This paper addresses the advantages as well as the obstacles in practicing photogrammetry based on archival photos of archaeological sites and examines how the results can be put to use for further research, preservation, restoration and monitoring rates of deterioration. While the extensive use of historic aerial photographs has been applied to photogrammetric modeling, archaeological excavation archives have been largely ignored. Historically archaeological excavations have been vigorously documented photographically and many of these photographs are available in archives. Not all photo archives are suitable for photogrammetry as they were not photographed with the intention of overlap and other photogrammetric qualities. By selectively choosing photographs with common points and manipulating exposures, cropping and other properties to enhance commonality, 3D models of past structures and excavations can allow us to revisit them, produce accurate measurements and view angles that were never photographed. Some sites are still available for modern comparison and surveying, allowing us to quantitatively compare conditions at the time of excavation with the current state of those sites. Given the right treatment, retrospective photogrammetry will have impacts in the preservation, restoration and monitoring of the deterioration of archaeological sites. Examples from the Athenian Agora: the state prison and Omega House, and Ancient Corinth: the Fountain of the Lamps, will be used to demonstrate these possibilities.

Archaeological monuments all over the world face problems of conservation and maintenance due to natural events and processes as well as human intervention, all of which lead to their alteration and deterioration. In particular, monuments and sites that have been excavated and left exposed to the elements experience decay, which would have taken centuries prior to excavation, in just a few years when left unprotected. Thus, the necessity to detect and observe changes over time becomes paramount. Legacy data and, in particular, retrospective photogrammetric modeling, are vital tools in this process. In this work we compare two photogrammetric 3D models of the Omega House, in the Athenian Agora, to assess how much the site has changed between the time of its first excavation in 1972 and its current state. Constructive Solid Geometry (CSG) is utilized to perform Boolean operations. Additionally, distance and volume calculations are performed. The software CloudCompare was used for this work. Overall, the state of Omega House monument proves to have been preserved from 1972 to 2017, except for certain differences that are highlighted as follows: The central north part of the monument in the model 2017 presents increased volume per 7.86% in comparison with the model 1972. The northeast part of the monument in the 2017 model shows decreased volume per 5.11% when compared to the model 1972. Moreover, the calculated distances between the two models from 1972 and 2017 present the greatest values in the case of the southwest and northwest parts of the monument, ranging between −17 cm to 5 cm.

This work presents the role of 3D visualization and analysis of monuments and archaeological sites in producing useful data regarding their preservation condition. The progress made in 3D digitization technologies, in combination with the development of new data processing algorithms, has enabled reliable and highly detailed digitization of the characteristics of different parts of the monuments. Due to both the effects of nature and human intervention, monuments and sites all over the world have undergone changes over time. The use of analog documentation data can help significantly towards this direction. In this work, we use as a case study a luxurious residential complex in the Athenian Agora, known as the Omega House. We use a retrospective 3D model, created with photographs taken in the late 60’s and early 70’s, in comparison with a 3D model made with contemporary digital photos, taken in 2017. All models are georeferenced. The old model is derived using analog terrestrial photographs and aerial photos taken by a blimp. The new one is created by terrestrial digital photographs in combination with images taken by an unmanned aerial vehicle, commonly known as a drone. The 3D models have been divided into smaller parts so that we can analyze them with greater accuracy separately, and then the whole models were compared as well. The Constructive Solid Geometry (CSG) modelling scheme is used and Boolean operations are applied to find the difference and intersection of the models. The comparison that is carried out in the current work elaborates on legacy data usefulness and their utility for monitoring the Omega House condition. The type of investigation proposed in this work proves that legacy data can be repurposed and can attain a new role through change detection techniques.