The geometric documentation of the Greek cultural heritage sites participating in the TRIQUETRA project

7 Charalabos IOANNIDIS(*), Kimon PAPADIMITRIOU(**), Alexandros TOURTAS(**), Sofia SOILE(*), Fotis BOUREXIS(*), Panagiotis TOKMAKIDIS(**), Konstantinos TOKMAKIDIS(**), Styliani VERYKOKOU(*) & Chryssy POTSIOU(*) (*)National Technical University of Athens - School of Rural, Surveying and Geoinformatics Engineering - Laboratory of Photogrammetry - Athens, Greece (**)Aristotle University of Thessaloniki - Faculty of Engineering - School of Rural and Surveying Engineering -Laboratory of Topography - Thessaloniki, Greece Corresponding author: [email protected] THE GEOMETRIC DOCUMENTATION OF THE GREEK CULTURAL HERITAGE SITES PARTICIPATING IN THE TRIQUETRA PROJECT ExTENDED AbSTRACT I siti del patrimonio culturale, su scala gobale, sono a rischio a causa dei cambiamenti climatici, dei processi geologici e delle condizioni ambientali estreme, che portano a una loro graduale degradazione. La documentazione geometrica sistematica del nostro patrimonio culturale è una procedura fondamentale che costituisce la base per gli sforzi di conservazione e le misure di mitigazione del rischio. Prodotti fotogrammetrici di altissima precisione svolgono un ruolo essenziale in questi interventi, grazie alle rappresentazioni geometriche dettagliate dei siti culturali, che supportano il monitoraggio a lungo termine, la pianificazione della conservazione e studi di valutazione del rischio molto precisi. Utilizzando modelli di mesh 3D, ortomosaici e modelli digitali di superficie (DSM), ingegneri, conservatori, ricercatori e tutti gli stakeholders interessati possono esaminare da vicino lo stato attuale dei siti culturali, rilevare eventuali cambiamenti nel tempo (a condizione che siano disponibili prodotti di documentazione geometrica passata) e mettere in atto strategie di conservazione mirate. Questo articolo presenta la documentazione geometrica di tre importanti siti archeologici in Grecia: Aegina Kolonna, la Città sommersa e il patrimonio culturale costiero di Epidauro Antica e il santuario di Kalapodi. Questi siti costituiscono i siti pilota del patrimonio culturale greco del progetto di ricerca europeo TRIQUETRA, essendo di grande importanza archeologica e, allo stesso tempo, esposti a diversi rischi ambientali, tra cui erosione costiera, sommersione, danni da gelo e degrado dei materiali. La documentazione è stata realizzata nell’ambito del progetto TRIQUETRA, il cui obiettivo principale è affrontare i rischi legati ai cambiamenti climatici e ai pericoli naturali che minacciano il patrimonio culturale, attraverso un set di attrezzi innovativa e un sistema di supporto alle decisioni (DSS) per l’identificazione, la quantificazione e la mitigazione dei rischi. Per documentare questi siti, è stata impiegata una combinazione di tecniche fotogrammetriche. Ad Aegina Kolonna, sono state utilizzate la fotogrammetria con droni (UAV) e rilievi a terra per documentare il sito archeologico e il paesaggio circostante, sempre più interessati dall’erosione costiera e dall’esposizione a scavi a lungo termine. Nell’Antica Epidauro, il patrimonio costiero è stato tilevato tramite tecniche UAV LiDAR e basate su immagini, mentre i resti sommersi della Città Sott’acqua sono stati registrati usando fotogrammetria subacquea e rilievi con ecoscandaglio multi-beam (MBES), affrontando rischi quali l’innalzamento del livello del mare, la bio-erosione e le attività umane. A Kalapodi, dove i danni da gelo rappresentano un rischio importante per la stabilità strutturale, sono stati effettuati rilievi a terra con tecniche UAV LiDAR e basate su immagini per documentare lo stato del sito e supportare una pianificazione conservativa adattata al clima. I set di dati raccolti durante queste campagne sono stati elaborati utilizzando workflow fotogrammetrici consolidati, tra cui il matching di immagini, la ricostruzione 3D tramite Structure from Motion (SfM), la generazione di nuvole di punti dense, la ricostruzione di superfici tridimensionali, la mappatura delle texture, la creazione di modelli digitali di superficie (DSM) e ortomosaici. I risultati fotogrammetrici finali rappresentano dati fondamentali per monitorare i cambiamenti strutturali, condurre analisi ambientali e guidare le decisioni sulla gestione dei siti e le misure di conservazione specifiche. Questo studio, insieme al lavoro più ampio realizzato nell’ambito del progetto TRIQUETRA, dimostra quanto sia importante un’acquisizione continua di dati e, soprattutto, una collaborazione interdisciplinare, per garantire la protezione dei siti del patrimonio culturale per le future generazioni. La fusione dei risultati provenienti da diverse discipline, come topografi, ingegneri civili, geologi, ingegneri chimici, oceanografi, meteorologi e climatologi, è essenziale per creare un quadro sostenibile volto a salvaguardare i siti archeologici, specialmente in un’epoca di crescenti sfide legate ai cambiamenti climatici. Italian Journal of Engineering Geology and Environment www.ijege.uniroma1.it IJEGE, Special Issue 1 (2025): 7-24, DOI: 10.4408/IJEGE.2025-01.S-01 E-ISSN 2035-5688 | ISSN 1825-6635 / ©Author(s) 8 Italian Journal of Engineering Geology and Environment, Special Issue 1 (2025) www.ijege.uniroma1.it C. IOANNIDIS, K. PAPADIMITRIOU, A. TOURTAS, S. SOILE, F. bOURExIS, P. TOKMAKIDIS, K. TOKMAKIDIS, S. VERYKOKOU & C. POTSIOU AbSTRACT In the light of threats including climate change, geological degradation and extreme weather conditions, the geometric documentation of cultural heritage sites plays a crucial role in their preservation. Photogrammetric techniques enable the production of highly accurate 3D models, orthoimages, and digital surface models (DSMs), which facilitate both site monitoring and conservation planning. This article presents the photogrammetric documentation of three archaeological sites in Greece, namely, the archaeological site of Aegina Kolonna, the Sunken City and the coastal cultural