# Methodology for the Efficient Progressive Distribution and Visualization of 3D Building Objects
## 1 Introduction  
The number of city models and their importance are increasing. In addition to the traditional commercial or institutional authorities' models, crowd-sourced models are also now available [1]. Three-dimensional (3D) city models as multi-purpose models of spatial environments are used in a growing number of different application domains [2]. For example, 3D navigation maps have become ubiquitous, both in automotive and pedestrian navigation systems, which include 3D city models--in particular, terrain models and 3D building models--in order to enhance visual depiction and simplify recognition of locations. In urban planning, 3D city models provide a means for project communication and better acceptance of development projects through visualization, and therefore avoid monetary loss through project delays. For emergency, risk and disaster management systems, 3D city models provide a conceptual framework. In particular, they are useful for simulating fire, floods, energy applications [3], noise analysis [4; 5], fine dust distribution modeling [6; 7], and explosions [8]. In Geodesign, virtual 3D models of the environment (e.g., landscape models or urban models) facilitate exploration and presentation, as well as analysis and simulation. 3D city models can be helpful in architecture engineering and construction, where modeling the full 3D space, including outdoor and indoor environments, is needed to provide an efficient total solution [9; 10; 11]. The amount of data in city models is often very large. This implies that there is a need for efficient methods to store, manipulate, distribute and visualize these models. The focus of this study isfinding an efficient method for 3D building objects which, in most cases, is the most prominent feature type of most city models. Distributing detailed 3D building objects to mobile devices, such as smart phones, is still limited by bandwidth. To circumvent this problem, we should only send a detailed description of the buildings that are close to the user; for other buildings, generalized versions are sufficient. However, as the user moves through the environment, new representations of the buildings are visualized on the screen, i.e., the user will notice an abrupt change in the scene. A method to avoid these abrupt changes is to use the progressive transfer of data from the server to the mobile device. In the progressive transfer of data, only generalized versions of the buildings are first sent to the mobile device. For objects that then need a more detailed representation, only the additional features of the objects have to be transferred. This also ensures that progressive transfer is efficient in terms the amount of data that has to be transferred. CityGML is an international standard for city models, proposed by the Open Geospatial Consortium [12]. A main focus of CityGML is to represent the semantic aspects of the features in a city. To support this, CityGML contains a comprehensive information model, divided into several sub-models of buildings, tunnels and bridges, city furniture (e.g., lamp poles), vegetation, etc. Furthermore, CityGML supports multiresolution modeling. The coarsest level, Level of Detail 0 (LOD0), is merely a digital surface model, while the most detailed level, LOD4, contains detailed representations of both outdoor and indoor features [13]. Its multi-resolution modeling capabilities enable CityGML to be used for multiple representation databases where representations of the same features in varying levels of detail are stored. However, CityGML is not designed for efficient visualization, while Collada or X3D can be employed for visualization. For 3D buildings which are inputted in different formats, they are firstly processed with their own parsers, respectively. Then the extracted geometry and, if applicable, semantic data about the building is converted into their counterparts in CityGML. In this study, CityGML is used as an intermediate format to integrate 3D city models into different formats, such as Collada, 3 DS and X3D.