1. Introduction
Historical sites are relics of human civilization, including buildings built for different purposes, as well as places affected by human utilization and the processing of the natural environment. Historical sites are the important material and spiritual heritage of a nation, as well as a witness to the development of a nation and even a civilization. They include archaeological sites with outstanding universal value from a historical, aesthetic, ethnological, or anthropological perspective [
1]. The preservation of historical sites is related to the future, and it is our unshirkable responsibility to protect them. We should understand the profound historical and cultural implications behind them so as to cultivate a new modern culture based on tradition [
2]. However, the rapid development of society has complicated the protection of historical sites, but fortunately, the development of digital technology, such as terrestrial laser scanning, close-range photogrammetry, and satellite remote sensing, has created new opportunities for the protection and utilization of historical sites with fragile and perishable characteristics [
3,
4]. In the past three decades, technical improvement has led to digital protection becoming a mainstream trend in the protection of historical sites, through digital archaeology, 3D modeling, geographic information systems, etc. [
5,
6]. Digital media technologies such as virtual reality and augmented reality have also revitalized the inheritance of historical sites [
7]. Breaking time and geographical restrictions, people can easily access global history and culture, enhance understanding between different cultures, and promote the diversified development of modern culture [
8].
Overall, digital technology has pushed the protection and utilization of historical sites to a new level. However, the increasing number of digital resources has also introduced new problems and opportunities, such as how to integrate and link digital resources and excavate heritage value. Additionally, digital protection and utilization involve mostly static data collection, storage, and processing, which lack a consideration of real-time intelligence and interactivity, as well as the intelligent diagnosis and prediction analysis of risks. Fortunately, digital twin (DT), a new technology, provides a new approach to solving the above problems.
The digital twin prototype was proposed for Product Lifecycle Management (PLM) by Michael Grieve at the University of Michigan in 2003 [
9]. In NASA’s 2010 technical report, digital twin was officially proposed as a system of aircraft simulation processes that integrate multiple physical quantities, scales, and probabilities [
10]. It was defined by Grieve as a digital replica used for virtual expression and real description, and he conducted model simulation tests of real environments, conditions, and states [
11]. As of 2023, research has made substantial progress in theory and application. The study field has gradually shifted from aerospace and the military to industrial manufacturing, heritage protection, etc. It has been expanded from designing and planning to modeling and manufacturing, management and maintenance, etc.
Especially in the past three years, digital twins have become a hot topic in cultural heritage research in theory and application. The focus of theoretical research is mainly on the models, frameworks, technologies, and management of digital twins. For example, a digital twin model was created by means of motion photogrammetry and depth camera mapping to generate a four-dimensional space of cultural heritage [
12]. A digital twin provided an accurate digital model for performing finite element analyses for preserving valuable assets, which was the structural vulnerability assessment of heritage structures and the pivotal part of a risk mitigation strategy [
13]. A digital twin research framework combined with the HBIM model is used for managing the preventive protection of heritage buildings and identifying the threats associated with their integrity, the corresponding mitigation strategies, the value assessment process, etc. [
14]. The function of digital twins is to support on-site managers in the preventive protection of their assets, predict threats to site integrity based on the DT environment, and propose corresponding prevention plans [
15]. The digital twin paradigm has significant advantages in integrating knowledge, protecting, restoring, and managing cultural heritage.
In applied research, the focus is mainly on the protection and utilization of key cultural heritage sites. A digital twin was used for the protection and utilization of the Great Wall through evidence-based analysis and scientific deduction using the information technology methodology framework [
16]. A digital twin method was constructed to consider the connotation of the whole life cycle and applied to the Great Wall to evaluate the application effects [
17]. The application of a digital twin was combined with artificial intelligence and 5G technology in the art design of digital museums [
18]. A protocol for the graphic and constructive diffusion with the digital twin method of building heritage was created to guarantee universal accessibility through AR and VR [
19]. The maintenance and preservation of art objects in a museum with digital twins highlighted the great advantages of the integration of traditional and novel procedures in the conservation of artistic assets [
20]. The Parco Archeologico dell’Appia Antica in Rome has been digitized in multi-temporal digital twins integrated with historical documentation, connecting physical and virtual models to provide an immersive experience beyond traditional reality [
21].
Based on the above research, this study aims to introduce digital twins into the digital protection and utilization of historical sites and build a digital twin system. With the characteristics of virtual–real integration and real-time interaction, it solves the problem of poor real-time intelligent interactivity and explores the coexistence of the physical entities and virtual twins of historical sites to produce a new protection and utilization mode for historical sites with intelligent interaction and management. This paper provides a comprehensive method for the protection and utilization of historical sites based on digital twins. This paper is organized as follows.
Section 2 describes the composition and function of this method in detail.
Section 3 is an introduction to the case study of the historical sites of the Northern Qi Dynasty in Taiyuan, Shanxi, China.
Section 4 provides a detailed analysis of the experimental results of the historical sites investigated by our team.
Section 5 discusses the methods and results of this study. Finally,
Section 6 reports the conclusions.
