Errors in Chain Surveying – Types and Causes

The errors formed in chaining can be either cumulative error and compensating error. The errors can be considered either as a negative value or positive value depending on whether the result is a higher value or smaller value.

Cumulative Error and Compensating Error

The error that occurs during the chaining process in the same direction is called as cumulative error. This type of error accumulates with the process of chaining.

An error that occurs in either directions during the chaining process is called as compensating error. As these errors takes place in either directions, the values compensate.

Causes of Errors

The basic reasons for errors caused in the chaining process in surveying are due to:

1. The Chain or tape with erroneous length

2. Inefficient Ranging

3. Inefficient Straightening

4. Careless holding and marking

5. Sag in Chain

6. Personal Mistakes

7. Variations in Pull

8. Variations in Temperature

9. Non-Horizontality

1. Erroneous Length of the Chain or the Tape

This error is a cumulative error that can either take a negative or a positive value. This is the error due to the wrong length of the chain which is considered as one of the serious error.

If the length of the chain is long, then the measured distance is smaller and the error is negative. And when the length of the chain is short, then the measured distance is long. Hence the error is positive. Time to time checking of the chain helps to provide adequate corrections.

2. Errors due to Inefficient Ranging

Inefficient ranging implies the measurement by placing the chain out of the survey line. This mistake always gives a longer distance value. Hence the error is a positive error which is also a cumulative error type. For each and every repetition of the mistake, the error is cumulative and the final effect on the result is large. If offset distance is also measured, then the error becomes very serious.

3. Errors due to Inefficient Straightening

While measuring a sloped or irregular ground, the chain must be held straight. Otherwise the resultant value measured is greater than the true length. This hence causes a cumulative positive error.

4. Errors due to Careless Holding and Markings

This error is caused due to an inexperienced chain man. Sometimes, the follower may hold the handle to one side of the arrow or to the other end. The leader trusts his activity and proceeds the work and marks the points. This error mostly compensates and hence is a compensating error.

5. Error due to Sag in Chain

This is a cumulative positive error. While measuring a sloped ground or stepped ground, there are chances for the chain to sag and the value obtained is higher. Hence the error is positive.

6. Personal Mistakes

Personal mistakes during the chaining results in irregular effects like:

Displacement of Arrows: Any change in position of the arrow during the chaining activities completely affects the original location of the arrow. Hence it is recommended to mark a point on the ground while fixing the arrow.

Misreading: Certain confusion while reading tally of a 5m and 15m chain, and confusing between 6 and 9 are some of the mistakes faced while reading the measurements.

Miscounting the Chain Length: This error can be avoided by following a systematic procedure to count the number of arrows.

Erroneous Booking: Sometimes, the surveyor may hear the reading wrong and write it on the book. To avoid this the chainman must say the reading loud and the surveyor should repeat it loud and enter the field book.

7. Errors due to Variation in Pull

A pull more than the calibrated pull of the chain brings some error. The chainman either apply more or less pull, which makes the error cumulative. If the pull applied is not known, the error is compensating error.

8. Errors due to Variation in Temperature

This is a cumulative error with either positive or negative value. A temperature other than the standard calibrated temperature of the tape results in length variation of the tape. This results in either showing a greater distance or shorter distance based on which the error can be positive or negative.

9. Errors due to Non-Horizontality

This is a cumulative positive error. While measuring the slope or irregular ground, if the chain is not held horizontal, it results in a longer distance than actual. Thus the error is positive.