RANGING AND CHAINING OF SURVEY LINES

RANGING A SURVEY LINE

Process of establishing intermediate points on a survey line, The process between two stations is known as ranging of survey lines or simply ranging. Ranging must be done before chaining a survey line, if the distance between two stations is more than the length of a chain or tape. Ranging may be done by the naked eye or by the line ranger or by theodolite. For important works theodolite can be used for ranging a survey line.
Ranging may be of two kinds:
(a) Direct ranging, and
(b) Indirect or reciprocal ranging.

DIRECT RANGING:

The ranging of a survey line done by direct observation from end stations, when the end stations are intervisible, is called Direct Ranging. The direct ranging is carried out by an eye or with line ranger.
1. Ranging by Eye:

The ranging with eye is carried out by following steps (Fig.)
(i) A and B be the two points at the ends of survey line.
(ii) Erect ranging rods at stations A and B.
(Ⅲ) The surveyor standing 1.5 to 2 m behind the ranging rod at A, directs the assistant to put a ranging rod approximately in line, between A and B.
(iv) The surveyor standing behind A, signals the assistant to move left or right until three ranging rods are exactly in a straight line.
(v) The surveyor check the verticality of the rods by sighting the lower ends of the rods.
(vi) Similarly, ranging is continued to put required no of Intermediate points, in between A and B
2. Ranging by Line Ranger:Line ranger is an optical instrument used for fixing intermediate points on a chain line. A line ranger consists of either two plane mirrors or two right angled isoscales prisms placed one above the other, as shown in Fig. 2.19 (a). The diagonals of the two prisms are silvered so as to reflect the incidental rays. A handle with a hook is provided at the bottom to hold the instrument in hand and to transfer the point on to the ground with the help of plumb – bob.
Procedure:
1. To fix an intermediate station ‘p’ on the line AB, the observer stands as near P as possible and holds the Instrument at his eye level.
2. A ray of light from A enters the upper prism and gets reflected from the hypotenuse ac, and visible to the surveyor at right angles to the AB.
3. Similarly another light ray from B enters the lower prism and gets reflected from the hypotenuse bd, and visible to the surveyor at right angles to the AB.
4. Surveyor observe the images of A and B in the upper and lower prisms at a time.
5. If the point ‘p’ is not in line of AB two images will be seen separated as shown in Fig(b).
6. The surveyor then moves the instrument side ways till the two images are visible in the same vertical line as shown in Fig (c).
7. The point ‘p’ is then transferred to the ground with the help of a plumb-bob.
8. Hence, the instrument can be conveniently used for fixing intermediate points on a survey lines without going to either end.
9. This method is most convenient, when there is no assistant is available, as it can be done only by surveyor

INDIRECT OR RECIPROCAL RANGING:

When both the end survey stations are not intervisible either due to high ground in between them or due to long distance between them, the ranging is done by Indirec or Reciprocal ranging.
Procedure:
1. A and B are two end stations, which are not intervisible due to a hill in between them.
2. Select two intermediate points P₁ and Q1, such that from each station point A and B are visible.
3. Two persons take up the positions P1, and Q₁ with ranging rods.
4. First the person standing at P1 directs the person at Q1 to come in line of P1*B and his new position will be Q2
5. Now, the person standing at Q_{2} directs the person at P1 to come in line of Q2*A and his new position will be P2
6. Now, the person standing at P2 directs the person at Q2 to come in line of P2*B and his new position will be Q_{3}
7. This process is continued until the intermediate points P and Q are located in such a way that the person standing at P, see Q and B in the line, and the person standing at Q. see P and A in the line.
8. Finally, the accuracy of work is checked from A and B.

CHAINING A SURVEY LINE

To chain a line two persons are essentially required. The first person (surveyor) who pulls the chain on the ground in the forward direction is called leader And the second person (surveyor) who holds the rear end of the chain is called follower. While chaining a line the follower play an important role and all the measurements taken are mainly dependent on the judgement and careful observation of the follower. Hence, the follower should be well known about the chain surveying.
(a) Duties of a leader :
The duties of the leader are as follows.
1. To pull the chain forward direction
2 To carry the arrows and ranging rods.
3. To fix arrow at the end of every chain length.
4. To obey the instructions of the follower.
(b) Duties of a Follower:
The duties of the follower are as follows.
1. To direct the leader at the time of ranging.
2. To carry the rear handle of the chain.
3. To pick up the arrows inserted by the leader after every chain length.
4. To give necessary instructions to the leader.