heritage of Ancient Epidaurus, as well as the sanctuary of Kalapodi. The geometric documentation of all three sites was conducted within the framework of the TRIQUETRA EU-funded project, through ground surveys and UAV-based photogrammetric techniques, either independently or in combination with underwater photogrammetry workflows, in order to capture the geometry of the cultural heritage sites and their surrounding environments. The produced results include 3D dense point clouds, 3D textured mesh models, DSMs and high-resolution orthomosaics. The generated datasets support detailed structural assessments, vulnerability analyses and risk assessment studies, providing a fundamental basis for protection efforts of the archaeological sites of interest. Keywords: geometric documentation, 3D model, cultural heritage, UAV-based photogrammetry, underwater photogrammetry, LiDAR INTRODUCTION The geometric documentation of cultural heritage sites is a fundamental process for their preservation. In the face of threats such as climate change, geological degradation or extreme water/ snow/ice, accurate photogrammetric products, like 3D models, orthoimages and digital terrain/surface models (DTMs/DSMs) of cultural heritage sites play a vital role for their conservation and monitoring. Advanced photogrammetric techniques can serve as a critical tool for capturing precise records of cultural heritage sites, facilitating conservation planning and promoting public awareness and engagement with our cultural heritage. This paper focuses on the geometric documentation of three prominent archaeological sites in Greece, as part of the TRIQUETRA project: in Aegina, Epidaurus and Kalapodi. TRIQUETRA is a research project funded by the European Union, that aims to address climate change risks and natural hazards threatening cultural heritage, through an innovative toolbox for risk identification, quantification and mitigation (Ioannidis et alii, 2023). Its primary objectives include the creation of a knowledge base platform for climate impacts and mitigation measures, the development of systematic approaches for assessing emerging risks, advancing technologies for quantifying threats and raising public awareness to involve citizens in cultural heritage preservation. Key activities of the TRIQUETRA project involve a new flash LiDAR for 3D mapping of underwater cultural heritage and erosion monitoring, novel spectroscopic sensors for water quality estimation, the refinement of climate and risk quantification models and the integration of remote sensing techniques. The basic outcome of the project is the development of a multi-hazard impact assessment platform, which functions as a decision support system (DSS). The TRIQUETRA project was launched in January 2023 and will conclude in December 2025, including validation testing at eight pilot sites. The geometric documentation of cultural heritage has evolved significantly over the years, from traditional surveying and photogrammetric techniques to automated workflows. Photogrammetric techniques have seen important advancements, while deep learning is now being integrated into traditional processes, like image matching and 3D reconstruction, hence improving accuracy and automation in cultural heritage documentation (Verykokou & Ioannidis, 2025). Early efforts were the foundation for integrating several different data acquisition methods to enhance the accuracy and completeness of the geometric documentation products. The importance of combining multiple data acquisition techniques for the geometric documentation of cultural heritage sites is addressed by Georgopoulos (2017), who provides a comprehensive overview of data acquisition methods, emphasizing both passive and active techniques for accurate geometric documentation. The need for high-quality data collection processes, to ensure the reliability of the resulting 3D models of cultural heritage sites for preservation and analysis is emphasized by Mavromati et alii (2019). In their study, Kasapakis et alii (2024) discuss the application of photogrammetry in cultural heritage, highlighting its effectiveness in creating accurate 3D models for documentation and preservation purposes. Also, the integration of reality capture technologies, such as 3D scanning and photogrammetry, with large-scale 3D printing has been explored as an innovative approach for the preservation and reproduction of historical and cultural heritage (Garcia-Espinel et alii, 2024). A comparative analysis of different software packages for 3D modeling of complex geometries of cultural heritage sites is presented by Verykokou et alii, (2021), where an overview of image-based and scanner-based 3D modeling techniques that may be applied for the geometric documentation of cultural heritage sites is given by Verykokou & Ioannidis (2023). In recent years, numerous geometric documentation applications have been reported in literature. Examples include the holistic 3D digital documentation of the Saint Neophytos Enkleistriotis Monastery in Paphos, Cyprus, through a comprehensive approach that integrates various data acquisition methods, to capture both the tangible and intangible aspects of the monument (Ioannides et alii, 2016); a multi-scale 3D modeling framework for damaged cultural heritage sites (Verykokou et alii, 2016); the creation of 9 Italian Journal of Engineering Geology and Environment, Special Issue 1 (2025) www.ijege.uniroma1.it THE GEOMETRIC DOCUMENTATION OF THE GREEK CULTURAL HERITAGE SITES PARTICIPATING IN THE TRIQUETRA PROJECT detailed models of the Saint Martyrs Constantin Brȃncoveanu and His Sons wooden church in Oradea, Romania, via photogrammetry and 3D scanning techniques (Herman et alii, 2020); a holistic 3D documentation approach applied to heritage monuments in Rhodes, Greece combining geodetic, photogrammetric, and laser scanning data acquisition methods (Tapinaki et alii, 2021); the generation of high-resolution 3D models for the multi-level documentation