**2. Related Work**

Preserving cultural heritage provides the reasoning to investigate the opportunities offered by digital technology. There is a vast body of work within the field of AR in CH contexts, especially for education, learning, and improving tourist experience [11–17].

From a historical perspective, AR was developed from VR, with the specificity that allows the visualization of both real and virtual objects at the same time. The first VR system is considered to be Sensorama machine, developed in the late 1950s [18], while the history of AR started in 1968 with the head-mounted display system developed by Ivan Sutherland. The term "augmented reality" was coined only in 1990 by Tom Caudell and David Mizell [19]. With the development of technology over time, the concept has evolved and the advancements in handheld computers have opened new opportunities for AR [1]. The first handheld mobile phone was presented by Motorola in 1973, and the first personal digital assistant (PDA) appeared on the market in the early 90s. Only a few years later, Bruce H. Thomas developed ARQuake, the first outdoor mobile AR game on PDA [20]. The mobility offered by handheld devices, and the increased progress in terms of software, led to the spread of AR applications as well as the development of new interfaces and new technologies. In 2014, Google announced Project Tango, an Android solution for AR that combines three core technologies: depth perception, motion tracking, and area learning [21].

Since then, stand-alone AR has gradually become more common as computing power on PDAs and smartphones has increased. AR applications implemented on mobile devices were named mobile AR, or handheld mobile AR (HMAR). Such a system is, in its basic form, a mobile device with a display to show an augmentation using the appropriate software [22], with the following characteristics [23]:


There are 2 key issues that need to be considered when developing MAR applications: the accuracy of user tracking and the registration of 3D models with real-world features [24]. The tracking, or the estimation of camera position, can be realized in two ways:


In the case of a CH application, placing markers in the scene is not practical; thus, marker-less methods are required for extracting features from the environment. These features are used for correct alignment of virtual information with real information, a process known as registration. There are two methods of registration:


The surveys on MAR work performed in different application domains [27], or strictly related to the touristic context [28], highlight some determinants and issues that have impact on the utilization of this type of application [15]. Although MAR applications were first designed and tested in indoor environments, the advantage of portability provided by mobile devices facilitated their expansion to the outdoors. This allowed visualization of 3D models of heritage sites to be superimposed over the video stream from the outside environment [29], visualizing virtual points of interest (POIs) overlaid on top of the phone's camera view [30], or textual annotation of outdoor locations based on GPS measurements [31]. Reference [32] proposed an application for outdoor site exploration throughout different periods of time, enriching travel experiences with important historic facts. In other research papers, the authors have used MAR applications "in situ" so that the visitors were able to explore and visualize building details in an intuitive way [33], to educate students visiting an outdoor archaeological site in a game-like approach—"Oracle of Delphi app" [34], or for huge reconstructed models of heritage monuments (e.g., Roman Theatre of Byblos) [35]. A novel paradigm is proposed in [36] based on presenting user artwork, in terms of 3D models, videos, and photos, in virtual kiosks augmented on the camera view of a handheld device.

AR has been introduced in the CH sector over the last decade primarily as a useful technology for assisting visitors inside museums or in heritage sites [37–39]. It allows visitors to explore and appreciate the objects showcased by overcoming barriers in time, space, and language [40]. It also improves the learning experience in CH with the aid of better user interaction methods [24,41]. For instance, in [42] the information is organized in finite triplets (visual representation, context, and corresponding audiovisual content) in order to be presented in a personalized, interactive manner on users' personal mobile devices. Using AR techniques, geomatic resources were also developed, allowing the valorization of the geological heritage [43]. MAR applications can provide real-time information based on the user's preferences and context [44], and can also generate revenue or economic returns [45]. A thorough review of AR applications for CH is provided in [46].

Moreover, AR technologies have proven their importance in the virtual reconstruction of historical monuments, having the potential to provide a new approach to the past by reproducing on-site historical experiences [13]. In the last decade, many objects, monuments, or historical sites were reconstructed into digital formats. The significance of reconstruction is to preserve, protect, and interpret culture and history [4], and to bring to life elements from the past [47,48]. According to [49], every two years the digital heritage is doubling in size and is expected to grow tenfold between 2013 and 2020. While digital heritage is primarily concerned with preservation of CH, virtual heritage (VH) involves the synthesis, conservation, reproduction, representation, digital reprocessing, and display [50] of monuments, artifacts, buildings, and culture, aiming to be more open to global audiences [11]. CH does not end with objects and collections. It also includes information such as traditions, performing arts, social practices, rituals, festive events, activities related to nature and the universe, or skills to produce traditional crafts [51]. According to UNESCO this is the intangible CH, and in order to be celebrated and contemplated more easily, requires the creation of specific conditions to ensure its viability when it is materialized in the form of a media resource [2].

However, the development of applications that targets reactivation of CH in local citizens remains a challenge [52]. In this sense, it is of the utmost importance to take into account the process of evaluation of digital cultural resources. Digital resources are a combination of cultural resources that are created using computer and multimedia technologies. They can be accessed and used in digital form [53].

Given their diversity, evaluation is not a simple process. In this paper we focus only on the evaluation of MAR applications. These should be carefully designed and improved based on user feedback [54]. An important component, which is studied in several papers, is the usability of the application. Relying on five usability principles developed in [55], and on a compilation of heuristic evaluation checklists for mobile interfaces proposed in [56], an MAR application was evaluated in [57] in terms of usability and user expectations. Usability was evaluated in [58] using four different methods. In [52] a methodological framework was proposed, which allowed developers to categorize the type of CH application and to determine which resources should be used. Santos designed a questionnaire for usability evaluation (HARUS), which was composed of two sub-questionnaires, namely the comprehensibility scale and the manipulability scale [59].

Tourist requirements for the development of mobile AR applications were investigated in [60], concluding that tourists are increasingly expecting methods to access information instantly. Other components of experience with AR applications in public were examined in [61], including designing interface features, augmentation features, and so on. Good results after an evaluation of user experience in a mobile AR tourist guide, in terms of ease-of-use and intuitiveness, were obtained in [62]. Users' perceptions and experiences were measured by applying a modified technology acceptance model (TAM) in [63]. Immersion, seen as a form of cognitive and emotional absorption in AR settings, was studied in [64].

In conclusion, there are various studies regarding augmented reality applications in the CH area. Most of them show that the use of new technologies creates innovative and attractive options that increases the users' interest in learning more about CH. However, there are just a few that try to target people from more than one country. Project Tango represents a high-end technology that provides stable and reliable motion tracking. Based on this technology, we've developed an application and assessed its adoption using a questionnaire adapted from [54] and [65], which characterizes the following four subjective parameters: perceived comprehensibility, perceived manipulability, perceived enjoyment, and perceived usefulness. To the best of our knowledge, this is one of the most extensive user studies in AR conducted in outdoor environments.
