1. Introduction

The rich and diverse archaeological heritage of Central Europe is, in parts, excellently developed and utilised. However, in contrast to finds in museums, a broad swathe of these heritage sites, even those of international importance, is very often invisible and intangible as far as the public is concerned. Moreover, archaeological heritage is affected by different human activities and spatial usage conflicts.

During the search for ways to provide effective protection for archaeological monuments, clear presentation of archaeological heritage and its communication to the general public have become essential topics in many European countries in the last few years (Olivier 2016). Emphasis is mainly placed on the active involvement of the general public. Such a 'new' approach represents, to a considerable extent, reaction to a strongly professionalised discipline that more or less alienated itself from the general public during the second half of the 20th century, whereby its promotional activities primarily relied on passive consumption of research (Willems 2014).

VirtualArch has focused on the practical application of innovative and trendsetting visualisation tools in the field of virtual and augmented reality. One of the aims is to unveil regional archaeological heritage located underground or submerged, some with global importance (UNESCO), for local and regional stakeholders that are responsible for economic development. It is hoped to increase awareness and acknowledgement of these sites by tailoring and implementing target-group-orientated and specially designed visualisation and presentations using virtual or augmented reality (VR resp. AR) (Pierdicca et al. 2015). Furthermore, virtual reconstructions could be used as an innovative visualisation tools during spatial usage conflict management and, hence, contribute to better heritage protection.

2. Partners and Pilot Sites

In contrast to many other VR/AR visualisation projects in archaeology, VirtualArch's approach is through transnational co-operation. Facing similar challenges and sharing the same objectives, although with diverse heritage, ten partners from eight countries have come together in an EU-funded project (Interreg Central Europe), running from 2017 to 2020. The partnership comprises regional and national archaeological institutes and heritage offices, two universities/research institutions and also two local communities as heritage owners. On eight selected pilot sites all over Central Europe, experiences were shared, and distinct innovative visualisation and communication approaches were discussed and introduced. Based on the experiences in these pilot regions, a transnational strategy for future projects, as well as guidelines for similar heritage sites, is planned.

This heterogeneous approach is also reflected in the diversity of the pilot sites, which are characterised by various archaeological cultures, areas, environments, impacts and challenges. All of these sites contain unique finds, often from organic material, which provide considerable insights into past life and are, therefore, of international importance for research and the general public. However, none of them are publicly accessible or visible, and because of their complex structures, they are somewhat intangible to non-professionals.

The pilot sites can be separated into three groups: urban areas, mines and underwater sites. The specifics of each group vary in the way primary data was gathered, but also in the way in which they are further presented to the public. Additionally, each pilot site has its own plans on how to achieve their goals.

Within the group of mining heritage sites, the prehistoric salt mines of Hallstatt, Austria, are pre-eminent; part of the UNESCO cultural landscape 'Hallstatt-Dachstein/Salzkammergut' since l997. Known to the scientific community for the famous cemetery excavated in the 19th century, Hallstatt is one of the most important sites in European archaeology, thanks to the outstanding results of excavations and experimental research undertaken by the Natural History Museum Vienna since the 1960s in the still-active salt mines (Reschreiter and Kowarik 2017). Nowadays, the Salt Valley is already a popular tourist destination with an excellent infrastructure, so the aim of this project is to develop more precise and attractive ways of presenting the finds or displaying them in a new light for the public. On the other hand, the heritage is seriously threatened by natural movements of the rock itself (Reschreiter et al. 2017).

Video 1: A model of the Bronze Age mine at Hallstatt was reconstructed and visualized for museum visitors. Credit: Interreg Project Partners

The second important mining site is located in Saxony, Germany, where unique and almost complete mines of the Middle Ages were found under the town of Dippoldiswalde. The Archaeological Heritage Office of Saxony has been working to record and preserve this outstanding heritage since 2008 (Hemker 2011; Hemker and Schubert 2018), which, for safety reasons, is not accessible or visible to non-professionals or tourists.

An important mining landscape lies in the mountainous area around Civezzano, near Trento in Italy. From the 12th to the 15th centuries, silver ore was exploited there intensively, and archaeologists discovered a mining area with sinkholes and gallery entrances over 12 sq. km (Casagrande et al. 2017). For safety reasons they are also not accessible as well as being at risk from threats posed by agriculture and forestry.

A big and important mining and metal-working settlement from the 13th and 14th centuries, associated with visible remains of mining activity, was discovered near Utín in the Bohemian-Moravian highlands (Hrubý et al. 2016). The settlement, known mostly thanks to geophysical surveys, includes interesting features, such as miners' houses, an ore mill, a stamping mill and furnaces, as well as a hospice and a filial chapel. The area is largely uninhabited today, but agriculture and forestry could endanger this unique site. Identifying its full extent, virtual reconstructions and target-group-orientated lectures and tools enables a better understanding and, consequently, protection, for example, by establishing special exclusion zones.