of the Meteora UNESCO site in Greece through photogrammetric and surveying techniques (Ioannidis et alii, 2022), integrated in a web-based framework with a responsive 3D viewer, data retrieval mechanisms and VR/AR functionalities (Boutsi et alii, 2023); the geometric documentation of the Mehmet Bey Mosque monument in Serres, Greece, using a multi-sensor approach integrating terrestrial laser scanning and photogrammetry (Tsiachta et alii, 2024); the digital documentation of the ambulatory of the Cathedral of Santiago de Compostela in Spain, using Structure from Motion (SfM) photogrammetry (Peña-Villasenín et alii, 2024); the 3D documentation of the Clock Tower in Tirana, Albania using terrestrial laser scanning (Llabani & Abazaj, 2024). These examples demonstrate the variety of methods and technologies that can be used for the digitization and the preservation of cultural heritage sites. Moreover, Colucci et alii (2024) showed how spatial and geometric data support risk assessment and vulnerability assessment of cultural heritage sites, by integrating geometric documentation into an INSPIRE-based 3D GIS framework for cultural heritage. Furthermore, Building Information Modeling (BIM) has become a useful tool for cultural heritage documentation, allowing the integration of both semantic and geometric data. Crisan et alii (2024) proposed a methodology for transforming 3D point cloud data into intelligent BIM models, enhancing the conservation, documentation and management of cultural heritage sites. These studies illustrate the continuous effort to use geometric documentation results for the scope of cultural heritage preservation. Moreover, the geometric documentation of underwater cultural heritage sites is the focus of many recent studies. Critical aspects of all stages of image-based underwater 3D modeling processes are discussed by Skarlatos & Agrafiotis (2020). Calantropio & Chiabrando (2024) show the effectiveness of underwater photogrammetry to create high-resolution 3D models of submerged archaeological sites. Additionally, the use of advanced technologies, like ultra-high-resolution multibeam echo sounders (UHR MBES), has been shown to improve the mapping of archaeological remains, as presented in the study by Abate et alii (2024). Several examples of 3D modeling underwater cultural heritage sites through marine remote sensing and photogrammetric techniques have been reported in literature, such as the recent applications of documenting underwater cultural heritage sites in Malta for the creation of a virtual museum (Gambin et alii, 2021); 3D reconstructing the M/S Helma wreck in Norway (Diamanti et alii, 2021) and the Christoforos Shipwreck in Greece (Collina et alii, 2022); documenting the underwater heritage of the Methoni Bay, Greece (Levy et alii, 2023); and reconstructing a cluster of cannons in the Gulf of Patras, Greece (Labrianidis et alii, 2024). Manglis et alii (2021) propose a roadmap for the sustainable valorization of accessible underwater cultural heritage sites, using augmented reality and virtual reality technologies, while the significance of shipwreck archaeology is highlighted by Briggs & Campbell (2023). All these studies demonstrate the continuous advancements in underwater 3D documentation techniques, thus contributing in preserving underwater cultural heritage sites and enhancing their accessibility. PHOTOGRAMMETRIC DOCUMENTATION This section presents the geometric documentation workflow followed for each site, detailing the site characteristics, data acquisition methods, photogrammetric processing steps and final results. Firstly, the documentation of Aegina Kolonna is presented, followed by the Sunken City and coastal cultural heritage of Ancient Epidaurus, and finally, the Sanctuary of Kalapodi is described. Aegina Kolonna In this section, the geometric documentation of Aegina Kolonna is presented, including a description of the site, data acquisition, photogrammetric processing and the final results. The site Aegina Kolonna is a major archaeological site at the northwest tip of the island of Aegina in the Saronic Gulf, Greece. It was one of the major hubs of the Aegean Bronze Age. The site lies next to the shore and opens to the sea on the west side, which is characterized by a steep cliff. The prehistoric settlement consists of an inner area and suburbs on the eastern side. The site was abandoned around 1200 BC and later occupied as a necropolis during the Iron Age. It continued to flourish from the Archaic to Roman times. From the 6th century AD until around AD 1000, the site was home to a large Byzantine settlement. Aegina Kolonna has been the subject of archaeological excavations since the 19th century. Since 1966, the University of Salzburg has undertaken annual research and restoration campaigns (Paris London University Of Salzburg, 2025). Since the 1970s, the site has undergone several restoration and consolidation efforts. The walls of the inner suburb were restored and consolidated mostly during excavations in the 1970s and 1980s. However, the outer suburbs and the western area remained largely unprotected until 2015. Due to extensive excavations since the late 19th century, many prehistoric to archaic walls in the outer suburb have been exposed for decades, leading to underwashing of walls and foundations. The deep excavation cuts, combined with high-standing walls, have posed significant stability concerns. 