## 2 Related Work
### Novel Interface to Support Exploratory Navigation  
As an interface between digital information and the physical world, the digital map always plays an important part in navigation systems and should be designed to be more adaptive to pedestrian needs. The information the map provides might shape and affect relationships between places and people and give pedestrians inspiration for searching and browsing on the way. Conventional navigation systems, such as Google Maps [45] and Apple Maps [46], have benefited daily navigation with efficient orientation and location. However, the main feature of these mainstream navigation applications is providing people with precise turn-by-turn navigation, but the exploratory feature is not widely designed and integrated. [PERSON] and [PERSON] [14] proposed a radar navigation interface to visualize the points of interest and provide a general exploratory directional navigation for urban exploration. [PERSON] and [PERSON] [13] proposed location-based augmented reality and object-recognition augmented reality to enable users to efficiently obtain directions to sightseeing spots and nearby facilities within urban tourist areas and sightseeing spot information. Some studies have also contributed to designing interesting location-based game mechanisms to engage in exploratory navigation in urban environments [11; 17]. Augmented reality can enhance people's perception of the real world by displaying additional virtual information. Recently, the studies on augmented reality navigation could be summarized into car navigation, pedestrian navigation, and indoor navigation according to the scenarios. Most studies on pedestrian augmented reality navigation are focusing more on the visualization of point-of-interest information and turn-by-turn navigation instructions of route planning on the hand-held devices [22; 47]. In these studies, the map interface usually performs as a split part of the screen to assist recognition of virtual marks of point of interest and digital route instruction. Moreover, the map interface design in these studies is similar to the conventional map view of the navigation system. With the development of technology on GIS data and modeling, the 3D digital map plays an important role in navigation, geographic data visualization, urban management, location-based service, etc. Some studies utilized mixed reality to augment the real world via virtual 3D content such as 3D architectures and buildings for supporting cultural heritage sightseeing [48; 49]. Differently from just overlaying the virtual labels and signs on the real world through augmented reality, mixed reality is a broader concept that can combine the virtual objects and the real world and make the virtual elements become anchored to specific locations in the physical world to make the interactions between human and virtual content more immersive. With the development of technology in the context of the smart city environment, the mobile mixed reality through head-mounted display might go outside and be applied in pedestrian exploratory navigation to provide a more immersive and interesting experience. However, because the user interface in a head-mounted display is far different than in 2D mobile devices, the design of the map interface needs to be explored and discussed in detail. For instance, what role does the map interface play in mixed-reality navigation, and how we should integrate the map interface into mixed-reality navigation outdoors? To our knowledge, the study on virtual 3D map interface design in mixed-reality navigation is not sufficient in the literature. In [50], the authors proposed the concept of the hybrid map in the virtual reality environment and discussed the interaction method in the preliminary study, but the detailed design of such a map interface has not been discussed a lot. In [51], researchers focused on hybrid mobile tools to visualize high-quality and complex 3D geometry in the real environment, and the design of a 3D map interface was not discussed.