Visualizing the Urban Environment
By William J. Starmer and Jefferey A. Shufelt, Ph.D.

Reconstructing the World

While many communities throughout the world have captured their infrastructure in GIS databases, they are faced with yet another challenge: visualizing the complex 3D urban environment–with its myriad structures and interconnections–out of 2D data layers that uniformly represent objects in the form of abstract colored points or lines. Experts learn to "reconstruct the world" mentally, but this reconstruction is not easily communicated to decision makers.

One solution is to depict the city in three dimensions from its GIS representation. However, important visual and geometric information is necessarily discarded in collecting this GIS data. To construct a realistic representation of the city we must reverse the abstraction process that first produced the GIS, fully leveraging GIS information while reconstructing the 3D world in a principled manner. In this way we can produce a visualization that is faithful to reality and representative of the physical world as we navigate through it.

A varied user community that is made up of city planners, commercial developers, emergency first-responders, utility companies, and A&E firms requires increasing levels of support for interactive presentations and analyses of the urban environment. For the past several years, TerraSim Inc. has been working on applications of its TerraTools® system to provide that support. This article discusses some of the considerations involved in constructing a 3D world.

Figure 1. From AutoCAD models to interactive site visualization

Remote Viewing

At its most basic level, 3D visualization allows us to view a site remotely. When employed to its fullest potential, it can be used as an urban planning tool to view things as they might be, for example, depicting a 40-story skyscraper where a 10-story building now stands, or visualizing relationships between urban structures in ways that are impractical in the real world, such as tracing connections through underground utility and subway systems.

Urban planning is intimately concerned with relationships among buildings, streets and neighborhoods. Three-dimensional visualization makes these relationships explicit, allowing careful consideration of the effects of a change in the urban fabric. Visualizing a proposed development within its urban context provides a precise idea of its impact on the neighborhood and the interplay between adjacent structures. Such visualization also greatly improves the public’s understanding of a proposed development and facilitates communications between all parties involved, including:

• City planning departments
• Housing departments
• Property appraisers
• Civic associations
• Private developers
• Utilities and public works.

Another important aspect of urban planning concerns emergency response, whether for fire or police calls or for disaster response and relief. A 3D simulation of the city can serve as a detailed planning map, a virtual training ground for scenario rehearsal, or as a situation monitoring display should an emergency occur. Effective counter-terrorism operations require coordination among numerous government agencies and private organizations, all of which can be facilitated by an accurate, shared model of the affected site.

Building the Virtual World

Many different types of computer-generated scenes are described as "virtual worlds," leading to confusion over data and software requirements and the quality of the visualization experience. Visualizations can be divided into four classes, grouped here in rough order of complexity and visualization quality.

• Imagery draped over a digital elevation model–Suitable for high-altitude flyovers, but not usable for urban visualization since buildings, trees, and other elevated objects are smeared on the ground instead of being shown in three dimensions.
• 2D/3D GIS features on terrain–Uses the geometry from the GIS data, without textures or additional models, to do a "blocks world" representation of the data. This format can be easily generated but lacks realism for the viewer.
• 2D/3D GIS features integrated into terrain–Uses the geometry from the GIS data, with added models and textures determined by attribution. This is the most practical type of visualization for real-time navigation and interaction, combining GIS data with realistically derived appearances.
• Photorealistic–Typically performed when making feature films and includes lighting and atmospheric effects, water and leaves in motion, and detailed textures. The majority of this modeling is done manually and, due to scene complexity, must have its rendering done offline; as a result, real-time interaction is impossible to achieve.

Until now, constructing a realistic visualization of a complex urban area has been an expensive undertaking due to two main factors: the lack of suitable input data, and the cost of construction due to the limitation of available tools. However, the increasing availability of more detailed data sets, plus major improvements in tools, processing algorithms, and graphics hardware have made the construction of 3D urban visualizations a realistic goal.

