Rods which are used for ranging (i.e. the process of making a line straight) a line are known as ranging rods. Such rods are made of seasoned timber or seasoned bamboo. Sometimes GI pipes of 25 mm diameter are also used as ranging rods. They are generally circular in section, of 25 mm diameter and 2 m length. Sometimes wooden ranging rods are square in section. The rod is divided into equal parts of 20 cm each and the divisions are painted black and white or red and white alternately so that the rod is visible from a long distance. The lower end of the rod is pointed or provided with an iron shoe (Fig. 1.2).
A chain is prepared with 100 or 150 pieces of galvanised mild steel wire of 4 mm diameter. The ends of the pieces are bent to form loops. Then the pieces are connected together with the help of three oval rings, which make the chain flexible. Two brass handles are provided at the two ends of the chain. Tallies are provided at every 10 or 25 links for facility of counting. ‘One link’ means the distance between the centres of adjacent middle rings (Fig. 1.3).
The following are the different types of chains:
(a) Metric chain,
(b) Steel band,
(c) Engineers’ chain,
(d) Gunter’s chain, and
(e) Revenue chain.
Metric chains are available in lengths of 20 m and 30 m.
The 20 m chain is divided into 100 links, each of 0.2 m. Tallies are provided at every 10 links (2 m). This chain is suitable for measuring distances along fairly level ground. The arrangement of tallies is shown in Fig. 1.4(a).
You may see from the arrangement of tallies that the central tally is round and that the other tallies have one, two, three or four teeth. So, each tooth may correspond to two different readings when considered from opposite ends. Therefore, during the measurement, the surveyor should bear in mind the position of the central tally.
As per ISI recommendations, tallies should be provided after every 5 m and brass rings after every I m. In Fig. 1.4(b), the central tally has two teeth and the tallies on opposite sides of it have one tooth each.
The 30 m chain is divided into 150 links. So, each link is of 0.2 m. The tallies are provided after every 25 links (5 m). A round brass ring is fixed after every metre. This chain is heavy and is also suitable for measuring distances along fairly level ground. Here the central tally has three teeth (Fig. 1.5).
It consists of a ribbon of steel of 16 mm width and of 20 or 3() m length. It has a brass handle at each end. It is graduated in metres, decemetres, and centimetres on one side and has 0.2 m links on the other. The steel band is used in projects where more accuracy is required.
The engineer’s chain is 100 ft long and is divided into 100 links. So, cach link is of I ft. Tallies are provided at every 10 links (10 ft), thc central tally being round. Such chains were previously used for all engineering works.
Gunter s’ Chain
It is 66 ft long and divided into 100 links. So, each link is of ().66 ft. It was previously used for measuring distances in miles and furlongs.
The revenue chain is 33 ft long and divided into 16 links. It is mainly used in cadastral survey.
Chains have the following advantages:
1. They can be read easily and quickly.
2. They can withstand wear and tear.
3. They can be easily repaired or rectified in the field.
They have the following disadvantages:
1. They are heavy and take too much time to open or fold.
2. They become longer or shorter due to continuous use.
3. When the measurement is taken in suspension, the chain sags excessively.
Steel bands have the following advantages:
1. They are very light and easy to open or fold.
2. They maintain their standard length even after continuous use.
3. When the measurement is taken in suspension, they sag slightly.
They have the following disadvantages:
1. If handled carelessly, they break easily.
2. They cannot be repaired in the field.
3. They cannot be read easily.
The following are the different types of tapes:
(a) Cloth or linen tape,
(a) Metallic tape,
(c) Steel tape, and
(d) Imar tape.
Cloth or Linen Tape Such a tape is made of closely woven linen and is varnished to resist moisture. It is 15 mm wide and available in lengths of 10 and 15 m. This tape is generally used for measuring offsets and for ordinary works.
Metallic Tape When linen tape is reinforced with brass or copper wires to make it durable then it is called a metallic tape. This tape is available in lengths of 15, 20 and 30 m. It is wound on a leather case with a brass handle at the end. It is commonly used for all survey works.
Steel Tape The steel tape is made of steel ribbon of width varying from 6 to 16 mm. The commonly available lengths are 10, 15, 20, 30 and 50 m. It is graduated in metres, decimetres and centimetres. It is not used in the field, but chiefly for standardising chains and for measurements in constructional works.
Invar Tape Invar tape is made of an alloy of steel (64%) and nickel (36%). Its thermal coefficient is very low. Therefore, it is not affected by change of temperature. It is made in the form of a ribbon of 6 mm width and is available in lengths of 30, 50 and 100 m. It is used at places sshere maximum precision is required. It is generally used in the triangulation survey conducted by the Survey of India department.
Arrows are made of tempered steel wire of 4 mm diameter. One end of the arrow is bent into a ring of 50 mm diameter and the other end is pointed. Its overall length is 400 mm. Arrows are used for counting the number of chains while measuring a chain line (Fig. 1.6).