10 Italian Journal of Engineering Geology and Environment, Special Issue 1 (2025) www.ijege.uniroma1.it C. IOANNIDIS, K. PAPADIMITRIOU, A. TOURTAS, S. SOILE, F. bOURExIS, P. TOKMAKIDIS, K. TOKMAKIDIS, S. VERYKOKOU & C. POTSIOU In 2011, a new extensive restoration campaign was initiated under the direction of K. Sporn and architect A. Tanner (Sporn et alii, 2017), focusing on the suburbs with scientific analysis. Since 2015, the restoration program has prioritized backfilling and consolidating the walls using various scientifically approved mortars. A particular challenge is the western area of Kolonna, which includes a sacred site overbuilt by Byzantine cisterns and houses. This area was excavated in the early 2000s and partially backfilled. However, it remains highly vulnerable to winter storms and constant wave action caused by the heavy maritime traffic between Aegina and Athens (with commercial ferry boats operating every half hour alongside heavy private traffic). Each year, the archaeological zone deteriorates, especially in the west, due to soil ruptures and the ongoing risk of collapse. To address these issues, TRIQUETRA technologies are being employed to measure geological hazards related to ground instabilities and to protect the cliffs from wave action and precipitation. Specific geophysical campaigns are being conducted at the site. The work plan followed within TRIQUETRA includes a thorough analysis of the factors contributing to the site’s ongoing deterioration. It is crucial to differentiate between natural wave action, waves produced by ship traffic and changes in current behavior. Additionally, climate studies are conducted to assess precipitation patterns over past decades. Methodologically, the project incorporates photogrammetric processing and remote sensing imagery analysis, as well as chemical and physical studies of the endangered cliffs. This approach facilitates the identification and quantification of risks and the implementation of risk mitigation strategies. Proven methods, along with those developed within TRIQUETRA, are applied to minimize risks and develop long-term strategies for protecting the cliffs from further damage. Data acquisition The Laboratory of Photogrammetry of the National Technical University of Athens (NTUA), led the photogrammetric documentation of the Aegina Kolonna site, producing a detailed 3D model, point cloud, DSM and orthomosaics. The photogrammetric survey was conducted on June 25, 2024 (Fig. 1). This documentation was implemented within the context of the TRIQUETRA project, with the aim to support long-term monitoring of the site and inform preservation strategies. For the aerial data collection, a DJI Mavic 3 Enterprise drone was used to capture 945 vertical aerial images, covering the entire site, providing an overview of the site’s topography (Fig. 2a), to ensure high-resolution documentation of complex structures such as slopes, walls, and intricate features, 4,900 oblique aerial images were captured at lower altitudes (Fig. 2b). Representative oblique images are shown in Fig. 3. Additionally, a DJI Mavic 2 Pro drone was deployed to capture 930 images of areas surrounding the Kolonna, as well as areas within the archaeological site and the museum facades (Fig. 4). Concerning the ground survey, 15 pre-marked Ground Control Points (GCPs) were placed around the site and measured in the Greek Geodetic Reference System (GGRS ‘87) using a Leica Fig. 1 - Images from the photogrammetric campaign in Aegina Kolonna, led by the Lab. of Photogrammetry, NTUA within the TRIQUETRA project Fig. 2 - Distribution of the UAV vertical aerial images (a) and the whole set of UAV vertical and oblique images (b) over the 3D model of the entire area surrounding the archaeological site of Aegina Kolonna Fig. 3 - Representative oblique images showcasing the slopes, walls and other site details of Aegina Kolonna 11 Italian Journal of Engineering Geology and Environment, Special Issue 1 (2025) www.ijege.uniroma1.it THE GEOMETRIC DOCUMENTATION OF THE GREEK CULTURAL HERITAGE SITES PARTICIPATING IN THE TRIQUETRA PROJECT 1200 GNSS receiver. The NTRIP RTK method was employed to receive corrections from the MetricaNet GNSS permanent stations network. Additionally, 10 characteristic points within the site were measured to further improve georeferencing accuracy. The distribution of the GCPs is illustrated in Fig. 5. Photogrammetric processing The photogrammetric processing was implemented using the Agisoft Metashape Professional software, following a multiview 3D reconstruction workflow. The first step included the estimation of the interior and exterior orientation of all 6,694 images and the computation of the coordinates of a sparse point cloud containing ~2.4 million points, consisting of the automatically extracted tie points. This process includes the steps of automatic identification of overlapping images, image matching and feature tracking as well as the main process of Structure from Motion (SfM). Georeferencing was conducted using 15 GCPs, while additional check points were used to validate positional accuracy. The georeferencing achieved a root mean square (RMS) error of ~4 cm for GCPs and 6 cm for check points. The stage of dense point cloud generation followed. This stage involves (i) performing dense image matching for selected pairs of overlapping images with known interior and exterior orientation parameters, thus producing a set of depth maps for the reference images, and (ii) converting them into 3D points, by projecting them into space to reconstruct the dense point cloud. 3D surface reconstruction was the next step. This process involves generating a polygonal mesh model using the dense point cloud. Texture mapping was the final stage, for producing a highresolution texture to the 3D mesh, using the oriented images. In addition to producing 3D geometric documentation products, the photogrammetric workflow led to the production of highresolution orthomosaics along with a digital surface model of the site. Results The 3D documentation of Aegina Kolonna resulted in the production of high-accuracy photogrammetric datasets, which support archaeological analysis, conservation planning as well as risk assessment within the TRIQUETRA project. The produced dense point cloud consists of ~61 million points (Fig. 6). It preserves fine details of the archaeological Fig. 4 - Distribution of images captured by the DJI Mavic 2 Pro drone around Aegina Kolonna along with the sparse point cloud Fig. 5 - Distribution of the GCPs measured