Chaining/Taping On Level Ground And Sloping Ground

The process of measuring the distance between any two points by using a chain is called chaining. If used tape instead of a chain then the process is called taping. Generally, we use tape nowadays due to its accuracy and portability. Chains help us to measure the distance between any two points, which ultimately means it supports us to measure the area of any ground, as the surface of grounds can be divided into many points.

Level grounds are also called uneven ground which is the contradiction of plane surface. It is a very cozy job to measure distance on a plane surface. But it gets complexed when the surface is uneven or on a certain level.

Instruments required for taping on level grounds are:

·       Tape

·       Ranging Rod (minimum 3)

·       Peg

·       Arrows (minimum 10)

If we want to measure the distance between two points AB. Let us consider the distance between A and B is L. And let L be the distance greater than the total length of the tape. If we are using tape of 30m then L is greater than 30m. So to measure L, we divide L into different segments. One segment could be in the range of 10-20m.

Even though we have 30m tape, we do not utilize the whole tape and only use a maximum of 20m. It is because if we use the whole 30m of the tape then there is a sagging effect. This means the tape does not get stretched to its required potential as its own weight causes it to sag. So we use a maximum of 20m of our tape to measure distance in each segment.

Then the procedure to measure distance on level grounds are:

Step 1: Install peg in the two endpoints. Here, we install one peg at

 mark A and another peg at mark B.

Step 2:  Fix two ranging rod at the end stations (points) i.e. one rod at

            station A and another at station B.

Step 3: 1^st person stands about 2m behind the ranging rod at the station  

A. 2^nd person stands at point B with the ranging rod. 3^rd person carries a ranging rod and moves forward from point A. 3^rd person starts to move the ranging rod to and fro under the command of 1^st person until the ranging rod lies on line AB and mark that point as C. Then the third person installs an arrowhead at point C.

The ranging rod will lie on the line AB when this ranging rod will coincide or blocks the ranging rod at B when viewed by 1^st person at station A.

Step 4: This process is repeated several times and required

intermediate points are determined. Let us divide AB into 3 segments pointed as points A, C, D, and B.

Step 5: Now, we use the tape or chain. For this, we have preferred

tape. We measure the distance between those three segments using tape each segment at one time, starting from point A.

Important Tip: While measuring any segment, say AC we measure or use tape from the nearest ground level as possible. It is done to avoid the sag effect or the inaccurate measurement of distance due to the inclined installation of the ranging rod. Also, while measuring, we should make sure that we measure the perpendicular distance between any two points.

Let the recorded measurements of AC, CD, and DB be L1, L2, and L3 respectively. And consider them as forward length.

Step 6: Now, again we start to measure different segments. But this

time we start to measure from point B. For this, again we divide the length AB into several segments. But this time we start from point B. Now, the 1^st person starts at point B and starts ranging. Let the intermediate points be D’, and C’.

We cannot use the previous intermediate points C and D while measuring from B, because we have to provide an independent check. If we had used points C and D, then it would provide us a dependent check and it would not make sense as we just measured the same distance segments two times.

Let the recorded lengths of BD', D'C', and C'A be L1' L2' AND L3' respectively. Consider those lengths as backward lengths.

 

Hence, total forward length(L)= L1+L2+L3

          Total backward length(L’) =L1’+L2’+L3’

          Average length= (L+L’)/2

          Discrepancy= │ L – L’ │

The average length is the required measurement of length L, only if the precision is greater than or equal to (≥) 1 in 2000. If precision does not lie within this range, then the whole process is carried out again, unless a precision is obtained. Then only the required length is obtained.

Precision=1/average length/discrepancy

What do you mean by precision is 1 in 2000 (1/2000)?

The precision is 1 in 2000. This means while measuring the distance of 2000m, the discrepancy should only be 1m. Or, while measuring the distance of 2000m, the fluctuating value (or change in value) between forward and backward length should only be a maximum of 1m.

 

Chaining/Taping on sloping ground

By the direct method,

The whole process is the same as above i.e. taping on level ground. For measuring the sloping ground, the precision should be 1 in 1000.

Here the distance between point A and B is L1+L2+L3 while starting from point A. Again we determine the different length segments initiating from point B. Finally, we check the precision using discrepancy and average length and obtain the required measurement.

Important Tip:

Q. How do we know the measurement reading we have taken is perpendicular value or exactly straight or accurate measurement?

Ans: suppose from the above figure, we have starting end of the tape is at A and we have to take perpendicular measurement with ranging rod at C. For this we fix, starting end of tape at A and lengthen tape towards the ranging rod at C. To obtain an exactly perpendicular measurement, we move the tape's end to and fro (or up and down) at in ranging rod at C until the shortest distance is measured at the tape in point C. This shortest distance in the tape is the exactly perpendicular measurement.

Q. What is the least count of engineering tape?

Ams: The least count of engineering tape is 2mm or 0.002m. Hence, while taking reading from tape in the field the third digit after the dot should always be a even number. For eg: 5.314m, 7.798m, etc

Here our reading is 5.314 means our length is  5.3m, 1cm and 4mm.