Generalization of buildings within the framework of CityGML

Abstract

For a better visual impression, 3D information systems and architecture need detailed, photo-realistic visualization of 3D data-sets. However, easy accessibility with efficient rendering becomes difficult due to the detailed data associated with 3D objects. Therefore, different applications demand different levels of detail (LoD). Currently, City Geography Markup Language (CityGML), as the Open Geospatial Consortium standard, is being used to model and represent buildings in different LoDs (LoD0-LoD4), but it does not provide methods to generate different LoDs automatically. Thus, generalized (abstracted) 3D scenes of buildings need to be generated to fulfill the demands of task-specific applications by reducing data volume. This paper discusses various ways to generalize building models, within the framework of CityGML, reducing the level of detail from higher LoD to lower. The LoD4 data is parsed and analyzed. Various heuristics are applied to simplify the ground plan and the results are then aggregated. The minimum length of an edge for simplification is restricted to the CityGML generalization specifications provided and is characterized by differing accuracies and minimal dimensions of objects for LoD1 and LoD2. This could maintain the accuracy of generalized objects and avoid the elimination or merging of important features. Second, the heights of the walls of the simplified ground plans are raised with the aim to construct simplified 3D building models. Algorithms for simplification and aggregation aiming to derive LoD2 and LoD1 are implemented and tested on a number of buildings of Putrajaya, Malaysia. The experiment results show that the minimum length of edges to be simplified is inversely proportional to the size of generalized models.Keywords: CityGML; 3D; simplification; aggregation; city models IntroductionThe demand for utilization of 3D city models is increasing as 3D geospatial data are commonly available. Cities are expanding due to population growth and require 3D models for planning purposes. Additionally, specific applications such as the simulation of air pollution, noise, and 3D navigation systems need 3D city models (1). Most 3D city models are used for visualization purposes. Digital City, Earth Viewer, and LandXplorer are some commercially available popular 3D visualization tools that operate based on X3D and KML as 3D visualization standards for online visualization of 3D city models. These standards are not useful to represent 3D city models for analysis but are only useful for visualization, as their standards do not support semantic information attached to city models. Alternatively, Open Geospatial Consortium, Inc. (OGC) have adopted version 2.0.0 of City Geography Markup Language (CityGML) as an official OGC Standard, which defines the shape and photo-realistic appearance of 3D building objects and additional boundary surfaces for the semantic classification of the outer shell of buildings and building parts (OuterCeilingSurface and OuterFloorSurface) (2). It also defines thematic...

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