the framework of survey. This primary investigations of the area is termed as reconnaissance survey or reconnoitre.

During reconnaissance survey, the surveyor should walk over the area and note the various obstacles and whether or not the selected stations are intervisible. The main stations should be so selected that they enclose the whole area. The surveyor should also take care that

The neat hand sketch of the area which is prepared during reconnaissance survey is known as the ‘index sketch’ or ‘key plan’. The index sketch shows the skeleton of the survey work. It indicates the main survey stations, sub-stations, tie stations, base line, arrangement for framework of triangles and the approximate positions of different objects. This sketch is an important document  for the surveyor and for the person who will plot the map. It should be attached to the starting page of the field book

3.4 DEFINITIONS AND ILLUSTRATIONS

A. Survey Stations

Survey stations are the points at the beginning and the end of a chain line. They may also occur at any convenient points on the chain line. Such stations may be:

1.     Main stations

2.     Subsidiary stations and

3.     Tie stations

2.     Main stations Stations taken along the boundary of an area as controlling points are known as  ‘main survey lines’. The main survey lines should cover the whole area to be surveyed. The main stations are denoted by ‘ ’ with letters A, B, C, D, etc. The chain lines are denoted by “__ … __ ... __...__...__...__”.

3.     Subsidiary stations Stations which are on the main survey lines or any other survey lines are known as “Subsidiary stations”. These stations are taken to run subsidiary lines for dividing the area into triangles, for checking the accuracy of triangles and for locating interior details. These stations are denoted by ‘’ with letters S₁,S₂,S₃, etc.

4.     Tie stations  These are also subsidiary stations taken on the main survey lines. Lines joining the tie stations are known as tie lines. Tie lines are mainly taken to fix 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. Sometimes tie lines are taken to locate interior details. Tie stations are denoted by ‘’ with letters T₁, T₂, T₃. Etc.

B.     Base Line

The line on which the framework of the survey is built is known as the ‘base line’. It is the most important line of the survey. Generally, the longest of the main survey lines is considered the base line. This line should be taken through fairly level ground, and should be measured very carefully and accurately. The magnetic bearings of the base line are taken to fix the north line of the map.

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 ‘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 perpendicular 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 perpendicular

(b) By setting a right angle in the ratio 3 : 4 : 5

(c) By setting a right angle with the help of builder’s square or tri-square

(d) By setting a right angle by 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 long distance from the chain line or when it is not possible to set up a right angle due to some difficulties. Such offsets are taken in the following manner.

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 apex point p will represent the position of the corner of the the building.

Perpendicular offsets are preferred for the following reasons:

(a) They can be taken very quickly

(b) The progress of survey is not  hampered

(c) The entry in the field book becomes easy

(d) The plotting of the offsets also becomes easy

3. Number of offsets The offsets should be taken according to the nature of the object. So, there is no hard and fast rule regarding the number of offsets. It should  be remembered that the objects are to be correctly represented and hence the number of offsets should be decided on the field. Some guidelines are given below:

(a) When the boundary of the object is approximately parallel to the chain line, perpendicular offsets are taken at regular intervals

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

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

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

(e) When a road crosses a chain line obliquely, the chainages of intersection points ‘a’ and ‘b’ are noted. Then at least one offset is taken on both sides of the inter-section points. More offsets may be taken depending on the nature of the road. Here, perpendicular offsets are taken at ‘c’ and ‘d’

(f) When the building is small, its corners are fixed by perpendicular or oblique offsets and the other dimensions are taken directly on the field and noted in the field book.

(g) When the building is large, zigzag in shape and oblique to the chain line, then the corners are fixed by perpendicular or oblique offsets. Then the full plan of the building is drawn on a separate page along with all the dimensions. This page should be attached with the field book at the proper place.

(h) When the object is circular, perpendicular offsets are taken at short and regular intervals

4. Limiting length of offset The maximum length of the offset should not be more than the length of the tape used in the survey. Generally, the maximum length of offset is limited to 15m. However, this length also depends upon the following factors:

(a) The desired accuracy of the map

(b) The scale of the map

(c) The maximum allowable deflection of the offset from its true direction and

(d) The nature of the ground

Problems on limiting length of offset

Problem 1  An offset was laid out 5⁰ from its true direction and the scale of the map was 20 m to 1 cm. Find the maximum length of offset for the displacement of a point on the paper not to exceed 0.03 cm.

Solution Let AB be the actual length of offset which was laid  out 5⁰ from its true direction. So, BC is the displacement of the point.

            Let the maximum length of offset,     AB = L m

                         

or         BC = AB sin 5⁰ = L sin 5⁰ m  (displacement of the ground)

Since the scale is 1 cm to 20 m, 20 m on the ground represents 1 cm on the paper.

             

     = 6.884 m

            Therefore, the maximum length of offset should be 6.884 m.

Problem 2  The length of the offset is 15 m and the scale of the plan 10 m to 1 cm. If the offset is laid out 3⁰ from its true direction, find the displacement of the plotted point on the paper

(i) perpendicular to the chain line, and

(ii) parallel to the chain line.

Solution Let AB be the actual length of offset, which is 15 m long and deflected by 3⁰ from its true direction.

Here,

            BC = Displacement parallel to chain line

                        CD = displacement perpendicular to chain line

(i)  CD = AD – AC = AB - AC

             = 15 – 15 cos 3⁰

             = 15 (1 – cos 3⁰) m (displacement on the ground)

            Since the scale is 1 cm to 10 m,

            10 m on the ground = 1 cm on the map

            

                                                          =  0.002 cm on the map

            Required displacement perpendicular to chain line

          = 0.002 cm (on paper)

(ii) BC = AB sin 3⁰ = 15 sin 3⁰ = 0.7850 m (displacement on ground)

     

E. Degree of Accuracy

Degree of accuracy is determined before the starting of any survey work. It is worked out according the following factors:

(a) Scale of plotting

(b) Permissible error in plotting

During reconnaissance survey, the length of the main survey lines are approximately determined by the pacing method. One pace or walking step of a man is considered to equal 80 cm. When the length of the survey lines or the extent of area to be surveyed is approximately known, the scale of the map may be assumed. Again, the permissible error in plotting may be obtained from the concerned department. Then the degree of accuracy in measurement is ascertained.

Let us now consider an example.

Suppose the scale of plotting is 5 m to 1 cm and the allowable error is 0.02 cm.

Then,  1 cm on the map = 500 cm on the ground

        0.02 cm on the map = 500 x 0.02 = 10 cm on the ground

So, the measurement should be taken nearest to 10 cm.

3.5 SELECTION OF SURVEY STATIONS

The following points should be remembered during the selection of survey stations:

1. The stations should be so selected that the general principle of surveying may be strictly followed.
2. The stations should be intervisible.
3. The stations should be selected in such a way that well-conditioned triangles may be formed.
4. The base line should be the longest of the main survey lines.
5. The survey lines should be taken through fairly level ground, as far as practicable.
6. The main survey lines should pass close to the boundary line of the area to be surveyed.
7. The survey lines should be taken close to the objects so that they can be located by short offsets.
8. The tie stations should be suitably selected to fix the directions of adjacent sides.
9. The subsidiary stations should be suitably selected for taking check lines.
10. Stations should be so selected that obstacles to chaining are avoided as far as possible.
11. The survey lines should not be very close to main roads, as survey work may then be interrupted by traffic.