in the archaeological site of Aegina Kolonna superimposed on the orthomosaic of the site Fig. 6 - Dense point cloud of the archaeological site of Aegina Kolonna 12 Italian Journal of Engineering Geology and Environment, Special Issue 1 (2025) www.ijege.uniroma1.it C. IOANNIDIS, K. PAPADIMITRIOU, A. TOURTAS, S. SOILE, F. bOURExIS, P. TOKMAKIDIS, K. TOKMAKIDIS, S. VERYKOKOU & C. POTSIOU site and serves as a critical dataset for archaeological studies and conservation efforts, enabling precise measurements and the detection of changes over time. The generated high-resolution 3D textured model accurately depicts the geomorphology and architectural details of Aegina Kolonna. The 3D mesh model consists of ~9.9 million vertices and 19.8 million faces. The textured rendered to the model led to a detailed visual representation of structural elements of the study Fig. 7 - Views of the 3D model of the archaeological site of Aegina Kolonna without texture (left) and with texture (right) Fig. 8 - Views of part of the 3D model of the archaeological site of Aegina Kolonna without texture (left) and with texture (right). 13 Italian Journal of Engineering Geology and Environment, Special Issue 1 (2025) www.ijege.uniroma1.it THE GEOMETRIC DOCUMENTATION OF THE GREEK CULTURAL HERITAGE SITES PARTICIPATING IN THE TRIQUETRA PROJECT area. The 3D model captures not only the main archaeological site, but also the surrounding terrain, providing a valuable resource for future monitoring of structural integrity and environmental changes. Views of the non-textured and textured 3D model are illustrated in Fig. 7 (entire model) and Fig. 8 (zoom-in views). Furthermore, a DSM was generated, allowing for detailed terrain analysis. The DSM, produced at 1 cm resolution, accurately captures the elevation variations across the site. The elevation values range from -4 m to 26 m, reflecting both the natural topography and the structural remains of Aegina Kolonna. The DSM enables hydrological and topographical assessments, contributing to the evaluation of erosion risks and water runoff patterns. Fig. 9 shows the produced DSM. Finally, high-resolution orthomosaics were produced for detailed planimetric analysis of the site. Specifically, orthmosaics with a ground sampling distance (GSD) of 1 cm and 2.5 cm were generated. The orthomosaics refer to the GGRS87 / Greek Grid (EPSG::2100) coordinate system and serve as a valuable tool for GIS-based analyses. The generated orthomosaic with a GSD of 1cm is illustrated in Fig. 10. The photogrammetric documentation of Aegina Kolonna provides a dataset of high geometric accuracy, allowing for structural assessments, archaeological analysis and conservation planning. The 3D textured model of the site enables detailed visualization and monitoring of the site’s architectural features, while the produced dense point cloud and DSM contribute to environmental assessments and long-term preservation strategies. Moreover, the generated 2D products (orthomosaics) along with the 3D outputs support archaeological mapping and aid in site management. Beyond their immediate applications, the generated datasets can serve as the basis for future monitoring of the site, through application of change detection methods that may support conservation efforts. What is more, the integration of the generated products into the TRIQUETRA Knowledge Base Platform can enhance risk assessment by providing accurate spatial data for evaluating the structural stability of the site and assessing environmental threats. The 3D photogrammetric products also form the foundation for the Digital Twin of Aegina Kolonna, enabling further applications such as the Augmented Reality (AR) app of the TRIQUETRA project for citizen engagement. The sunken city of Ancient Epidaurus and its coastal heritage In this section, the geometric documentation of the Sunken City of Ancient Epidaurus and its coastal cultural heritage is presented, including a description of the site, data acquisition, photogrammetric processing and the final results. The site Ancient Epidaurus comprises a cultural heritage site of highly significant underwater and coastal remains dating back to the 12th century B.C. The Sunken City is located in the bay of Agios Vlasios, near Ancient Epidaurus. Bio-erosion is one of the most significant risks affecting these underwater and coastal archaeological findings. The Sunken City and the surrounding coastal heritage are slowly deteriorating due to water exposure and microorganisms. Additionally, sea level rise poses a severe threat to coastal archaeological remains, while coastal erosion is another major risk. Flooding on the coastal land, sediment deposition, and embankment construction for port development further endanger the site. Damage is also caused by illegal mooring of tourist boats, while exposure to chemicals, seismic activity and potential vandalism represent additional threats. Systematic conservation and promotion efforts have already begun, alongside ongoing excavations across the site. The 3D modeling of CH sites, particularly underwater antiquities, presents significant challenges due to the complexity Fig. 9 - DSM of the archaeological site of Aegina Kolonna Fig. 10 - Orthomosaic of the archaeological site of Aegina Kolonna 14 Italian Journal of Engineering Geology and Environment, Special Issue 1 (2025) www.ijege.uniroma1.it C. IOANNIDIS, K. PAPADIMITRIOU, A. TOURTAS, S. SOILE, F. bOURExIS, P. TOKMAKIDIS, K. TOKMAKIDIS, S. VERYKOKOU & C. POTSIOU and specific conditions of the aquatic environment. The detailed 3D survey of underwater and coastal CH resources in Ancient Epidaurus, combined with literature reviews and archaeological studies, will serve as the knowledge base for the TRIQUETRA DSS. These efforts aim to: (i) prevent further deterioration of the site due to overtourism, natural hazards and human-induced threats, (ii) ensure structured protection of the CH site and (iii) develop sustainable tourism strategies and propose