County manages floodplains with updated photos

Until recently, Maricopa County, Ariz., used semirectified aerial photography in a GIS to help manage floodplains. However, the photographs lacked positional accuracy, and the county’s flood insurance maps overlaid poorly on them. Floodplain managers were reluctant to use the photographs internally and in public forums because of discrepancies. The county decided it needed to upgrade its GIS to include orthophotographs — map-accurate aerial photographs — of 7,931 square miles.

In late October 2000, its decision was validated when unexpected heavy rain fell in Wenden, a town of 700 residents in the northwest part of the county. The town is in a region that barely averages six inches of rain a year, but remnants of Hurricane Nora sent a wall of water through the many farming communities in the area. The county needed to compile flood data quickly to qualify for a grant from the Federal Emergency Management Agency.

“We usually conduct the annual aerial imaging project in December and January and share the information with the county tax assessor,” says GIS Manager Marta Dent. “Suddenly we needed the data turned around in less than half the time we normally require.”

That November, the county contracted with Irvine, Calif.-based Landata Airborne Systems to create black and white digital orthophotography at one-foot pixel resolution. For the purpose of flood analysis, the firm also provided 3-D digital images of the entire hydrographic network in the county.

The firm used a high-accuracy control network of points known as the GDACS (geodetic densification cadastral survey) points to spatially locate the photographs, providing a common background for data shared by flood analysis, tax assessment, transportation, planning and disaster preparedness agencies. County surveying crews verified that the photographs matched the GDACS points well within plus or minus five feet.

The county has used the one-foot pixel resolution photographs to determine flood hazards for properties, to determine where the road network needs improvements and to assess property taxes. It also is using the photographs to verify the accuracy of its parcel maps, which are being converted into digital format.

Using orthophotography has transformed the county’s approach to floodplain management. Now, the county can conduct studies using the newly generated terrain models and the photography, without the need for conducting additional expensive fieldwork. In addition, the improved image quality and accuracy has allowed the Flood Control District to significantly enhance its public information effort in floodplain management.

For example, the district recently created a floodplain management plan in a developing area where there previously had been none. With the new orthophotography, the GIS staff quickly assembled photo-based maps showing the floodplain plan and published them on the district’s Web page.

This article was written by Tom LaMarche, IT division manager for the Maricopa County Flood Control District.