The lack of sufficient input data still creates limitations on visualization construction. Both 3D geometry and appearance information are needed but, in some cases, a GIS lacks appearance information and contains only 2D geometry. It follows that an effective toolset for 3D visualization must include a full suite of tools to model 3D objects and features from 2D data, as TerraTools does.

Two-dimensional features such as roads may have no associated elevations; therefore they must be dropped to the DEM surface and integrated into the terrain. With 3D features represented in 2D, there are many different ways of turning the 2D data into three dimensions. A simple extrusion can be sufficient if attributes such as height are available. Geometry or appearance information can often be inferred from the attribution, so long as the construction tool has sufficient capabilities.

Buildings represented by 2D footprints and rooflines must be extruded to form a 3D volume, a simple process if building heights are stored within the GIS data. Otherwise, heights can be inferred from other attributes. For example, if zoning or building-type information is available, residential structures may be assumed to be two stories and commercial structures three stories. Similar procedures can be used to assign building appearances.

For visualization to be realistic at street level it must contain utility poles, street signs, traffic signals, and the typical "street furniture" that populates city streets. Placing models at GIS point locations given for these objects is straightforward, but determining proper model orientation requires sophisticated reasoning about adjacent objects. For instance, streetlights must be oriented so that the light is over the street, and traffic signs must face the flow of traffic. In other cases, objects not normally included in a GIS–newspaper boxes or trash cans–may be placed at their typical locations to provide a more realistic street appearance. Three-dimensional geometry can also be generated based upon standard geometric templates. For example, curbs and sidewalks can be created in TerraTools with specified widths, heights and materials.

In addition to modeling considerations, an urban visualization tool must often handle a heterogeneous mix of data types. In some cases, standard cartographic or DEM data from the U.S. Geological Survey may be required to provide the context around a smaller area of interest for which GIS or CAD data is available. CAD data for 2D planimetrics or 3D models, most often in AutoCAD or MicroStation format, may be available for specific sites or structures of interest, especially in planning applications. Different types of CAD data require different kinds of processing for real-time visualization.

• 2D CAD data is typically represented as simple linear and point features. While this representation is appropriate for printing maps and diagrams, a lack of topology and polygonal features make them less ideal for the automated generation of 3D visualization. However, TerraTools possesses both automated and manual tools to address these deficiencies in order to use them for visualization.
• 3D CAD models tend to be highly detailed, typically containing too much detail for real-time inspection. There are often additional problems inherent in the CAD representation such as disconnected polygons, a lack of lighting normality, and an absence of appropriate textures that render them unsuitable for real-time visualization. While these 3D CAD models can be used directly within TerraTools, they are typically processed through a manual 3D modeling software package before being placed in the 3D scene, thereby optimizing and improving their appearance for real-time 3D rendering.

Limitations on urban visualizations include the fact that they do not allow the viewer to step off the street directly into a building. The CAD data necessary to support construction of interior floors, stairways, doors, and windows is seldom available and, even when it is, rarely is suitable for use in real-time visualization because of its high level of detail and lack of topology. TerraTools allows for the automatic generation of plausible building interiors given the building type, exterior building shape, and general parameters that describe the desired interior.

Much of a city’s complexity is hidden below ground, with utilities, drainage, and transportation forming a complex, invisible 3D web. Including this underground infrastructure in an urban visualization allows it to be seen and understood in relation to aboveground structures, without excavation. The ability to inspect and manage underground assets with an interactive 3D visualization is a cost-effective alternative to excavation for water departments, electric companies, metropolitan transportation authorities, and telephone and cable providers.

Traveling the Virtual World

The viewer’s sense of reality in a virtual world is determined both by the realism of the scene and one’s interactivity with it. The number of included polygons is one determining factor for scene realism, with more polygons implying a more realistic rendering. However, realistic interactivity requires that the scene have as few polygons as possible for graphics hardware to maintain realistic rates of motion throughout the scene.

