Adding Another Dimension to Municipal Management
By Lionel Dorffner and Martin Ludwig, City of Vienna, Austria, and Gerald Forkert, Geodata IT GmbH

Three-dimensional digital map enhances planning, management, and security processes.

In the City of Vienna, Austria, three-dimensional geoinformation is being used for noise protection and city planning tasks and for managing the underground train system.

Ideally, this information should be shared with the city's GIS users. For this reason, the City of Vienna began moving its existing geodata into a database that manages a three-dimensional city model. This data will be regularly updated and made available to ArcGIS users. The surveying department, as the central service agency for all survey-related inquiries, has taken over the task of maintaining the city model.

Left: Building represented by structure lines from the 3D database. Center: The surface model derived. Right: Photo texture applied.

The three-dimensional model of the City of Vienna currently consists of a multipurpose digital city map known as MehrZweckKarte (MZK), a digital terrain model, several hundred thousand three-dimensional buildings, and an underground train network. The digital city map and terrain model are managed in ArcSDE. The three-dimensional buildings and the underground train system are modeled and managed in the CityGRID system from Geodata IT GmbH.

Esri's multipatch data format allows three-dimensional models to be accessed using ArcSDE. Multipatch is a shape type stored in an Esri shapefile that uses a type of geometry to represent the outer surface of features that occupy a discrete area or volume in three-dimensional space.

The city model displayed in ArcScene along with geodata provided by the digital map MZK.

The City of Vienna also uses ArcSDE to manage elements of MZK, the digital terrain model, and the supply line cadastre. This 2.5-dimensional geodata is mainly used in ArcGIS. Presently, approximately 100 workstations are equipped with ArcGIS. The city's geoinformation technicians wanted a three-dimensional solution for buildings and subterranean structures that was compatible with the existing system. The CityGRID system (www.citygrid.at) was the solution. This system was implemented in several stages beginning in 2003. Using this system, geodata is topologically processed and the result saved in a three-dimensional database so that modeled objects can be continuously updated.

Generating the City Model

In the first phase, the aboveground roof scenery of the city model was established by completing the existing geodata using photogrammetric aerial restitution. This task required only two employees from the surveying department. In contrast, the tasks associated with MZK

Noise propagation modeling based existing settings

At the same time, the city's 500,000 buildings represented by simplified flat roof models and 25,000 buildings represented by detailed roof shape models were available. On demand, the roof shape models in a scene can be textured quickly using photographs that were taken on the ground.

In 2005, an ArcGIS interface was developed. Using this interface, various items can be derived including

• Three-dimensional buildings and modeling structures in multipatch format that can be viewed in the ArcScene and ArcGlobe applications in the ArcGIS 3D Analyst extension.
• A two and one-half grid height model representing the terrain and rooftops for use in visibility calculations performed in the ArcGIS 3D Analyst extension.
• Polygon layers for use in the two-dimensional display of building areas and rooftops in ArcMap. Derived attributes, such as building heights, allow for spatial analyses.
• Fa�ades as a line theme with links to the fa�ade textures.

The city model can be used with the two-dimensional geodata in ArcGIS and three-dimensional data in ArcScene and ArcGlobe.

Using the City Model for Noise Protection

A simulation of noise reduction using noise barriers (shown in green)

The aboveground three-dimensional city model is used primarily by noise protection officials and city planners. The entire city model is required for simulating noise propagation over a wide area. The city's noise protection departments are responsible for documenting existing noise pollution in noise maps and simulating the effect of building restrictions and protection measures. Topography and buildings affect noise propagation. A three-dimensional model of the city is therefore essential for calculating noise pollution in a city.

For a typical noise protection department project, using the city model represents a time savings of more than 90 percent as compared with the time needed for preparing the geometric input needed when calculating noise using an expert system.

Visibility analysis for a high-rise project in Vienna. All parts of the city where the four high rises would be visible to pedestrians are shown in red.

When using three-dimensional data as input to noise protection systems, the city model must be provided as a block model in shapefile format. A simplified roof can be included as a prism on top of the building if attribute information on the average height of the eaves and ridge height is available. Information on road areas and areas with foliage is stored in MZK. This is used to identify differences in acoustical absorption for these areas.

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