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Dec 11, 2005

FEMA Flood Map Modernization: A Plan For Local GIS Cooperation

The recent busy hurricane season has brought new attention to efforts by the Federal Emergency Management Agency (FEMA) to modernize their flood maps. Using Cobb County, Georgia as a example, an article in Government Technology shows how cooperation among federal and local governments can speed up and improve those efforts.

FEMA will use Cobb County's digital terrain model to update and modernize area Flood Insurance Rate Maps. The county's Information Services department created the countywide GIS base maps with the use of digital orthophotography -- aerial photography that's been rectified so GIS maps can be created from the photos.
The maps are based on 10 million ground control points across the county's surface that each have values relating to horizontal and vertical geographic positioning. The county's topology is built wherein the points are related to each other, creating the contours of the Earth's surface in their proper proportion.
Turning to local governments enables FEMA to utilize larger-scale data with a lower error rate, which improves accuracy when determining flood zones, said Tim Scharff, Cobb County's GIS manager.

GIS map scales relate distance on the map to distance on the ground. The concept is similar to taking a photo: the farther away a photographer is from the subject, the less detail the picture contains, even though more area is captured. A small scale of 1:100,000 -- where one unit on the map represents 100,000 units on the Earth's surface -- would have a larger acceptable error rate than a large scale of 1:1,200, because detail is sacrificed to cover more ground.

"On a scale of 1:100,000, data is going to be tens or hundreds of feet off," said Scharff. "So you can see how quickly that could be the size of a lot [of land] or more." He also said FEMA's maps suffer from small-scale error rates.

"You take a look at the scale developed for FEMA and you begin to question if that should be used for a particular house as opposed to a broader area," said Scharff.

Localized maps are more conducive to large-scale accuracy, something that Scharff said can further FEMA's attempt to produce accurate floodplain maps.