3. Case Study Introduction
In this case study, we investigated a historical site of the Northern Qi Dynasty (557–587 AD) in Taiyuan, Shanxi, China, because of the diversity and complexity of its historical sites with high symbolic and cultural value, as shown in
Figure 4. These features make it a relevant case study for explaining and analyzing the results of digital twin.
Taiyuan is the capital city of Shanxi Province in China, with an area of about 7000 km
2 and a population of 5.4 million. It is the political, economic, cultural, and international exchange center of Shanxi. Taiyuan is a national historical and cultural city, an ancient city with a history of more than 2000 years. In 557–587 AD, Taiyuan was the alternate capital of the Northern Qi Dynasty, which was also important for commercial exchanges and cultural integration. World economic and cultural exchanges were unprecedentedly prosperous, with envoys and merchants from various Asian and Mediterranean countries gathering [
28]. Foreign religious ideas and cultural arts were widely absorbed, greatly enriching China’s traditional culture and art [
29]. In an era of great ethnic and cultural integration, a rich historical and cultural heritage was left behind, such as the Xuxianxiu Tomb, Lourui Tomb, Tongzi Temple, etc. However, due to natural or human factors, most of the sites are not completely preserved, and the original history cannot be completely presented.
3.1. Xuxianxiu Tomb
The Xuxianxiu Tomb, the only original protected historical site in China, is located in Wangjiafeng Village in the southeast of Taiyuan City. It was one of the most representative historical sites and the best preserved mural tomb of the Northern Qi Dynasty (550 A.D–577 A.D), excavated in 2000 and rated among “China’s Top Ten Archaeological Discoveries” in 2002, as shown in
Figure 5a. In 2006, it was designated as a key cultural relic protection unit in China and is one of the few burial sites protected in its original location [
30]. The owner of the tomb, Xuxianxiu, was the King of Wuan in the Northern Qi Dynasty. Therefore, it was a royal mural tomb with a grand scale and rich contents, a brick chamber tomb with a square surface facing south, dome-shaped brick coupons, and a single-room long slope. The tomb was composed of a corridor, tunnel, patio, and tomb, measuring 30 m long and 8.5 m deep. The 330 m
2 painted murals on the tomb wall are well preserved, magnificent, and spectacular, with vivid and realistic imagery, retaining their original color [
31]. They were a masterpiece in the history of Chinese art, representing the highest level of painting at that time, and containing much historical and cultural information. In addition, there are more than 550 buried objects, including pottery figurines, porcelain, etc. [
32]. At present, most of them are placed in the Shanxi Provincial Museum, Taiyuan Museum, and Shanxi Provincial Archaeological Institute. As shown in
Figure 5b, the Xuxianxiu Tomb, under original protection, is located in the Taiyuan Northern Qi Dynasty Mural Museum.
Since its discovery, the Xuxianxiu Tomb has received much attention from academia and society. In particular, its protection concepts and practices have been highly recognized by experts and various sectors of society, such as protection during excavation, minimal intervention protection, environmental control protection, scientific monitoring and preventive protection, digital protection, and scientific protection of the original site. At present, it is the only mural tomb in China that has been scientifically protected through building a museum on its original site after archaeological excavation, and its digital protection and utilization have become a new focus of attention. Therefore, we conducted research on the digital protection and utilization of historical sites based on a digital twin and took the Xuxianxiu Tomb as a case study.
3.2. Lourui Tomb
The Lourui Tomb is located in the southwest of Wangguo Village, southwest of Taiyuan City, and was excavated in 1979, as shown in
Figure 6a. It is another well-preserved mural tomb and a representative historical site of the Northern Qi Dynasty (550 A.D.–577 A.D.) [
33]. The owner of the tomb, Lourui, was the Dongan King of the Northern Qi Dynasty. Therefore, it is a grand and rich royal tomb, consisting of a tomb passage, courtyard, corridor, and tomb chamber, over 22 m in length and over 12 m in depth. It is over 200 m
2 and contains 71 picture murals in the tomb, mainly distributed in the tomb passage and chamber. The content of the mural is a grand and colorful large scroll, depicting the magnificent scenes of the tomb owner’s life before his death [
34]. It can be regarded as a representative work of the Northern and Southern Dynasties period, filling a gap in the history of art. Due to the low-lying and humid terrain, the tomb murals have been exposed and are now preserved in the Northern Qi Dynasty Mural Museum, as shown in
Figure 6b. In addition, there are over 870 funerary objects, including pottery figurines, porcelain, etc., which are the most excavated tomb artifacts of the Northern Qi Dynasty [
35]. Most of them are stored in the Shanxi Provincial Museum, Taiyuan City Museum, and Shanxi Provincial Institute of Archaeology.
Since its discovery, the Lourui Tomb, like the Xuxianxiu Tomb, has received much attention from the academic community and society, not only focused on its painting, but also focused on research on the Northern Qi Dynasty culture. However, due to the backfilling of the Lourui Tomb and the distribution of funerary objects in various museums, research on it is restricted. Therefore, we researched the digital protection and utilization of historical relics based on a digital twin and used the Lourui Tomb as an example for digital restoration and VR/AR display.
