## 1 Introduction  
In recent years there has been a widespread penetration of location-based services (LBS), also known as geo-services, into people's daily activities, triggered by the increasing affordability of smart devices (tablets, mobile phones) for the average end-user/consumer [1]. A typical example of usinggeo-services is the navigation to a place using freely available maps and the simultaneous association of current location with other complementary information. At the same time, significant developments in geoinformatics technologies reflect the progress occurred in hardware and software technologies. Nowadays, it is possible to map a region with high topographical accuracy and create high-resolution digital terrain/surface models (DTM/DSM) by employing unmanned aerial vehicles (UAV) [2; 3; 4]. Likewise, new technologies allow the creation of three-dimensional models that simulate inanimate or moving/animated spatial objects (e.g., buildings or virtual humans respectively) [5]. The above, combined with high quality three-dimensional (3D) modeling visualization and animation capabilities offered by computer graphics technologies, enable the development of custom virtual geospatial worlds. A custom virtual geospatial world simulates real-world spatial objects in a number of overlaid geo-referenced thematic layers [6]. These layers may include raster surface textures, DTM/DSM models and 3D spatial entity models moving in pre-specified or dynamically defined motion paths. The implementation of such virtual worlds can be based on open technologies and standards and on functionalities provided by free software libraries. Moreover, they can be demonstrated on smart device platforms and/or standard interfaces of the World Wide Web. The ever increasing data transfer speed in communications and wireless networks [7], the widespread use of geospatial web services [8] and the evolution in the augmented reality (AR) technologies [9] are boosting the provision of custom virtual geospatial worlds for a mobile end-user and offer novel opportunities for the deployment of numerous smart applications (apps). Building on the above-mentioned, the presented work aims to introduce \"Mergin' Mode\"--a system comprising an authoring tool and an app, able to support overlaying of (a) highly detailed virtual terrain environments and three-dimensional models representing animate or inanimate objects, placed or moving over these environments and (b) the real world as captured by the camera of a smart device. For example, during the physical presence of a user in an archaeological site, it will be possible, through storytelling techniques, to \"revive\" on the screen of the mobile phone, historical events represented by a custom virtual geospatial world. These events along with dynamic reconstructions are evolving based on the users' actual position (spatial reference) and the route they follow on the site. That way, the digital material must also possess a spatial reference and be provided under LBS. Possible application scenarios include information provision regarding both the history and the historical events that took place in monuments and archaeological sites. These scenarios can be used both for educational and recreational purposes for the local communities of the monuments and archaeological sites areas, and for the development of cultural tourism in those areas. Technically, the system is based on the merging of the real with the virtual in mixed reality (MR), assisted by geoinformatics technologies, to be applied on monuments with the goal of demonstrating them. The final output is a set of web services that will enable the visualization of an archaeological site, a monument or a set of monuments in its current form as captured by the camera of a smart device in conjunction with virtual objects that can represent historical events and that can narrate stories to the end-users/visitors of the site.