Street addresses were useless last week when officials in New Orleans needed to direct helicopter crews to rooftops where stranded people waited to be rescued.

Before and after Katrina With help from Carnegie Mellon University software, Google Earth is taking aerial photographs of areas damaged in Hurricane Katrina and turning them into overlays for its standard satellite imagery. The Google Earth service allows emergency workers, as well as home and business owners, to search the database for an address or locale and then view up-to-date imagery to see the extent of damage from the storm.

Source: Google Earth Here, a portion of hard-struck Waveland, Miss., can be seen in satellite imagery before the storm and in an aerial view afterward. Click photos for larger image. More info For more information online: Google Earth: earth.google.com/katrina.html NOAA photos of hurricane areas: ngs.woc.noaa.gov/katrina

But by linking to the Google Earth service, searching for the address and then viewing post-Katrina aerial photos, they were able to provide descriptions of the buildings that were meaningful to the chopper pilots.

By last Friday, Google Earth had amassed more than 7,100 post-Katrina aerial images from the National Oceanic and Atmospheric Administration and used them to create overlays for its geographic database. Aligning those aerial images with Google Earth's satellite imagery and maps was accomplished with special software developed by NASA and Carnegie Mellon University.

As Randy Sargent, the project lead at CMU's West Coast campus, recalls it, Google officials contacted him at midday on Sept. 2. "We actually had the first [post-Katrina] photos out by 6:30 p.m.," he added, though admitting "it's all a blur to me now."

He and his wife, Anne Wright of NASA Ames Research Center, have since worked around the clock overseeing the process.

"Anecdotally, I know that every single government agency that I can name is using this data," said Brian McClendon, director of engineering for Google Earth. And no one knows for sure how many residents and business owners in the hurricane areas have used the site to see if their buildings are still standing -- or, in the case of New Orleans, surrounded by water.

NOAA, which began taking photos using its Cessna Citation aircraft the day after Katrina made landfall, also has the photos available for download from its site. From Sept. 1-7, 5 million photos were downloaded from the site each day -- a total of 27 terabytes of data.

But the NOAA photos are huge and, for dial-up users, can take a long time to download. And the photos are laid out geographically so it's not always easy for users to find the one they need, said Greg Hernandez, a NOAA spokesman.

Hernandez said one of his contacts at the "CBS Evening News" suggested that NOAA get together with Google Earth, in hopes that the photos could be made more accessible by linking them to Google's searchable data base.

"From there, it just mushroomed into this great partnership," Hernandez said.

Hernandez said the system not only allows the images to be searched, but also reduces the amount of data that needs to be sent to each computer user. By sending only the portion of the image that a user needs, the download time is reduced significantly, he explained.

Aligning the photos with Google Earth's satellite imagery and maps would have been straightforward if the photos had included standard geospatial data included in standard aerial surveys, McClendon said. But the post-Katrina photos were taken with fast-moving aircraft and, rather than recording multiple Global Positioning System coordinates for different parts of the aircraft, each photo included only one GPS coordinate, along with information on the aircraft's yaw, pitch and roll.

That's where the NASA/CMU software came in. The software automatically aligns each photo with the satellite image, using both the GPS coordinates and features from the image itself -- shorelines, large buildings, streets or other things that both images have in common.

An early version of this software was used during the Mars Exploration Rover mission to Mars, comparing orbital images with images snapped by each rover as it descended through the Martian atmosphere. The analysis helped NASA scientists figure out where they landed and build three-dimensional models of the terrain.

Carnegie Mellon has been working on an enhancement to Google Earth, to be called Global Connections, which will tie aerial photography and cultural information to elements of the Google Earth map, said Illah Nourbakhsh, associate professor of robotics and the principal investigator for Global Connections.

It took a fair bit of reworking of the original software to accommodate the NOAA photos, said Sargent, who moved from NASA Ames to Carnegie Mellon in June, "but something crude came together pretty quickly."

Though each image doesn't have to be individually placed, he and Wright are continually tweaking the software to improve the alignments. Even a one-degree change in the plane's roll, for instance, can have a significant effect on how the aerial image aligns with the satellite image.

"The images are streaming in," said Nourbakhsh, who recently completed a two-year sabbatical at NASA Ames. "There's no end in sight."