Urban archaeology is represented by the pilot site of Nitra, Slovakia, a princely residence since the 9th century and of national importance as the oldest centre of early Christianity (Ruttkay and Bednár 2018). In fact, the urban area of Nitra was first settled in the Neolithic period. The settlement stratigraphy and phases of rebuilding made the archaeological layers invisible to the visitors´ eyes. The small finds and excavations carried out over 30 years by the Slovak Academy of Sciences, will be visualised to present the importance of the site — right down to the smallest cup.

In contrast to the previous example, the Slovenian pilot site Ljubljansko barje is a large wetland area near Ljubljana, containing several prehistoric pile dwellings, a World Heritage Site since 2011. The pile dwellings are a tremendous source of information, not only for archaeology, but also for dendrochronology, botany, climatology, geology and other fields of interest. The preservation of this archaeological heritage of global importance is, however, heavily endangered. Ljubljansko Barje constitutes a very attractive area from an agricultural point of view and is, therefore, highly endangered because of farming activity, such as digging new, deep drainage channels and deep ploughing. Interactive historic landscape visualisations and AR applications, aim to make stakeholders aware of these hidden settlement structures, and so provide better protection.

Finally, the field of underwater archaeology is represented by two important harbours. The ancient Roman harbour of Barbir in Sukosan, located on the Adriatic Sea coast of Croatia. There are several submerged remnants of stone piers or breakwaters, as well as pottery and small finds from the 3rd to 4th century. Although the International Centre for Underwater Archaeology is situated near Zadar, little research and few surveys were conducted, and the site is not well known. Secondly, a large site from the 10th to 14th centuries in the Baltic Sea is known offshore from the Polish town of Puck (Pomian et al. 2016). The remains of the harbour structures, four shipwrecks, potsherds and bones dating from the 10th to the 14th centuries are spread over an area of 12 hectares. Puck was probably the most important early medieval port in the southern Baltic coast, more prominent than well-known places like Haithabu, Schleswig or Lübeck.

Video 2: A model of the Roman port of Babir was developed to show the site in its moden setting as well as in Roman times. Credit: Interreg Project Partners

3. From Field Survey to 3D Models

Although there are significant differences between the heritage sites mentioned, activities in the pilot regions are based more or less on the same multi-stepped strategy. Firstly, all partners gathered and digitised data obtained from the archaeological pilot studies, including field surveys and aerial archaeology. Finds and archaeological features were 3D recorded, using different techniques ranging from structured-light scanners to photogrammetry and 3D scanning of finds (for an example of mining archaeology see Elburg et al. 2014), as well as hydro-acoustic survey methods for underwater sites.

The processed data provide the basis for modelling the virtual reconstructions, representing the second step. Depending on the visualisation options and the 'storytelling' behind the picture, the high-resolution meshes had to be reduced, missing items added, or situations and textures exchanged.

The third step aims to produce a realistic virtual model of each heritage site, visualised and presented by using various VR/AR options. In 2018, the project partners met together with other interested parties (such as external experts, stakeholders) to create a coherent vision for digitisation and visualisation of the pilot sites. Among the suggestions was the application of interactive panoramic views of, for example, prehistoric or medieval settlements that are currently invisible and concealed beneath modern towns or farmland. In addition, interactive 3D models of small finds, or even entire segments of a landscape, allow us to better understand and interpret the subject matter of our research. The AR methods would enable users to take a virtual walk through past settlements and mines directly on or over the heritage site. Finally, new VR data glasses enable immersive experience of inaccessible sites using 'ancient items' that would otherwise be hidden in archives or exhibited in showcases of archaeological museums.

4. Why use Augmented and Virtual Reality Tools?

The biggest challenge in presenting the earliest history of human culture is that its cognition largely remains encapsulated within the highly professional environment of archaeological monument care and particular scientific projects. Therefore, an alternative approach, i.e. the effort to make knowledge in our field virtually accessible, is highly desirable. Augmented reality technologies can quickly provide access to archaeological artefacts as well as entire sites.

It is very likely that virtual presentations of archaeological objects and features present a distinct trend for the future. This approach guarantees many benefits, including easy and fast dissemination of information, as there is already an established and functioning infrastructure, namely a massive expansion of smartphones and affordable Internet connection. Thus, such possibilities open the way to address the computer-literate young generation, who already perceive virtual space as a natural part of their real world. Building virtual presentations is much easier, as far as organisation and financial demands are concerned, than the physical presentations. Moreover, the costs of operation and maintenance of mobile applications are virtually zero. Virtual open-air museums can also be created in an environment where other forms of presentation cannot be envisaged, such as directly at the site of a protected cultural monument or in a city centre.

The rapid development of information technologies has been significantly facilitated by the widespread use of virtual and augmented reality for the presentation of archaeological sites without substantial financial costs. Moreover, there is a wide range of free tools. If 3D data from reconstruction models or digital non-contact documentation is available, it can be presented free of charge in augmented reality by using the Sketchfab platform. After you install the application on your phone, it allows you to present and view all the 3D models that are loaded on the platform. Similarly, it is possible to directly present 360° panoramic images generated from a 3D model using a mobile application such as VR Media Player.

