Machining of free-form surfaces is an important place in terms of design and performance in transportation, electronics and aeronautical industry. Problems such as form error and surface roughness were frequently encountered in the manufacture of these surfaces. For the solution of the problems, reprocessing is done, which causes time and resource loss. To eliminate these losses, the parts must be produced in the desired quality at one time. In order to find out the causes of form errors and to remove the adverse effects, the distribution on the surface must be determined correctly. Form errors should be examined in a way to cover the point, region and the whole part.