Just a few months earlier, Hurricane Dennis dropped about nine inches of rain on Cobb County, damaging more than 400 homes. When Dennis struck, many homeowners were still repairing damage from Hurricane Ivan 10 months earlier.
For these homeowners, and countless others across the nation, flood insurance helped pick up the pieces. Maps from the Federal Emergency Management Agency (FEMA) are the authority when assessing flood risk. Mortgage lenders require homebuyers to purchase flood insurance based on flood risks indicated on FEMA's maps.
Outdated maps become a risk as new development and land erosion alter drainage paths and floodplains, and FEMA's maps are more than 10 years old. Last year, however, the agency embarked on a $1 billion map modernization project, set for completion by fiscal 2008. The new maps will be compiled using GIS technology.
Updating an entire nation's floodplain maps is time-consuming and expensive, so FEMA is requesting help from all levels of government, especially at the county level, which often have very detailed, local maps created by their IT departments.
The United States has about 40,000 licensed surveyors who use FEMA's floodplain maps to help property owners assess the best locations on which to build, with the lowest flood risk.
As the maps became more outdated, surveyors questioned their reliability.
"Surveying in particular is a very precise profession where we measure things very, very closely. So in our community, we're accustomed to doing things very accurately, and it concerns us sometimes when we see data being used for purposes that it may not have been created for," said Curtis Sumner, executive director of the American Congress on Surveying and Mapping.
Maps that aren't meant for surveying might not be up to par with surveyors' standards.
Local GIS maps could provide the solution -- as long as they contain high-quality, advanced topographic data, Sumner said. Then FEMA could build upon them without having to collect the topography data, saving the agency time and money.
Many states and local communities, including Cobb County, are already partnering in the efforts to minimize community flood risks as part of FEMA's National Flood Insurance Program. For those wanting to contribute local GIS maps, FEMA established the Cooperating Technical Partners Program, through which states and communities can develop their own data to help FEMA update their area flood maps.
New Elevation
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.
Orthorectification removes warping -- or distortions -- caused by topography variations and camera angles, which affect the apparent distance of features and objects represented in 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.
FEMA uses topology information to acquire detailed elevation data -- an important factor in determining floodplains. Elevation data shows not only topography, but also how water will flow and drain across the land.
Another important consideration is surface type -- impervious surfaces such as pavement do not absorb water, so water will drain as determined by the surface's slope.
Theoretical models help determine flood risk by studying which conditions would alter water levels and to what degree, said Sumner.
"In other words, the intensity of a storm, the duration of a storm -- all those things that contribute to extraordinary water flow -- would be placed into a model, and that model would determine which areas would be flooded under that particular condition," Sumner said.
This is what FEMA does, he added. It builds scenarios using topographic data, and determines possible outcomes. For this reason, FEMA will be involved in floodplain assessment across the country, even though the topographic base maps are coming from local sources.
Less Room for Error
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.
"Let's say this has the potential to allow FEMA to map at a different scale for this county because we've got some nice elevation data layer developed at a scale that's larger than what they typically develop data in," Scharff said. "As long as whatever else they combine it with isn't at a much smaller scale -- because the strength of any map is only as good as its weakest link -- then they can get larger-scale data that provides more detail."