restoration solutions for the site. Cutting-edge data collection techniques in surveying, photogrammetry and remote sensing have been implemented. The authors worked closely with the local municipality, the Archaeological Service of the area, the Marine Antiquities Service, local guides and travel agents. High-accuracy photorealistic 3D models of the coastal and underwater antiquities were generated. For the coastal part of Ancient Epidaurus, a combination of aerial and close-range photogrammetry, terrestrial laser scanning, and aerial LiDAR was used. For the underwater part, hydrographic multi-beam sonar surveys and underwater photogrammetry were employed. A common reference network was established for all reality capture surveys, ensuring accuracy at every step. A key focus of this work is the detection and monitoring of changes, damage, or deterioration, with a systematic approach to identifying changes at an early stage to allow for timely interventions. All collected survey data were uploaded to the TRIQUETRA online platform for further processing, analysis and collaboration. The high-accuracy 3D survey of Epidaurus antiquities and their surrounding area will serve as a reference framework for scientific research and for integrating the outcomes of the project. Data acquisition The Laboratory of Topography (LabTop) of the School of Rural and Surveying Engineering of the Aristotle University of Thessaloniki (AUTh) undertook the photogrammetric documentation of the underwater and coastal CH resources in Ancient Epidaurus, aiming to capture the geometry of the cultural heritage assets of the Sunken city and the surrounding coast for assessing areas vulnerable to environmental and structural risks. The data collection mission in Epidaurus was conducted from May 13th to May 16th, 2024 and its goal was to collect data of four different types (Fig. 11), i.e., aerial LiDAR data, aerial images, underwater images and multibeam SONAR data. Implementing and measuring GCPs was also a prerequisite for some of the above-mentioned surveys. A DJI Matrice 300 RTK UAV was used for the aerial LiDAR survey, in combination with DJI’s L1 LiDAR sensor. Matrice 300 RTK integrates dual GNSS sensors and antennas and can fly with survey grade accuracy, but also geotags the images with survey grade accuracy. For this to happen, the controller must be connected to either a network of GNSS reference stations via NTRIP or to a base GNSS receiver casting NTRIP. The second scenario was used for all cases in Ancient Epidaurus. L1 according to the manufacturer has an accuracy of 5cm, a 20Mpixels camera for coloring the point clouds captured, integrated gimbal and IMU and records triple return for each pulse, so it can penetrate even heavy vegetation and capture ground points underneath. The coastal area of Ancient Epidaurus was surveyed in five autonomous flights. Three of these flights utilized the terrain-follow function due to significant height variations in the terrain, while the remaining two flights did not use this function, considering the height differences in those areas to be insignificant. The Digital Elevation Model (DEM) files needed for the terrain-follow function were created from data provided by the national Fig. 11 - Images from the campaign in Epidaurus, led by the Lab. of Topography, AUTh within the TRIQUETRA project 15 Italian Journal of Engineering Geology and Environment, Special Issue 1 (2025) www.ijege.uniroma1.it THE GEOMETRIC DOCUMENTATION OF THE GREEK CULTURAL HERITAGE SITES PARTICIPATING IN THE TRIQUETRA PROJECT captured from the two cameras and an additional 308 images of details were captured with a medium format Hasselblad X1D II in a waterproof housing (Fig. 13). The sensors on the GoPro cameras are 1/1.9” and a resolution of 27Mpixels, while X1D II has 50 million effective pixels and was combined with a 21mm lens, cadaster and uploaded to the flight controller. Throughout the five flights, a total of 340 million points were collected, with a spatial resolution of approximately 170 points per square meter. The resulting LiDAR point cloud is illustrated in Fig. 12. The same UAV was used for capturing aerial images (Fig. 13), for the photogrammetric survey of the area. Again, five flights were conducted, three with the terrain-follow function and two without. In total, 4,949 vertical images were captured during the five autonomous flights. To capture the excavation area in greater detail, two additional flights were carried out: one involved manually capturing vertical images at a lower altitude, and the other involved manually capturing oblique images of the excavated ancient ruins. Together, these two flights contributed an additional 668 images to the dataset. All the necessary flight permits from the Hellenic Civil Aviation Authority, were granted well in advance. For the underwater photogrammetric survey of the “Mansion” at the south bay of Epidaurus, a variety of cameras and scuba diving equipment have been used. The reason for that is that due to the very shallow depths in most parts of the mansion, a big, heavy camera system with lights would be very difficult to operate. Additionally, the mansion’s ruins were overpopulated by the venomous urchin species “Diadema Setosum”, requiring the divers to avoid contact with the ruins under any circumstances. Finally, a method previously employed by LabTop was adapted for these conditions: two GoPro Hero 11 Black cameras were mounted on a 1.20m rod, rigidly fixed at both ends. In addition, two underwater lights each producing 21,000 lumens, were attached to the rod, which was then secured under a diver’s buoy. With this configuration, the cameras were capturing slightly unsynchronized (thus the “pairs” of images cannot be used as a rigid camera system) images (Fig. 13) in timelapse mode every 2 seconds and the diver stayed safely on the surface navigating the buoy in the same pattern that drone surveys are conducted in dry land. The two cameras with their ultra-wide