## 1 Introduction
In addition, the visitor app may be freely available on the marketplaces and in combination with the virtual objects offered in special repositories, may enable virtual tours of the monuments remotely. In a hypothetical scenario, the visitor of the monument opens the mobile camera and aims at a point of interest. Through a free application and a set of data made available by the wireless infrastructure of the archaeological site or the Internet, the cultural tourists can visualize and acquire, according to their wishes, in-depth information regarding the archaeological site, the monuments or historical events pertinent to them, enhancing, that way, their cultural experience. At the same time, the Global Positioning System (GPS) receiver of the smart device will accurately approximate the visitor's position within the 3D virtual geospatial world. Alternatively, the visitor may be able to make use of the virtual content remotely without having to visit the site and receive information about the cultural product of the area of interest (Figure 1). Having described all the above capabilities provided by the system in a laboratorial-experimental level, the real-world conditions at an operational level will be presented. There are two distinct factors to be considered, which both highlight the potential contribution of the presented work:  
* The authorities responsible for promoting specific cultural-touristic resources may exploit the portion of the system dealing with the development of the digital material. \"Mergin' Mode\" authoring tool utilizes geographic information systems (GIS)-based functionalities to assist the development of virtual custom geospatial worlds presenting historical representations of the monuments. Beyond the typical thematic layers that may include the site terrain and vector graphics with areas, lines and points, additional themes may be added to compose sophisticated 3D scenes [6]. Such themes may include 3D models of natural spatial objects (e.g., trees or plants) or of cultural objects of historical importance either moveable (e.g., amphore) and immovable (e.g., temples) or \"living\" ones (e.g., people and animals). Other thematic layers may specify the routes of motion for moving objects, or the points/areas of placement for the stable ones. Although the overlaying of numerous thematic layers to form a photorealistic 3D scene is a relatively old technique, since the era of the first multimedia projects, such as Flash [10], the geospatial reference of all the involved objects nevertheless requires a GIS-based approach. Besides, the spatial reference is the key property of an object that specifies its behavior in response with the end-user location. Moreover, although virtual reality and 3D computer graphics technologies have evolved over the last decades, their co-existence with LBS is an issue that invites further research and development projects. * The visitors/end-users perceive the digital material provided by the authorities of a monument as rendered over their camera, along with the monument in its current condition in a MR app. Considering the ubiquity of LBS in a vast number of smartphone apps, \"Mergin' Mode\" invests in this evolving capability of establishing an on-demand, direct connection between the user of cultural digital material and the provider, without the need of specialized equipment except for a smart device. Although this issue is already raised [11; 12; 13], the developments so far justify additional research efforts and allow significant improvements. Someone could imagine the whole venture as being similar to that of the Google Maps: the user may download the maps for an area of interest and use them offline for routing purposes, whenever the device is located in that area. In our case, the material concerns cultural heritage resources and the representations of a monument along with historical events, which are triggered at the time of the user's georeference in the area of the monument.