Which of the following statements is incorrect ? A. Error due to refraction may not be completely eliminated by reciprocal levelling. B. Tilting levels are commonly used for precision work. C. The last reading of levelling is always a foresight. D. All of the above statements are incorrect.

The correct answer is: C. The last reading of levelling is always a foresight.

A foresight is a reading taken on a point that is higher than the instrument. The last reading of levelling is always a backsight, which is a reading taken on a point that is lower than the instrument.

Refraction is the bending of light as it passes from one medium to another. In levelling, refraction can cause errors in the readings. Reciprocal levelling is a method of levelling that can help to reduce the effects of refraction. In reciprocal levelling, the instrument is set up at two different points and readings are taken on a series of points between the two points. The readings are then used to calculate the difference in elevation between the two points.

Tilting levels are levels that have a built-in compensator. A compensator is a device that helps to keep the line of sight level even when the instrument is not perfectly level. Tilting levels are commonly used for precision work because they can help to reduce the effects of errors due to refraction and instrument tilt.