It's one of the first questions that occurs to a visitor at George Washington's Mount Vernon estate: How much of the mansion really dates back to Washington's time, and how much has been replaced?

The estate is in the midst of a project that will help answer that question, down to every nail in the floorboards.

Architects and preservationists are at the estate building a computerized database of every piece of the mansion. Laser scans are recording three-dimensional images to the tiniest detail, and workers are crawling on their hands and knees to photograph and document key components.