In the past most buildings have a set of paper plans, floor plans or the like on which the architects or consultants made notes on construction changes and other modifications.
These "as built's" usually are a good record of construction changes, but they are often a poor representation of the actual floor plan of the building.
The owner, when faced later on, sometimes years later, with a need for his plans, usually has to give his architect or his consultants a set of these "as built's". If they plan to use these on a computer as a basis for a remodel or whatever, they currently have two options: the "as built's" can be digitized or they can be scanned. Both procedures put paper plans into a computer format usable by a CAD (computer aided drafting) procedure. However, both systems produce computer plans only as accurate as the paper plan they came from. Any additions made to these plans on a computer will be very precise. This is the inherent nature of the computer drafting system. But the plans will never be current because the base information was not accurate.
The designer can take these plans to the field to verify them. With plan and tape measure in hand he attempts to determine the accuracy of the plan. It is relatively easy for him to determine proper room sizes, but only the most thorough will check wall thicknesses, squareness and details. In total absence of "as built's" drawings this job of field tape measuring can be monumental and mistakes can easily be made.