After reading this article you will learn about the errors occur in chaining along with its limits.
1. Instrumental Errors:
They occur due to faulty adjustments or imperfections of the instruments or devices such as chain or tape may be too long or too short etc.
2. Natural Errors:
ADVERTISEMENTS:
They arise due to variations in the phenomenon of nature such as temperature etc.
3. Personal Errors:
They are due to chain or tape not being straight or applying different tension in the chain than the standard one etc.
These errors are classified as:
ADVERTISEMENTS:
(i) Compensating errors, and
(ii) Cumulative errors.
The compensating errors are those which occur in either direction and tend to compensate at the end. These errors do not affect the survey work seriously. The cumulative errors are those which occur in one direction only and go on cumulating. These errors have got a serious effect on the accuracy of the survey work. The errors in chaining are regarded as +ve or -ve according as they make the result too great or too small.
Sources of Error and Their Prevention:
ADVERTISEMENTS:
(i) Erroneous length of the chain or tape (cumulative, + or -):
The chain or tape may either be too long or too short and the error in length of the line is negative or positive accordingly. This is the most serious source of error in chaining. This may be eliminated either by standardising the chain before commencement of the survey or by applying correction to the measured distance by the use of formula,
(ii) The chain getting out of line due to wrong ranging:
ADVERTISEMENTS:
If the ranging is wrong, the chain will be stretched out of line measuring more distance than the real one thus making the error +ve. The error is not considered to be serious if only the length of line is needed but it becomes very serious if offsetting also has to be done. This can be remedied by careful ranging.
(iii) Chain or tape not being straight (cumulative, +):
While chaining through grass, bush or when wind storm is blowing, the chain or tape cannot be stretched straight. The effect will be the same as that while measuring with the chain or tape which is too short. This error is always positive. This can be minimised by applying the proper pull and clearing the obstacles.
(iv) Sag in the chain or tape (cumulative, +):
ADVERTISEMENTS:
This type of error generally occurs during the process of stepping on sloping ground. This is always positive. This can be minimised by decreasing the length of the step and pulling the chain firmly.
(v) Variation in temperature (cumulative, + or-):
The length of a chain or a tape changes when it is used at a different temperature from that at which it was calibrated. The length of chain increases due to rise in temperature and decreases due to fall in temperature making the error -ve and + ve respectively. For minor changes in the temperature, it is neglected because a change of 15° F in temperature will produce a change in length of about 0.01 link for a 100 link chain. However when great accuracy is required, correction has to be applied.
(vi) Variable tension in the chain (compensating + or -):
If the pull applied at the ends of the chain is greater than the standard pull, the length increases while if it is less, the length will decrease. The error may be negative or positive and is generally neglected for ordinary surveys. It can be prevented by applying only the desired standard pull.
Mistakes in Chaining:
Mistakes are generally made by inexperienced chairmen. These can be avoided by careful working.
The following are the common mistakes made in the field:
(i) Miscounting the chain length:
This is the most serious mistake and occurs due to wrong counting or due to loss of arrow. It can be guarded against if leader takes full number (usually ten) of arrows at the commencement of work and at each transfer, the follower and the leader count them.
(ii) Displacement of arrows:
When once an arrow is displaced, it may not be replaced correctly. To avoid this, the end of the chain length should be marked both by scratching a cross on the ground and fixing an arrow.
(iii) Misreading:
This happens due to reading from the wrong end of the chain e.g. reading 8 m for 12 m in 20m chain. It can be avoided by carefully noticing the position of the central tag.
(iv) Some mistakes like trasposing figures e.g. 58.38 for 58.83 ; reading tape upside down e.g. 6 for 9 or 68 for 89 ; calling numbers wrongly e.g. calling 20.1 as 21 and wrong booking e.g. 264 for 246 are also common, these can be avoided by care and good power of judgement of the surveyor.
Limits of Errors in Chaining:
1. For measuring over rough or somewhat hilly ground, the permissible error is 1 in 250.
2. In Ordinary Chain Survey, the maximum permissible error is 1 in 1000. The probable error in any single measurement should be much less than this amount.
3. If a steel tape or a steel band chain is used to obtain greater accuracy in measurements, the limiting error may be 1 in 2000.
4. When the standardised steel or invar tape is used and corrections for pull, temperature, sag, slope and alignment etc. are applied, the maximum error should not exceed 1 in 5000.