When measuring a cam profile, the datum mark doesn't always coincide with its real reference, and there is a difference Dd0 between them, resulting in measuring errors of the cam profile. In the paper with a simple instance, the relationship between the maximum difference of the measuring and real data, and the difference Dd0 is discussed; and the appropriate formulation of interpolating function is investigated. Eventually the value of the difference Dd0 can be got, which can be used to obtain the measuring data with desirable precision by using the method of angle searching.