A finished structure seldom corresponds exactly to the original plans in every detail. Unexpected, usually unforeseeable difficulties often make variations from plans necessary or, occasionally, variations may occur accidentally that are economically unfeasible to correct.
The purpose of an AS-BUILT SURVEY is to record these variations. The as-built survey should begin as soon as it becomes feasible: meaning that the actual horizontal and vertical locations of features in the completed structure should be determined as soon as the features are erected.
At times, variations from the original plans are recorded on new tracings of the working drawings, on which as-built data are recorded in the place of the original design data when the two happen to differ. Sometimes, reproductions of the original drawings are used with variations recorded by crossing out the original design data and writing in the as-built data.