UNFOLDING AND FOLDING A CHAIN

(a) Unfolding a chain:
1. Before taking measurements the chain should be unfolded
2. To unfold a chain, the strap is unfastened.
3. Now, the chain is lifted into right hand, keeping brass handles in left hand, the chain should be thrown with right hand
4. Then follower stands at the starting station by holding one handle and leader moves forward by holding the other handle until the chain is completely opened.
5. Now, the chain is checked for correct length by observing if there is any kinks or bent in links.
(b) Folding a chain :
1. After the days work is completed the chain should be folded and fastened with a leather strap.
2. To fold a chain, the two handles are to be brought to one end by pulling the chain at middle.
3. After this, commencing from the middle of the chain, two pairs of links are taken at a time with the right hand and placed on the left hand alternatively in both directions.
4. Finally, the two brass handles will appear at the top. The bunch should be then fastened with the leather strap.

METHOD OF CHAINING ON LEVEL GROUND

1. Before starting the chaining, two ranging rods are fixed on the chain line, at the end stations.
2. An intermediate stations can be fixed near the end of each chain length, by ranging the line.
3. To start chaining operation first of all the chain should be unfolded properly and checked for its correctness.
4. Then, the follower places the zero end of the chain in contact with the peg at the beginning of the survey line.
5. The leader moves forward by taking handle of the chain in one hand, ten arrows and a ranging rod in the other hand.
6. When the leader reaches approximately one chain length, the follower directs the leader, using code of signals, to fix the ranging rod in line of the two end stations.
7. Then the follower stands firmly on the handle with the heels on it, and the leader stretches the chain by moving it up and down with both the hands and finally places it on the line.
8. He then inserts an arrow on the ground at the end of the chain and marks a cross (‘X’).
9. Now, the leader picks up the ranging rod with remaining nine arrows. He moves forward by swinging the chain a little off the line, so that the arrow placed in position may not be disturbed.
10. The follower carrying the rear handle of the chain comes upto the arrow inserted by the leader at one chain length.
11. The leader fixes another arrow at the end of the chain length. As the leader moves further, the follower picks up the arrows which were inserted by the leader.
12. During chaining, the surveyor or an assistant should conduct the ranging operation.
13. The process is repeated in this way until the complete length of the line is measured. The number of arrows in the hand of follower indicates the number of chain lengths completed.
14. To measure the fractional length at the end, the leader should pull the chain beyond the station and the follower should hold the zero end of the chain at the last arrow. Then the links should be counted by the follower or surveyor.
15. The total length of the survey line can be found by multiplying the number of full chains completed with the length of chain plus the fractional length of the chain at end.

METHOD OF CHAINING ON SLOPING GROUND

Chaining on sloping ground gives the sloping distance. But for all plotting works, horizontal distances between the point are required. Hence, it is necessary either to directly measur the horizontal distances between the points or to measure th sloping distance and calculate its horizontal equivalent. Thus there are two methods for determining horizontal distance o sloping ground (a) direct method or stepping method ( indirect method.
(a) Direct Method or Stepping Method:
This method is applied when the slope of the ground is ve steep. In this method, the sloping ground is divided into number of horizontal and vertical strips, like steps. The lengt of the horizontal portions are measured with chain or ta and added to get the total horizontal distance between points. The steps may not be uniform, and they would depen on the nature of the ground.
(b) Indirect method :
This method is adopted when the slope of the ground surface is uniform and the stepping method is not suitable. In this case the horizontal distance can be calculated by measuring the sloping distance and the angle of slope or the difference in elevation between the points. To calculate the horizontal distance indirectly, the following methods can be applied.
1. By measuring the angle of slope with clinometer,
2. By measuring the difference in level with a leveling instrument and
3. By applying hypotenusal allowance.