Most of the variation in linear measurements is caused by lack of proper consideration of the variables that can and do enter into any measurement. No matter how precise the instrument is, it does not correct the variables that the operator is supposed to take account of and make the proper corrections for. This paper shows that, under realistic conditions, a temperature gradient of 0’7°C across the slab of a granite surface table contributes to the uncertainty of flatness measurement by as much as 25-37% of the permissible tolerance on the granite surface table under consideration.