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What is a 3D City Model?
A 3D city model is a three-dimensional representation (3D) of a city or an urban environment. A common understanding is that every city model consists of a digital elevation model (ground height) and 3D building data (building heights). Usually a 3D city model represents an existing city but in some applications, especially in gaming and entertainment, it may have no counterpart in the real world.
Due to the rapid development of computer hardware, and progress in (semi-) automatic data acquisition, it is now possible to create 3D city models at a reasonable cost. This development has led to the broad application of three-dimensional spatial information in a variety of fields including urban planning, telecommunications, ecology, tourism and entertainment.
In Germany, the mobile phone network was one driving force behind the collection of comprehensive 3D city models in metropolitan areas. With the aim of achieving a maximum net coverage for their mobile phone network, 3D-city models are used for simulation purposes so that effects such as dead-spots and reflections of electromagnetic waves by buildings can be taken into account (Ragia and Laing 2000).
A study by the Organisation Europeene d’Etudes Photogrammetriques Experimentales highlighted the demand for accurate 3D city models (Fuchs et al. 1998). 95% of the participants identified three-dimensional building data as the most interesting feature in digital city models, followed by information about traffic systems (85%) and 3D information about vegetation (75%).
Methods of creating 3D city models
Several methods are currently used to create 3D city models. The simplest is to extrude building footprints from 2D maps to a given height based on building attributes such as the number of storeys. A 3D terrain model is usually added to provide the landscape context for the buildings. Most GIS systems support this method but the 3D city model that is generated does not contain detailed information, such as facade geometry or textures, and the height of the buildings may not be very accurate. To overcome the problem of inaccurate building height data, photogrammetry or laserscanning methods have been developed to capture 3D city models from aerial images (Stuttgart, Darmstadt) or airborne laserscan images (LiDAR).
Using photogrammetry, a 3D city model is produced from stereo aerial imagery in a semi-automatic way using special software tools. The 3D data can be exported into several commercial 3D formats for visualisation. Laserscanning data provides an accurate height model with semi-automatic extraction of buildings. Current research is focusing on extending both methods to capture facade geometry and building material texture.
The VEPs project is using city models in two open standards: VRML (Virtual Markup Language or X3D) and CityGML (City Geographic Markup Language).
References
Ragia, L. And R. Laing (2000): Qualitätsprüfung von Gebäudedaten aus photogrammetrischen Auswertungen, PFG -Photogrammetrische Fernerkundung Geoinformation 3/2000, Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, 2000, pp 212-220.
Fuchs, C., E. Gülch and W. Förstner (1998): OEEPE Survey on 3D-City Models, OEEPE publication, No 35, Bundesamt für Kartographie und Geodäsie, Frankfurt, 1998.