lenses were able to capture images with enough overlap for photogrammetry even in depths of less than 0.5m, which was the case in several parts at the Epidaurus submerged mansion. In total 13,529 images were Fig. 12- LiDARpointcloudderivedfrom5LiDARflightsinEpidaurus Fig. 13 - Images captured with the UAV (up), GoPro camera (middle) and Hasselblad X1D II (down) in Epidaurus 22 Italian Journal of Engineering Geology and Environment, Special Issue1 (2025) www.ijege.uniroma1.it C. IOANNIDIS, K. PAPADIMITRIOU, A. TOURTAS, S. SOILE, F. bOURExIS, P. TOKMAKIDIS, K. TOKMAKIDIS, S. VERYKOKOU & C. POTSIOU heritage—TRIQUETRA”, which is a Project funded by the EU HE research and innovation program under GA No. 101094818.” CONCLUSIONS The aim of this study was to apply state of the art photogrammetric techniques for the geometric documentation of the three Greek pilot sites of the TRIQUETRA project, namely, Aegina Kolonna, the Sunken City in Epidaurus and its coastal cultural area as well as the Kalapodi site, contributing to their preservation. The main stages of the photogrammetric campaigns consist of data acquisition and photogrammetric processing. The results of the geometric documentation of the three cultural heritage sites are further used and analyzed within the TRIQUETRA project for assessing their structural conditions and monitoring the impact of external factors, like climate change, on these monuments. The produced geometric documentation results are of high importance, so they will be shared with the scientific community and the public, raising awareness of the challenges that our cultural heritage faces under the threat of climate change and natural hazards. ACKNOWLEDGMENTS This work is based on procedures and tasks implemented within the project “Toolbox for assessing and mitigating Climate Change risks and natural hazards threatening cultural Fig. 27 - Detailed view of the delivered orthomosaic of Kalapodi REFERENCES Abate N., Violante C. & Masini N. (2024) - A semi-automatic-based approach to the extraction of underwater archaeological features from ultra-highresolution bathymetric data: The case of the Submerged Baia Archaeological Park. Remote Sensing, 16: 1908. https://doi.org/10.3390/rs16111908. Boutsi A.-M., Tallis I., Pastos I., Verykokou S. & Ioannidis C. (2023) - 5dMeteora framework: Management and web publishing of cultural heritage data. ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences, X-M-1-2023: 33-40. https://doi.org/10.5194/ isprs-annals-X-M-1-2023-33-2023. Bilis T. & Sotiropoulos A. (2024) - Ancient sanctuary at Kalapodi, Fthiotis, Central Greece: Presentation of restoration works and the implementation of TRIQUETRA programme. EasyChair Preprint, 14920. Zenodo. https://doi.org/10.5281/zenodo.14192248. Briggs L. & Campbell P.B. (2023) - Six shipwreck archaeology studies in the past 10 years. Archaeological Reports 69: 131-145. https://doi.org/10.1017/ s0570608423000054. Calantropio A. & Chiabrando F. (2024) - Underwater cultural heritage documentation using photogrammetry. Journal of Marine Science and Engineering, 12(3): 413. https://doi.org/10.3390/jmse12030413. Collina M., Lagudi A., Bruno F., Cosentino V., Peluso R., Ktistis A. & Manglis A. (2022) - Exploring modern shipwrecks using digital technologies: The case study of the Christoforos Shipwreck. 2022 IMEKO TC-4 International Conference on Metrology for Archaeology and Cultural Heritage, University of Calabria, Italy, October 19-21, 2022. Colucci E., Matrone F., Noardo F., Assumma V., Datola G., Appiotti F., Bottero M., Chiabrando F., Lombardi P., Migliorini M., Rinaldi E., Spanò, A. & Lingua A. (2024) - Documenting cultural heritage in an INSPIRE-based 3D GIS for risk and vulnerability analysis. Journal of Cultural Heritage Management and Sustainable Development, 14(2): 205-234. https://doi.org/10.1108/JCHMSD-04-2021-0068. Crisan A., Pepe M., Costantino D. & Herban S. (2024) - From 3D point cloud to an intelligent model set for cultural heritage conservation. Heritage, 7: 1419-1437. https://doi.org/10.3390/heritage7030068. Diamanti E., Snefjellå Løvås H., Kvisvik Larsen M. & Ødegård Ø. (2021) - A multi-camera system for the integrated documentation of underwater cultural heritage of high structural complexity: The case study of M/S Helma wreck. IFAC-PapersOnLine, 54(16): 422-429. https://doi. org/10.1016/j.ifacol.2021.10.126 Gambin T., Hyttinen K., Sausmekat M. & Wood J. (2021) - Making the invisible visible: Underwater Malta—A virtual museum for submerged cultural heritage. Remote Sensing, 13: 1558. https://doi.org/10.3390/rs13081558. Garcia-Espinel J.D., González J.M.L.G. & Torres M.F. (2024) - 3D Heritage: Preserving historical and cultural heritage through reality capture and large-scale 3D printing. In: Moral-Andrés F., Merino-Gómez E. & Reviriego P. (eds.) - Decoding Cultural Heritage. Springer, Cham. https:// 23 Italian Journal of Engineering Geology and Environment, Special Issue 1 (2025) www.ijege.uniroma1.it THE GEOMETRIC DOCUMENTATION OF THE GREEK CULTURAL HERITAGE SITES PARTICIPATING IN THE TRIQUETRA PROJECT doi.org/10.1007/978-3-031-57675-1_16 Georgopoulos A. (2017) - Data Acquisition for the Geometric Documentation of Cultural Heritage. In: Ioannides M., Magnenat-Thalmann N. & Papagiannakis G. (eds.)