# Automatic Generation of Guidance for Indoor Navigation at Metro Stations
## 3 Materials and Methods
### Spatial Data Model  
The model of spatial data used in the LIFT project, with clear division into three layers, was based on earlier work of the research team [57, 58, 59] with modifications specific to metro stations. The classes of the model are presented in Figure 2.  
The graph representing the path network in the building consists of objects of _Path Node_ and _Path Edge_ classes. Geometries corresponding to _Path Edges_ represent users' movement path fragments. They are modelled in accordance with natural pedestrian movement flow and should have geometries that support the creation of unequivocal navigational directions. _Path Nodes_ model the connectivity of _Path Edges_ and represent nodes in the graph. The principles of modelling the indoor path network and creating geometries to obtain the natural trajectory of the route were studied in [51]. The building topology graph Is modelled with the _Interior Space_ and _Space Connection_ classes. In regular buildings, _Interior Space_ represents rooms, corridors, or other well-defined parts of the building. At metro stations, most _Interior Spaces_ are logical parts of the floor that are not necessarily separated by walls. However, some spaces can also be marked out by visible borders like turnstiles or dissimilar characteristics like differences between platforms turning into corridors. _Interior Spaces_ in metro stations usually do not have names like rooms in regular buildings, but there are other features like ticket zones, which are restricted to ticket holders. Passages between _Interior Spaces_ are represented by _Space Connections_. These can represent logical connections between logical spaces as well as physical connections like doors, gates, or turnstiles. Connections are also located between _Interior Spaces_ on different levels or between exits from stations. Attributes of _Space Connections_ model the rules of traffic flow. Some connections have a limited time of crossing--doors and turnstiles are related to the metro operating times. Figure 1: LIDAR point cloud of the southern part of metro station ’Politechnika’. Figure 2: Class diagram for spatial data model used in the LIFT project (only the key classes and important associations are presented). Numbers define a multiplicity of classes in associations; \({}^{\star}\) is equivalent to ’0.-\({}^{\star}\)’ and means ’zero or more instances’. Others preclude some groups of people with special needs--small steps or turnstiles are a real barrier for people in wheelchairs. Based on the type, if necessary, some costs can be added to avoid situations when the route with the shortest distance crosses the ticket zone. The classes from the topology layer of the model provide information on the location of other spatial objects. Associations with _Path Nodes_ and _Path Edges_ divide path networks by logical sub-spaces. This allows the algorithm to detect change in a room and supports the generation of a navigational direction. Change in a room is the most important direction for a user who expects the system to provide information about events such as passing through a door/gate or moving to another space with different characteristics. The topography layer classes can enrich guidance with spatial information about a room or a door or provide complex information involving descriptions of landmarks or PoIs and spatial relations. Although the system does not implement a hierarchical landmark system, simple information about the presence of landmarks is included in the indoor guidance. The next two classes, _Indoor Installation_ and _Anchor Connection_, are also important. The _Indoor Installation_ class represents connections between levels, such as stairs, elevators, escalators, or ramps. Such objects are modelled with three nodes--the entrance, the middle, and the exit. This type of modelling enables the connection of many floors and maintains the direction of the installation. The _Anchor Connection_ class represents exits from the building.