Chain surveying operations

CHAIN SURVEYING OPERATIONS

Steps involved in conducting chain surveying are: 1. Ranging, 2. Chaining and 3. Offsetting.

 

1. RANGING A SURVEY LINE

Ranging is done prior to Chaining. Ranging is necessary when the chain line is too long. Sometimes, chain line exceeds one chain length. In such cases, it is necessary to fix an intermediate point in line with the end points to measure the length accurately.

Ranging a Survey Line is the process of locating intermediate points in between the terminal stations or extending a survey line beyond the terminal stations in a straight line. It is done by fixing a series of pegs such that they all lie on a straight line. Ranging can be done by Eye Judgment or by using Optical Instruments such as Line Ranger or Theodolite.

1. Methods of Ranging

(i) Direct Ranging: Direct Ranging is the process of fixing intermediate points in line with the two end points by making direct observations from either of these points. This is possible when the end points are inter-visible. It is carried out by following two methods:

• Direct Ranging by Eye Judgment: (Fig. 14)

The surveyor uses his eyes to fix an intermediate point C in line with the two end points A and B. For this, two persons, Leader and Assistant, are required. Leader stands with a ranging rod at A. Another ranging rod is erected at B.

Assistant then goes with another ranging rod C at a distance not greater than one chain length from A. Leader at A signals the Assistant to move his rod to the right or left as required until the three rods appear exactly in the same straight line.

• Direct Ranging by Line Ranger: (Fig. 15)

When the distance is very long, Line Ranger is used. Line Ranger is a small reflecting instrument used for fixing an intermediate point C between A and B. The line ranger consists of two rightangled isosceles triangular prisms placed

To locate C in line with the ranging rods of A and B, stand near C approximately in line with A and B. Hold the line ranger at eye's level. Turn to see the image of the ranging rod at A. Now, the ray from A enters the upper prism, gets reflected from its hypotenuse and enters the Eye Hole E at right-angles to AB.

Similarly, the ray from B enters the lower prism, gets reflected from its hypotenuse and enters the Eye Hole E at right-angles to AB. Thus, the images of A and B are seen in upper and lower prisms.

See Fig. 16(a). If the position of C is not in line with AB, the images are seen separated.

See Fig. 16(b). Move the instrument forward or backward at right-angles to AB to bring the images in the same vertical line, one above the other. C is in line with AB. Transfer C to the ground using a plumb bob.

(ii) Indirect Ranging

It is the method of fixing an intermediate point between two end points, when they are not inter-visible.

Inter-visibility between the end points are obstructed due to (a) an obstacle such as a tall building or a thickwooded forest or a valley and (b) the distance between the end points may be too great to have a distinct visibility.

See Fig. 17. A and B are not inter visible due to the rising ground between them. C and D are the intermediate points to be fixed in line between A and B.

Select two points C₁ and D₁ in between A and B at random such that A and B are visible from them. Ask two persons with ranging rods to occupy the positions C₁ and D, such that A is seen from D₁ and B is seen from C₁.

Now, the man at C₁ directs the man at D₁ to come to D₂ in line with C₁B. The man at D₂ directs the man at C₁ to come to C₂ in line with AD₂. Again, the man at C₂ directs the man at D₂ to come to D₃ and so on. Continue the operations until both C & D are in line with AB.

 

2. CHAINING A SURVEY LINE

It is the process of measuring horizontal distance between two points in a survey line with a chain or tape. It is done after the process of ranging. For chaining operation, two Chainmen - Leader and Follower - are required. The chained distances can be plotted only when measured horizontally. But, the ground may be flat or sloping. Even if the points are on a sloping ground, chaining means the measurement of horizontal distance between them.

1. Chaining along FLAT Ground

Ranging rods are fixed at the ends of the line to be chained. Intermediate points are located at less than one chain length. The leader takes the handle of the chain and moves forward. The follower takes the other handle, places it in contact with the starting station of the line.

Now, the leader stretches the chain by aligning it with the above intermediate points. He inserts an arrow into the ground at the end of the chain to indicate one chain length. The leader then drags the chain in forward direction. The follower follows him. He places the handle of the chain in contact with the arrow and directs the leader to stretch the chain in alignment with the line.

The leader now inserts another arrow at the end of chain handle to indicate second chain length. The operation is repeated until the end station of the line is reached.

2. Chaining along SLOPING Ground

The horizontal distance of a sloping ground is measured by Direct Method, when the ground is not very steep and by Indirect Method, when the ground is steep. i)

i) Direct Method or Stepping Method: See Fig. 18.

In this method, the horizontal distance L between A and B along a sloping ground is obtained directly by placing short horizontal lengths in the form of steps. Hence, it is also known as Stepping Method..

Follower holds the zero end of the chain at the starting point A. Leader stretches the chain to a smaller length of l₁ (=AC₁) horizontally by eye judgment. Leader then transfers the position Ci to C on the ground by dropping a plumb bob or stone from C₁.

Then, the leader moves forward and the follower occupies the position C. The follower again holds the zero end of the chain at C. The leader now aligns the chain in line with AB and stretches it to a length of 12 (=CD₁) horizontally. The above procedure is repeated until the last point B is reached.

Therefore, L = (1₁ + 1₂ + 1₃ + 1₄ + 1₅)

ii) Indirect Method - By Measuring the Sloping Length and Difference in Elevations

In this method, the horizontal distance D between A and B along a sloping ground is obtained indirectly by calculation. See Fig. 19.

When the slope of the ground is not uniform, divide AB into a number of sections such as AC and CB. Measure the sloping lengths l₁ and l₂ of AC and CB respectively.

Also, measure the difference in elevations h₁ h₂  between A, C & C, B respectively. d₁ and d₂ are found as:

 

3. OFFSETTING (Refer Fig. 13)

Offset is the lateral measurement taken to the left or right of a survey line to locate the details such as buildings, boundaries, roads, etc. The process of marking the offset is Offsetting.

1. Perpendicular Offsets: The offset taken at right angles to the survey line is called Perpendicular Offset. A cross-staff or an optical square is used to locate the foot of the perpendicular from any point or to erect a perpendicular at any point.

2. Oblique Offsets: An offset taken at an angle other than 90° to a survey line is calle Oblique Offset. The operation of taking oblique offsets to locate a point involves the measurement of (i) Length and Angle or (ii) Length only or (iii) Angle only.

 

4. USES or ADVANTAGES OF CHAIN SURVEYING

1. Chain surveying is the simplest form of surveying. It requires no sophisticated equipment.

2. It is suitable when the ground is fairly level. 3. It is used to secure necessary data for making plans of small areas. 4. It is used to determine the area of land and to divide the land into a number of units.

 

5. LIMITATIONS or DISADVANTAGES OF CHAIN SURVEYING

1. It is not suitable for larger areas with more and crowded details.

2. It is not suitable for thick, bushy and wooded areas and areas with large ups and downs.

3. Errors: Errors in chaining are (i) Personal Errors caused due to inability of the surveyor to direct the line of sight of the instrument exactly towards the center of the object, (ii) Instrumental Errors caused due to imperfections in the instruments, (iii) Natural Errors caused due to natural phenomena like wind, temperature changes, etc.