-MixedRealityandGamificationforCulturalHeritage.Springer, Cham. https://doi.org/10.1007/978-3-319-49607-8_2. Herman G.V., Caciora T., Ilies D.C., Ilies A., Deac A., Sturza A., Sonko S.M., Suba N.S. & Nistor S. (2020) - 3D modeling of the cultural heritage: Between opportunity and necessity. Journal of Applied Engineering Science, 10(1). https://doi.org/10.2478/jaes-2020-0005. Ioannides M., Ioannidis C., Archimadrite N., Castrillo D., Chatzigrigoriou P., Papageorgiou E., Leventis G., Nikolakopoulou V., Athanasiou V., Bourexis F., Soile S., Verykokou S., Costi De Castrillo M. & Sovis C. (2016) - Towards monuments’ holistic digital documentation: The Saint Neophytos Enkleistriotis case study. In: Ioannides M., et al. (eds.) - Digital Heritage. Progress in Cultural Heritage: Documentation, Preservation, and Protection. EuroMed 2016. Lecture Notes in Computer Science, vol. 10058. Springer, Cham. https://doi.org/10.1007/978-3-319-48496-9_36. Ioannidis C., Verykokou S., Soile S., Istrati D., Spyrakos C., Sarris A., Akritidis D., Feidas H., Georgoulias A.K., Tringa E., Zanis P., Georgiadis C., Martino S., Feliziani F., Marmoni G.M., Cerra D., Ottinger M., Bachofer F., Anastasiou A., Charalampopoulou V., Krebs P., Mizaikoff B., Roulet J-C., Bulliard X., Dudnik G. & Anyfantis G.C. (2024) - Safeguarding our heritage - The TRIQUETRA Project Approach. Heritage, 7: 758-793. https://doi.org/10.3390/heritage7020037 Ioannidis C., Soile S., Boutsi A. M., Verykokou S., Pastos I., Tallis I., Tokmakidis P., Tokmakidis K. & Potsiou C. (2022) - 5dMeteora: Information system for multi-level documentation of religious sites and historic complexes. FIG Congress 2022 - Volunteering for the Future: Geospatial Excellence for a Better Living, Warsaw, Poland, 11-15 September 2022. https://www.fig.net/resources/proceedings/fig_proceedings/fig2022/papers/ ts01e/TS01E_ioannidis_soile_et_al_11447.pdf Kasapakis V., Gavalas D. & Dzardanova E. (2024) - 3D modelling through photogrammetry in cultural heritage. In: Lee N. (ed.) - Encyclopedia of Computer Graphics and Games. Springer, Cham. https://doi.org/10.1007/978-3-031-23161-2_206 Levy T., Papatheodorou G., Geraga M., Christodoulou D., Georgiou N., Kordella S., Gkionis P., Spondylis I., Michalis M., Dimas X., Tsoulakou M., Kylafi M., Vouvalidis K., Tamberino A., Clark L., Reece J., Cantu K., Walsh R., McDonald C. & Shtienberg G. (2023) - Digital underwater technologies in the Methoni Bay cultural heritage project, Greece: Interdisciplinary approaches and sustainability. Science & Culture, 9: 51-88. https://doi.org/10.5281/zenodo.7265745 Llabani A. & Abazaj F. (2024) - 3D documentation of cultural heritage using terrestrial laser scanning. Journal of Applied Engineering Science, 22(2): 267-271. https://doi.org/10.5937/jaes0-50414. Manglis A., Fourkiotou A. & Papadopoulou D. (2021) - A roadmap for the sustainable valorization of accessible underwater cultural heritage sites. Heritage 4: 4700-4715. https://doi.org/10.3390/heritage4040259. Mavromati E., Stamatiou E., Chrysaeidis L. & Astaras K. (2019) - Improving the geometric documentation of cultural heritage: Combined methods for the creation of an integrated management information system in Greece. In: Moropoulou A., Korres M., Georgopoulos A., Spyrakos C. & Mouzakis C. (eds.) - Transdisciplinary Multispectral Modeling and Cooperation for the Preservation of Cultural Heritage. TMM_CH 2018. Communications in Computer and Information Science, vol. 962. Springer, Cham. https://doi.org/10.1007/978-3-030-12960-6_1. Paris Lodron University Of Salzburg (2025) - Aegina Kolonna. Available at: https://www.aegina-kolonna.at/. Accessed on: February 12th, 2025. Peña-Villasenín S., Gil-Docampo M. & Ortiz-Sanz J. (2024) - Digital documentation and architectural heritage restoration with 3D geometry: The ambulatory of the Cathedral of Santiago de Compostela. International Journal of Architectural Heritage: 1-21. https://doi.org/10.1080/15583058.2024.2366998. Skarlatos D. & Agrafiotis P. (2020) - Image-based underwater 3D reconstruction for cultural heritage: From image collection to 3D. Critical steps and considerations. In: Liarokapis F., Voulodimos A., Doulamis N. & Doulamis A. (eds.) - Visual Computing for Cultural Heritage. Springer Series on Cultural Computing. Springer, Cham. https://doi.org/10.1007/978-3-030-37191-3_8. Sporn K., Berger L. & Tanner A. (2017) - Mapping Aegina Kolonna. In: Lang F. & Wohlmayr W. (eds.) - 50 Jahre Archäologie an der Paris LodronUniversität Salzburg. ArchaeoPlus - Schriften zur Archäologie und Archäometrie der Paris Lodron-Universität Salzburg, 9: 81-98. Tapinaki S., Skamantzari M., Anastasiou A., Koutros S., Syrokou E. & Georgopoulos A. (2021) - 3D holistic documentation of heritage monuments in Rhodes. International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, XLVI-M-1-2021: 739-744. https://doi. org/10.5194/isprs-archives-XLVI-M-1-2021-739-2021. Tsiachta A., Argyrou P., Tsougas I., Kladou M., Ravanidis P., Kaimaris D., Georgiadis C., Georgoula O. & Patias P. (2024) - Multi-sensor image and range-based techniques for the geometric documentation and the photorealistic 3D modeling of complex architectural monuments. Sensors (Basel), 24(9): 2671. https://doi.org/10.3390/s24092671. Verykokou S., Doulamis A., Athanasiou G., Ioannidis C. & Amditis A. (2016) - Multi-scale 3D Modelling of Damaged Cultural Sites: Use Cases and Image-BasedWorkflows.In: Ioannides M., et al. Digital Heritage. Progress in Cultural Heritage: Documentation, Preservation, and Protection. EuroMed 2016. Lecture Notes in Computer Science, vol. 10058. Springer, Cham. https://doi.org/10.1007/978-3-319-48496-9_5. Verykokou S. & Ioannidis C. (2023) - An overview on image-based and scanner-based 3D modeling technologies. Sensors, 23: 596. https://doi. org/10.3390/s23020596 24 Italian Journal of Engineering Geology and Environment, Special Issue 1 (2025) www.ijege.uniroma1.it Verykokou S. & Ioannidis C. (2025) - Image matching: A comprehensive overview of conventional and learning-based methods. Encyclopedia, 5: 4. https://doi.org/10.3390/encyclopedia5010004. Verykokou S., Soile S., Bourexis F., Tokmakidis P., Tokmakidis K. & Ioannidis C. (2021)-Acomparativeanalysisofdifferentsoftwarepackagesfor3Dmodelling of complex geometries. In: Ioannides M., Fink E., Cantoni L. & Champion E. (eds.) - Digital Heritage. Progress in Cultural Heritage: Documentation, Preservation, and Protection. EuroMed 2020. Lecture Notes in Computer Science, vol. 12642. Springer, Cham. https://doi.org/10.1007/978-3-030-73043-7_19 Received February 2025 - Accepted July 2025 C. IOANNIDIS, K. PAPADIMITRIOU, A. TOURTAS, S. SOILE, F. bOURExIS, P. TOKMAKIDIS, K. TOKMAKIDIS, S. VERYKOKOU & C. POTSIOU