Objective Question & Answer

If the vertical circle verniers do not read zero when the line of sight is horizontal, the vertical

angle measured will be in correct. The error is known as the index error and this can be

eliminated by taking both face observations or by applying index correction. 

The error is introduced when the zeros of the vernier are not at the ends of the same

diameter. Thus, the difference between the two vernier readings will not be 180°, but there will

be a constant difference of other than 180°. The error can be eliminated by reading both the

vernier and taking the mean of the two. Hence option (a) is correct.