An open platform for creating mobile applications that can be used for the presentation of archaeological sites is being created within the scope of the VirtualArch project. The aim is to enable even complete computer novices to create mobile applications. All information and content would be imported via a website interface, and the user can then upload texts, accompanying images, 3D models and 360° panoramic images to the application and display them. Thus, users will be able to view 3D models interactively, as well as 360° panoramic outputs from computer reconstruction models, just by swiping their fingers across the screen. The application will also include a map with points of interest and the current position of the user.

Another outcome of the project under preparation, which should facilitate the creation of computer models, is the so-called '3D Home Kit'. It is, in fact, a catalogue of digital models of objects and features (buildings, technological equipment, movable objects) that have been prepared to simplify computer visualisation of archaeological sites. The 3D Home Kit is designed primarily for smaller archaeological institutions or amateur interest groups to facilitate the creation of low-budget computer models.

Presentation of the medieval archaeological site at Buchberg, a 13th- and 14th-century silver mine near the village of Utín in the Czech Republic, is among the pilot project studies using game elements. The user will have the opportunity to become acquainted on the site itself with the appearance and function of the mine galleries and the adjoining processing district, through the prepared 3D reconstruction model available in the mobile application. To draw the public more intensively and actively into the topic, the application includes a game with a detective plot called the 'Devil's Adit' (Figure 1), the story of which takes place in medieval Buchberg in 1269. The player takes on the role of a young knight named 'James of Týn' and will gradually reveal the terrible secret concealed within the mining area. During the young knight's journey the user encounters different historical figures, known from written sources and directly connected to the site, and visits particular places in the mines, which are connected, in various ways, to the mining and processing of silver ore. The whole plot will be unravelled in the underground galleries of the mine, and the player will be able to enter them via augmented reality. The game, which has the potential to quickly draw the visitor into the action, mediates in a humorous and original way the computerised reconstruction and visualisation of the medieval mining area, and also conveys information about everyday life there and the operation of the entire silver mining process in the Middle Ages.

5. Specifics of Computer Reconstructions in Archaeology

The presentation of archaeological heritage to the public through virtual reality also involves a number of risks. Archaeological features are inherently incomplete, and their interpretation is overwhelmingly ambiguous. Thus, computer visualisations are pre-conditioned to, somehow, cope with the relatively high degree of uncertainty of archaeological data. The London Charter, which states, among other things, that visualisation should accurately determine the differences between real data and hypotheses, and also between different levels of probability, may serve as a guide for identifying appropriate practices (Denard 2012, 60).

Since the 1990s, many publications have been dedicated to the use of computer visualisations and their professional value (Reilly 1991; Miller and Richards 1995; Sims 1997; Barceló 2001; Sanders 2014). A concern that visualisations would only be used to produce attractive images and, thus, become another version of PC games or fantasy movies, runs like a silver thread through these publications. In this respect, one of the essential tasks of the VirtualArch project is to respect the London Charter recommendations and look for specific technical or visual solutions.

We can only regret that no methodology of visual communication of computer reconstructions has yet been developed. Frameworks created for digital reconstructions, such as the London Charter, represented an essential step in evaluating the creative process and its objectives, but they do not pursue any consequential possibilities and creative potential that 3D computer visualisations can bring to archaeology. One possible approach represents projection into photorealistic models of an actual archaeological field situation obtained by laser scanning or multiple photogrammetry. In such environments, it is possible to distinguish existing structures and those that have been modelled based on interpretation (Figures 2 and 3). Another possibility is the application of principles of technical illustration utilising diagrams, simplified technical sketches, plans or graphs. Such types of data can convey more information and explain the context of the situation.

Each ordinary amateur 'consumer' of computer visualisations or outputs of the VirtualArch project will be aware of several essential characteristics of the archaeological heritage, namely of the incomplete nature of archaeological data and the possibility of alternative interpretations of archaeological contexts. The impossibility of creating a single correct interpretation of an archaeological context based on the field excavations should support the arguments for maintaining any archaeological site in situ as far as possible.

6. Conclusion

Rapid technological development and, thus, easier accessibility has significantly transformed the role of virtual reconstructions from a mere illustrative complement of archaeological popularisation to the position of a standard part of the interpretative process of archaeological data. Inevitably, such a process always involves subjective imagination, even if the reconstruction is only verbal. Contrary to interpretative texts, virtual visualisation can very quickly and clearly determine the boundary between attested, anticipated and imaginative features. It is crucial to supplement virtual reconstruction models with metadata that explain the selected reconstruction steps and interpretative methods. Thus, the virtual visualisation could become a standard part of the process of learning about the past in many fields, not only in archaeology.

Reconstruction and direct interpretation of the past is a crucial factor in making it accessible to people in the present. Visualising the past in virtual space with all the available options described above will undoubtedly continue to strengthen and broaden its potential. If we can dare to predict, then such a form of presentation will challenge the prevailing and conventional forms based more or less on textual communication. Why would an image significantly supplant the word? Because the text demands understanding (the text is a language-dependent medium) and requires the consumer to concentrate. On the other hand, an image is generally comprehensible, and it is up to the viewer to decide how much attention he or she is willing to give it.