1. A method of determining the physical dimensions and configuration of a structure or system comprising: scanning a structure or system with an x-ray backscatter unit; collecting data from x-ray photons scattered by the structure or system; combining and reconstructing the collected data into a 2-D, 2-D panoramic or 3-D data set; generating 3-D representations of hidden objects from the data set; mapping the 3-D representations of the hidden objects into a structure or system reference coordinate system; combining the 3-D representations mapped into the reference coordinate system with models to form a 3-D CAD model of the scanned structure or system; and designing modifications to the structure or system using the 3-D CAD model by planning routing and installation of hardware in the structure or system.