Accurate measurement with photogrammetry at large sites

Photogrammetry has become increasingly popular as a low-cost method for documenting cultural heritage and archaeological excavations. However, we have yet to establish best practices for its implementation at the site, or methods for assessing the accuracy of the resulting 3D measurements. This article presents a recent study of the Temple of Hera at Olympia, where a 25 × 55 m area was recorded at 1 mm resolution using photogrammetry both for survey and 3D reconstruction. Coded targets were set up throughout the site, which was then photographed in two phases. First, a site-wide survey established the locations of the network of targets. Second, sets of close-up photographs for detailed 3D reconstruction of the site were registered to the global survey via the targets. This technique developed at Olympia improves measurement accuracy by an order of magnitude compared to previous implementations, with a precision of at least 1 part in 50,000, and 95% of the surfaces located accurately within 2-3 mm.