Change Detection between Retrospective and Contemporary 3D Models of the Omega House at the Athenian Agora

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.