How Many 3D City Models Are There? - A Typological Try

Three-dimensional (3D) city models are intended to represent the geometry along with the appearance of the reality. In some cases, photos are precisely mapped as texture onto the geometric model, which then renders a realistic impression of the underlying environment. In other cases, the models are visualized in a more abstract way. This paper is dedicated to investigating the relation of level of abstraction, information density and storage capacity for the representation of a 3D city model. Increasing the level of abstraction usually leads to the decrease of information density to be processed. Accordingly, the general storage capacity will decrease as well. Since a higher level of abstraction is less bound to photorealistic aspects, it may allow further semantic information to be accommodated in the visualisation process. If the level of abstraction in a city model increases in a well-controlled way so that the characteristic features are enhanced while non-relevant information is inhibited, the user's cognitive load during task completion may be reduced. This implies an increased efficiency of information communication. Using image segmentation algorithms, the elements within a facade per unit of area are successively reduced while preserving the characteristic structures within the image. The segmented image holds the sufficient information to be recognized in the environment. We treat the three concepts, level of abstraction, information density and storage capacity, as fundamental variables for the visualisation of building facades. To deal with these concepts, we organizing them on a continuum, which enables us to find a tradeoff between a potential loss of information and the level of usage for various tasks.