Definitions & Illustrations - Chain Surveying, Surveying and Levelling | Surveying and Levelling Notes- Agricultural Engg - Agricultural Engineering PDF Download

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
4. 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 “__ … __ ... __...__...__...__”.
(i) 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.
(ii) 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. 

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.