1) Reading error:  It is the error of observation resulting from incorrect reading of the indication of a measuring instrument by the observer.

2) Parallax error:  It is the reading error which is produced, when, with the index at a certain distance from the surface of scale, the reading is not made in the direction of observation provided for the instrument used.

3) Interpolation error:   It is the reading error resulting from the inexact evaluation of the position of the index with regard to two adjacent graduation marks between which the index is located.