Three-dimensional city models include Digital Elevation Models (DEM) of ground surface and 3D models (geometry and attributes) of buildings and other objects. Applications such as analysis of sunlight and shadow of buildings in densely populated areas require, in addition to 3D city models, appropriate mathematical methods, usually in addition to other types of information. Such mathematical methods include geo-statistical analysis, time-series analysis and analysis of moving platforms (dynamic analysis). The focus here is on examples of the use of 3D city models, without comprehensive treatment of the mathematical methods used to compute the results.

Wireless Communication
Intervisibility means that one object is visible from the other, and vice versa. Intervisibility is widely used in urban planning, for example for determining the locations of telecommunication antennas and analysis of sunlight and shadow of buildings. To compute whether an antenna can receive signals from a receiver in a certain position the following information has to be known: height of receiver, antenna and objects obstructing the line of sight between antenna and receiver, and elevations of their base at ground level. All this information can be extracted from 3D city models.

Location-based Services
Intervisibility is also important for establishing location-based services in a town. A typical example of LBS would be a base-station broadcasting information picked up by a moving platform, such as a truck. In this case the Electro- Magnetic (EM) signal transmitted by the base-station should reach every street of the town. Buildings and other obstacles will obstruct the propagation of EM signal by reflection, absorption, refraction or penetration. So the establishment of LBS requires analysis of how EM signals interact with objects present in the town. Also, the strength of an EM signal weakens with increasing distance from the base-station, so factors to be used in the analysis include:

• distance between base-station and moving platform
  
• antenna height of base-station and height of moving antenna
  
• heights of buildings and other obstructing objects
  
• distances between buildings in the street and the moving platform.

• source of pollution: in cities mainly industry (usually point pollution), traffic/transportation (usually linear pollution), and living stove (usually plane pollution)
  
• gravity causing polluting dust to nestle easily on objects between atmosphere and ground
  
• height of gas layer beneath which pollution concentrates.

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• Klungboonkrong P. and M. A. P. Taylor 1, 1998, a microcomputer-based system for multi-criteria environmental impacts evaluation of urban road network computation, Environ. and Urban Systems, 22, pp 425-446.
  
• Sivacoumar R., Thanasekaran K., 1999, Line source model for vehicular pollution prediction near roadways and model evaluation through statistical analysis, Environmental Pollution, 104, pp 389-395.
  u Yilmaz Yildirim, Nuhi Demircioglu, Mehmet Kobya , Mahmut Bayramoglu, 2002, A mathematical modeling of sulphur dioxide pollution in Erzurum City. Environmental Pollution, 118, pp 411 –417.
  
• Zhu Qing, Li Deren, Gong Jianya, 2001, The Design and Implementation of Cyber-City GIS (CCGIS), Journal of Wuhan University (in Chinese), 26, pp 8-11.