5. Discussion
This approach emphasizes the process of preserving and utilizing historical sites through digital twins. Through digital twins, the historical site information of the real world is mapped to the virtual world, providing support for preventive protection and cultural transmission, and facilitating interdisciplinary collaboration. In practice, it was found that this method has certain advantages in protection and utilization.
In the case of the Northern Qi Dynasty historical sites in Taiyuan, a five-dimensional digital twin system was established to form the hierarchical research on protection and utilization. By collecting a large number of monitoring data and protection cases, the system can simulate and predict possible risks based on the current monitoring data and give protection suggestions. For example, in
Figure 14a, through an analysis of the temperature and humidity monitoring data at the entrance, middle, and bottom of the Xuxianxiu tomb passage in the digital twin system, the change trend of monitoring temperature and humidity was found to be greatly affected by the external environment. Due to the rainy season, the humidity at the entrance and bottom of the tomb remains high, which easily causes plaster disruption, flaking, detachment, and other symptoms.
Figure 14b shows the symptoms of plaster disruption, flaking, and detachment found at the bottom of the east and west walls of the tomb passage, consistent with the mural symptom warning issued by the system.
In 2021, we relied on the temperature and humidity monitoring data from 2018 to 2020 to conduct risk assessment simulation calculations on the site, predict risks, and put forward corresponding protection suggestions. We implemented a fully enclosed, constant temperature and humidity protection system for the Xuxianxiu Tomb site, with a temperature of 15–16° and a humidity of 55–60%. More than a year after the adjustment, the temperature and humidity of the ruins were still controlled and monitored. The preservation results of the ruins’ space and murals were good, and no hollowing, crispness, or cracking occurred again. As shown in
Figure 15a, a mural on the east wall of the tomb passage suffered from hollowing and fell off due to alkali damage in 2019.
Figure 15b shows that after restoration, protective measures were taken to maintain the effect to this day.
In addition, the knowledge graph constructed from digital twin data brought us a significant amount of information. Taking the funerary objects of historical sites as an example, due to the different excavation times of historical sites, some of which were nearly 30 years apart, they are all stored in different museum warehouses and recorded in different ways. For example, the Lourui Tomb was excavated in 1979, and the Xuxianxiu Tomb was excavated in 2003, and their funerary objects were stored in different museums. As a result, it was difficult for us to determine the correlation between different artifacts, and it was also difficult to explore the information behind them. However, all funerary objects can be identified and mapped through digital twins, and we can discover much hidden information. For example, we were pleasantly surprised to find that the “Cage-crown figurine” from the Xuxianxiu Tomb and the “ Maidservant figurine” from the Lourui Tomb were exactly the same in shape, color, texture, and size material and were fired in the same kiln, as shown in
Figure 16a. In addition, we found some regular features. For example, in
Figure 16b, which depicts a sunburst chart of funerary objects, ceremonial objects are the most common, accounting for 53%, and burial objects are the least common, accounting for 3%. Among the caravans, there are already more camel figurines and fewer horse figurines in the western regions, indicating that multi-ethnic integration had become very prosperous. A camel figurine from the Hanzunian Tomb is shown in
Figure 16c. The discovery of numerous potential sources of information provides a scientific basis for research in history, sociology, religion, and philosophy.
However, this method still lacks many functions. In protection, we have only achieved simulation and the prediction of risks, whereas how to use digital twins to repair damaged historical sites has not yet been achieved. In utilization, we have only implemented knowledge graphs and digital displays, while other areas such as planning and development, cultural and creative design, etc., need to be implemented. In addition, this method has limitations. For example, we have only achieved text recognition but not image recognition, so the knowledge graph can express the physical characteristics of funerary objects, but it is difficult to describe their symbolic characteristics and environmental characteristics. This will inhibit them from becoming integrated into an overall scheme in the correlation comparison [
37]. The image features of objects can only be compared based on textual descriptions, such as image elements in murals and decorative elements in utensils. Therefore, in future research, more functions and more effective methods need to be researched and developed.
6. Conclusions
Current methods of historic heritage conservation and utilization lag far behind advances in digital technology, and this study provides an innovative approach to the digital preservation and utilization of historic sites. Through the digital twin method, it realizes the preservation and utilization of historical sites from three aspects. Firstly, the simulation and prediction of historical sites are realized in the data twin virtual entity. Secondly, in digital twin, knowledge maps are constructed to mine potential connections between sites that appear to be unrelated on the surface. Finally, through digital twin, historical sites that have disappeared can also be restored and displayed through VR/AR and other technologies, and multidisciplinary researchers can collaborate efficiently to create digital display works.
By investigating the historical sites of the Northern Qi Dynasty in Taiyuan, Shanxi, the method was further applied to the protection of historical sites. The results show that the digital twin method is effective. It can also be applied to different types of cultural heritage. These digital achievements can be widely disseminated through websites, mobile phones, and museums and galleries.
In the future, we will further improve this method in terms of interactive display, simulation, digital repair, cultural mining, pattern recognition, etc. We will also survey more historical sites to form a giant digital twin system that integrates site protection and cultural inheritance. In addition, AI has grown, and we can also integrate it into our innovation research, which is an important task for our future work.