# Research on the Symbolic 3D Route Scene Expression Method Based on the Importance of Objects
## 2 Materials and Methods
### Object Importance Evaluation Model
#### 2.1.2 Spatial Importance Calculation of Objects in 3D Route Scenes  
The importance level is the basis for the selection of objects for the 3D route scenes. It is based on certain indicators that classify entities into different levels and orders them from highest to lowest. The importance rating of an entity object must take into account a variety of indicators such as type, class, density, and spatial distribution characteristics in order to ensure that the evaluation is as comprehensive and scientific as possible. In practice, it is not easy to obtain a satisfactory selection result if only one specific indicator is used. Therefore, several indicators are usually used for comprehensive assessment and combined implementation. Semantic and spatial information can be expressed directly in the 3D scene by visualisation. The semantic information includes administrative levels, building materials, population numbers, etc. It is highly subjective and generally not computable or comparable. Therefore, the method of mapping functions must be used to make it into a numerical type that can be mathematically calculated and compared. The spatial information includes location, shape, size, distribution density, spatial structure, etc. A comprehensive calculation of multiple spatial information indicators can effectively compare the importance levels of similar objects in terms of spatial attributes [46]. In this research, four importance level indicators were considered to calculate the spatial importance of objects, namely, centreline offset distance, volume, spatial distribution density, and distances of key points. These provide an objective basis for the construction of object importance evaluation models in 3D route scenes. 1. Centreline offset distance Distance from the centre point of the object's base to the projection point of the road centreline. The smaller the result, the more important it is. 2. Volume Volume  
Volume is the most intuitive indicator of an object. As a rule, the larger volume is more important than the smaller one. The formula is given in Equation (1). \[f_{\text{size}}=\frac{V}{\frac{1}{n}\times\sum_{t=1}^{n}V_{i}} \tag{1}\]
3. Spatial distribution density  
The totality of a 3D route scene is generally greater than the individual characteristics of the objects. Spatially distributed objects are therefore more representative of the overall situation of a 3D route scene and are more important than densely distributed objects. The average step length T is calculated according to the formula T = L/C, where T is the average step length, L is the length of the road segment where all objects of that type are located, and C is the total number of objects of that type. The density of the spatial distribution can be obtained from the size of the average step size T. As the average step length increases, the spatial distribution widens while the density decreases. 1. Distances of key points (junctions and corners)  
In a 3D route scene, there are key points on the route that can be used as critical factors in the calculation of spatial importance. Road junctions and corners are key spatial feature locations in the construction of 3D scenes, as often the geographic entity objects at road junctions and corners are more important. Road junctions are uncertain due to the number of roads at the intersection and the type of road. Therefore, it is first necessary to determine the importance level of road intersections according to different situations and then calculate the spatial importance of objects according to their distance from the intersection [47]. In order to standardise the importance rating of road junctions, a crossing importance target (CIT) is used. The CIT has a clear and prominent calculability and reasonableness for all types of junction situations. The methodology is as follows:1. If no other road crosses or borders this junction, then \(CIT=1\times CI\) for this junction. 2.