KEEPING track of progress on the PAA's Site Preparation contract is done by a combination of aerial photography and computer imaging. Each month, light aircraft photograph the project from 3,000 feet, taking 20 to 25 photos to cover the island. The photos are scanned into digital form by the PAA Survey Section, and then fed directly into an Intergraph Image Station. Using a process called digital photogrammetry, a digital terrain model is produced. It automatically corrects distortions caused by the perspective geometry of the aerial photographs. An ''Orthophoto'' is then produced which may be superimposed on the terrain model. The process enables the PAA engineers to distribute up-to-date maps for overall planning. Another process compares the terrain model with the one taken the previous month. By comparing the two, the computer system calculates exact quantities of earth moved. The computer screen can also be viewed through 3-D glasses, presenting a three-dimensional image. According to Intergraph, both earthmoving subcontractors in the Airport Platform Contractors' Joint Venture - Roche Brothers and Yamazaki - have accepted the quantity ports of the PAA survey section.