Therefore, every construction step must be taken with its effect on viewing in mind–the number of polygons, the amount of texture memory required and disk I/O, to name but a few. These numbers change as graphics hardware becomes more powerful, but the problem remains the same. Increased graphics capability can be used either to render an unoptimized visualization, produced with less effort, or to render a more realistic or detailed visualization that has been optimized for display. Given this choice, it has been found that users of 3D visualization technology generally prefer to construct virtual environments that are as faithful to reality as possible, implying that highly optimized construction is a critical component of any 3D visualization toolset.

The greatest consideration is interactive versus pre-planned viewing. Are viewers able to fly or drive freely through the scene, or are they restricted to viewing non-interactive videos produced offline? Offline rendering can produce highly realistic views because the polygon count can be high. But these renderings are not interactive, thereby eliminating the possibility of seeing the scene from another viewpoint, or traversing another path. In addition, offline renderings may take hours or even days to complete.

Smarter database design circumvents the polygon count dilemma. More efficient representations, such as triangulated irregular networks (TIN), maintain the appearance of the terrain without adding unnecessary polygons. The fact remains that less detail is visible on an object as one moves away from it, which is also advantageous. By including levels of detail (LODs) in the database and support for them in the associated viewer, less-detailed versions of models can be automatically substituted as the user moves farther away in the virtual world, providing the illusion of detail without sacrificing performance. TerraTools uses these methods to manage dense urban environments while maintaining real-time viewing rates.

Beyond Visualization

While looking at and walking through a virtual world is useful in many applications, underlying GIS data and associated databases often contain additional information which would be much more valuable if it were accessible through 3D visualization. Imagine being able to click on a building in a visualization to obtain construction history, a fire marshal’s inspection report, tax appraisal records, or a directory of occupants. The TerraSim GISLink product delivers this level of capability.

GISLink™ technology ties together the powerful information access capabilities of a GIS database and the intuitive interaction of 3D visualization. In conjunction with the ITspatial VIO-GIS™ viewer, GISLink enables users to seamlessly interact with both a standard GIS (ArcView) and a 3D visualization of the same scene. When features are selected in the 2D window the 3D view flies to that point, and features are highlighted in a flashing box. Alternatively, objects selected in the 3D window are highlighted in the 2D view. Object selection can also be accomplished by using standard GIS queries, since VIO-GIS is built on an ArcView platform. This enables ArcView users to immediately exploit 3D visualization tools without suffering a steep learning curve or undergoing costly training on an unfamiliar toolset.

For example, a user could drive down a virtual street and select a building in the visualization to query its tax assessment information. Conversely, he or she could write a GIS query to select every utility pole with a certain type of transformer, and then fly to the location of each one to examine surrounding buildings or terrain.

Visualizing the Future

Three-dimensional urban visualization is now at the same relative point in its development that GIS technology was several years ago. Thanks to graphics card developments fueled by the video game market, sufficiently powerful PC graphics hardware is already widely and inexpensively available. As capable software tools such as TerraTools become available and more powerful, the user community will grow more rapidly. City planners can interactively judge the impact of proposed developments, commercial developers can communicate site plans dynamically to interested constituencies to facilitate project acceptance, utility companies can manage existing underground infrastructure, and multi-agency crisis response teams can rapidly form and execute plans in a high-fidelity, 3D geospatial visualization. It is indeed an exciting future for 3D visualization, and TerraTools will remain at the forefront of that ongoing development.u

About the Author:
William J. Starmer is a geographic technologies specialist at TerraSim Inc., where he is in charge of the Database Construction Group and also provides customer training and support. He has more than eight years experience in GIS and remote sensing and is currently completing his master’s degree in geography at the University of California (Santa Barbara).

Jefferey A. Shufelt is director of corporate research and a co-founder of TerraSim Inc. Dr. Shufelt supervises the research and development of advanced visualization tools at the company. He has more than thirteen years experience in cartographic feature detection and extraction and received his Ph.D. in computer science from Carnegie Mellon University.

The authors may be contacted via e-mail at [email protected].

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