Residents who see white vans moving slowly through their neighborhoods with photographers snapping pictures shouldn't call the city.

That's because the city sent the vans.

The District's Office of Tax and Revenue is using teams in the vans to complete a new assessment of real estate in the city.

The project began last month and is expected to continue until the end of September, said Lucy Murray, office spokeswoman.

The goal is to get a more efficient and effective accounting of real estate values and to aid in public safety issues, said Tom Branham, acting director of the Office of Tax and Revenue's Real Property Tax Administration.

As part of the project, the city is sending three D.C. Property Data Verification Project vehicles out to 140,000 parcels of residential and commercial land throughout the District.

The vans are labeled and crews all have city-issued identification cards.

Each van will have three people on board: a driver, a professional appraiser and a person who will take a picture of the front door at each parcel. Using a geographic information system in the vehicle, the city will obtain the precise geographical coordinates for each piece of property examined. The contractors will have computers in the van that allow them to check what they see against existing sketches of each building to make note of any discrepancies.

They will also assess the condition of the property, including any garages and pools, and the quality of materials used to make the building.

"The idea is to upgrade our data," Branham said.

Taking photos of homes and other property is not new in the District; the city has pictures going back decades, and the practice is used in other municipalities.

But much of the city's information is dated, and Branham said the District wants to have the most current data available to assist residents in times of trouble and to make accurate assessments of properties.

One problem the city has encountered is that several downtown businesses have multiple addresses because parts of their buildings are on different streets.

Another difficulty is that some apartment buildings and other similar complexes are incorrectly listed in city files.

When the project is complete, the city will have a more precise location for each building when it is sending emergency workers to respond to a call, Branham said.

In the event of a terrorist attack or other disaster, knowing how many floors buildings have and how many occupants they can hold could be useful to city officials in coordinating relief efforts, Branham said.

The District once relied on aerial photos taken at 30,000 feet to look for property alterations not listed in city records, either by mistake or by an owner deliberately failing to report changes.

Now, modern photography allows photos to be taken at 40-degree angles at 17,000 feet. The technique can show new additions, such as pools, that may not have been seen before, Branham said.

"We can do a virtual walk-around," he said.

The current aerial photographs can be seen by visiting the District's Web site, www.dc.gov, and typing in an address in the D.C. Guide section under "Where You Live."