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Microsoft released a new 3-D version of its popular Virtual Earth Web service today.

Virtual Earth representatives said they expect their existing government customers, including the Defense Department, to enhance current Virtual Earth mapping applications with the new 3-D features.

Right now, 3-D models are available for 15 U.S. cities: San Francisco; San Jose, Calif.; Seattle; Boston; Philadelphia; Los Angeles; Las Vegas; Detroit; Phoenix; Houston; Baltimore; Atlanta; Denver; Dallas; and Fort Worth, Texas. Users can virtually fly above, around and down into realistic, computer-generated simulations of skyscrapers, stadiums, dams and other city structures.

The consumer version is free and accessible from any Web browser. Microsoft officials said 3-D Virtual Earth does not require a large amount of storage space to download, unlike the rival Google Earth Web service, which needs at least 400M of free space, according to Google’s system requirements. Virtual Earth users need to install a 5M plug-in to view the 3-D images, Microsoft officials said.

But Microsoft’s system requirements say the 3-D version of Virtual Earth needs 250M or more of hard disk space.

Microsoft officials did not have a response to the apparent discrepancy in the system requirements.

3-D Virtual Earth is a byproduct of Microsoft's May acquisition of Vexcel, a company that specialized in remote sensing and aerial mapping.

"The 3-D models are accurate to within as good as a meter in all directions," said William Gail, a former Vexcel vice president who is now strategic development director in the Virtual Earth Business Unit at Microsoft.

Imagination is the only boundary for government customers, other Microsoft officials said. Users cannot, however, fly through buildings and hills. Developers omitted such maneuvers because they are not realistic.

Government employees can access the tool from a Web browser on any computer.

Virtual Earth representatives listed several possible government applications for the service. The Homeland Security Department could simulate the impact of a chemical dispersion. DOD officials could train recruits with 3-D movies of troops fighting in urban areas.