The process of establishing intermediate points on a straight line between two end points is known as ranging. Ranging must be done before a survey line is chained. Ranging may be done by direct observation by the naked eye or by line ranger or by theodolite. Generally, ranging s done by the naked eye with the help of three anging rods.
Ranging may be of two kinds:
l. Direct, and
2. Indirect or reciprocal.
When intermediate ranging rods are fixed on a straight line by direct observation from end stations, the process is known as direct ranging. Direct ranging is possible when the end stations are intervisible. The following procedure is adopted for direct ranging.
Assume that A and B are two end stations of a chain line, where two ranging rods are already fixed. Suppose it is required to fix a ranging rod at the intermediate point P on the chain line in such a way that the points A, P and B are in the same straight line. The surveyor stands about 2 m behind the ranging rod at A by looking towards the line AB. The assistant holds a ranging rod at P vertically at arm’s length. The rod should be held lightly by the thumb and forefinger. Now, the surveyor directs the assistant to move the ranging rod to the left or right until the three ranging rods come exactly in the same straight line. To check the non-verticality of the rods, the surveyor bends down and looks through the bottom of the rods. The ranging will be perfect when the three ranging rods coincide and appear as a single rod. When the surveyor is satisfied that thc ranging is perfect, he signals the assistant to fix the ranging rod on the ground by waving both his hands up and down. Following the same procedure, the other ranging rods may be fixed on the line (Fig. 1.7).
Indirect or Reciprocal Ranging
When the end stations are not intervisible due to there being high ground between them, intermediate ranging rods are fixed on the line in an indirect way. This method is known, as indirect ranging or reciprocal ranging. The following procedure is adopted for indirect ranging.
Suppose A and B are two end stations which are not intervisible due to high ground existing between them. Suppose it is required to fix intermediate points between A and B. TWO chain men take up positions at RI and Sl with ranging rods in their hands. The chainman at RI stands with his face towards B so that he can see the ranging rods at Sl, and B. Again, the chainman at Sl stands with his face towards A so that he can see the ranging rods at RI and A. Then the chainmen Proceed to range the line by directing each other alternately. The chainman at RI directs the chainman at Sl to come to the position S2 so that RI, S2 and B are in the same straight line. Again, the chainman at S2 directs the chainman at RI to move to the position at R2 so that S2, R2 and A are in the same straight line. By directing each other alternately in this manner, they change their positions every time until they finally come to the positions R and S, which are in the straight line AB. This means the points A, R, S and B are in the same straight line (Fig. 1.8).
UNFOLDING AND FOLDING A CHAIN
To open a chain, the strap is unfastened and the two brass handles are held in the left hand and the bunch is thrown forward with the rieht hand. Then one chainman stands at the starting station by holding one handle and another tnoves forward by holding the other handle until the chain is cornpletely e x tended.
To fold the chain, a chainman should move fonsard by pulling the chain at the middle. Then the two halves of the chain come side by side. After this, commencing from the central position of the chain, two pairs of links are taken at time with the right hand and placed on the left hand alternately in both directions’ Finally, the two brass handles will appear at the top. The bunch should be then fastened by the strap.
TESTING A CHAIN
Due to continuous use, a chain may be elongated or shortened. So, the chain should be tested and adjusted accordingly. If full adjustment is not possible then the amount of shortening (known as ‘too short’) and elongation (known as ‘too long’) should be noted clearly for necessary correction applicable to the chain•
For testing the chain, a test gauge is established on a level platform with the help of a standard steel tape. The steel tape is standardised at 200C and under a tension of 8 kg. The test gauge consists of two pegs having nails at the top and fixed on a level platform a required distance apart (say 20 or 30 m). The incorrect chain is fully stretched by pulling it under normal tension (say about 8 kg) along the test gauge. If the length of the chain does not tally with standard length’ then an attempt should be made to rectify the error. Finally, the amount of elongation or shortening should be noted (Fig. 1.9).
The allowable error is about 2 mm per 1 m length of the chain. The overall length of the chain should be within the following permissible limits:
ADJUSTMENT OF CHAIN
Chains are adjusted in the following ways:
l. When the chain is too long, it is adjusted by
(a) Closing up the joints of the rings,
(b) Hammering the elongated rings,
(c) Replacing some old rings by new rings, and (d) Removing some of the rings.
2. When the chain is too short, it is adjusted by
(a) Straightening the bent links,
(b) Opening the joints of the rings,
(c) Replacing the old rings by some larger rings, and
(d) Inserting new rings where necessary.
DEGREE OF ACCURACY IN CHAINING
The degree of accuracy in chaining is expressed as a ratio called the chaining ratio. The chaining ratio may be 1/1000, 1/2000, etc.
For example, if there is an error of 0.25 m during the measurement of a total length of 500 m,
Chaining ratio= 0.25/500 =25/500× 1OO=1/2000
Some permissible limits of error:
l. For measurement with steel band— 1/2,000
2. For measurement with tested chain— 1/1000
3. In normal conditions— 1/500
4. For rough work— 1/250