Errors in Chaining - Errors in Chain Surveying, Surveying and Levelling | Surveying and Levelling Notes- Agricultural Engg - Agricultural Engineering PDF Download

Errors in Chaining: - The errors that occur in chaining are classified as (i) Compensating, (ii) Cumulative.  These errors may be due to natural causes such as say variation in temperature, defects in construction and adjustment of the instrument, personal defects in vision etc.

Compensating Errors:- The compensating errors are those which are liable to occur in either direction and hence tend to compensate i.e. they are not likely to make the apparent result too large or too small.
In chaining, these may be caused by the following:
The follower may not bring his handle of the chain to the arrow, but may hold it to one or other side of the arrow.
Fractional parts of the chain or tape may not be correct if the total length of the chain is adjusted by insertion or removal of a few connection rings from one portion of the chain, or tape is not calibrated uniformly throughout its length.
During stepping operation crude method of plumbing (such as dropping of stone from the end of chain) is adopted.
When chain angles are set out with a chain which is not uniformly adjusted or with a combination of chain and tape.

Cumulative Errors: - The cumulative errors are those which occur in the same direction and tend to add up or accumulate i.e. either to make the apparent measurement always too long or too short.
Positive errors (making the measured lengths more than the actual) are caused by the following:
The length of the chain or tape is shorter than the standard, because of bending of links, removal of too many links in adjusting the length, ‘knots’ in the connecting links, cloggings of rings with clay, temperature lower than that at which the tape was calibrated, shrinkage of tape when becoming wet.
The slope correction is not applied to the length measured along the sloping ground.
The sag correction is not applied when the tape or the chain is suspended in the air.
Measurements are made along the incorrectly aligned line.
The tape bellys out during offsetting when working in the windy weather.
Negative errors (making the measured lengths less than the actual) may be caused because the length of the tape or chain may be greater than the standard because of the wear or flattening of the connecting rings, opening of ring joints, temperature higher than the one at which it was calibrated.
The final error in a linear measurement is composed of two portions:
cumulative errors which are proportional to L and
compensating errors which are proportional to √L, where L is the length of the line.

Illustration: Suppose a line 1280 m in length is measured with a 20 m chain which is 0.02 m too long, and error in marking a chain length is say ±0.03 m.
Compensating error of marking

The latter error though smaller has a greater effect than the former though it is larger.
Mistakes in Chaining: - The mistakes are generally avoidable and cannot be classed under any law of probability.  The following mistakes are commonly made by inexperienced chainmen.
Displacement of arrows: - When the arrow is displaced, it may not be replaced accurately.  To guard against this mistake, the end of each chain length should be marked both by the arrow and by a cross (+) scratched on the ground.

Failure to observe the position of the zero point of the tape: The chainmen should see whether it is at the end of the ring or on the tape.
Adding or omitting a full chain or tape length (due to wrong counting or loss of arrows): - This is the most serious mistake and should be guarded against.  This is not likely to occur, if the leader has the full number (ten) of arrows at the commencement of chaining and both the leader and follower count them at each transfer.  A whole tape length may be dropped, if the follower fails to pick up the arrow at the point of beginning.
Reading from the wrong end of the chain: - e.g. reading 10 m for 20 m in a 
30 m chain, or reading in the wrong direction from a tally, e.g. reading 9.6 m for 10.4 m.  The common mistake in reading a chain is to confuse 10 m tag with 20 m tag.  It should be avoided by noticing the 15 m tag.
Reading numbers incorrectly: Transposing figures e.g.37.24 for 37.42 or reading tape upside down, e.g. 6 for 9, or 36 for 98.
Calling number wrongly: e.g. calling 40.2 as “forty two”.
Reading wrong metre marks: e.g. 58.29 for 57.29.
Wrong booking: - e.g. 345 for 354.
To guard against this mistake, the chainmen should call out the measurements loudly and distinctly, and the surveyor should repeat them as he books them.