view of some properties. To account for obstructions and changes that may occur between aerial photo shoots, Blossom said the WMD systematically selects and investigates properties to verify the imagery's correctness.

"Anytime houses are built and then occupied, we map those properties," he said. "Anytime a parcel is split or two parcels are combined, we remap the impervious to verify that we have the correct amounts. Anytime property is bought or sold in the city, we do an inspection to make sure the storm bill is correct for the seller going to the buyer. Anytime customers call in and want to verify their bill, we map those."

Additionally the WMD conducts routine investigations where there has been a 40 percent to 50 percent change from previously collected imagery, Blossom said. On their computers, investigators search the overlaid images of their assigned areas and find changed properties by eye.

"That's a time-consuming process," Blossom said.

Using DigitalGlobe's satellite imagery and algorithm, properties can be flagged automatically.

"It's not up to the human eye to detect it and estimate 40 to 50 percent," Blossom said. "You can get exact numbers. I can identify all properties that have a 50 percent difference, all properties that have a 29.5 percent difference or whatever I specify in the queries I want to run."

Blossom said having more current data will also help the WMD give better customer service by providing a more accurate bill.

"A lot of people are being billed, and they've made changes to their land and their bill is incorrect," he said.

DigitalGlobe's Liedtke noted that more revenue could be realized by obtaining data more often than every two years. "There's a lot of growth in two years, and that's unrecognized revenue for the city and county of Denver if they don't assess those fees," he said.

Currently investigators are often delayed in getting to new construction sites. "It might be months before we get around to mapping those properties and adding them into our billing database," said Blossom. Streamlining the process could help the city keep up with Denver's rapid growth.

"The time-savings amounts to a more productive environment and a higher rate of revenue generation for wastewater," he said.

Blossom said engineers who plan storm water infrastructure could also gain from the maps. "Having a complete impervious map for the entire city gives them a great data set, and they can really refine and improve their models so they can be much more accurate," said Blossom. "Then they will know exactly what pipe size they need instead of an estimate now."

Rising Costs

The impervious surface product, which Liedtke said would go for a standard price of $300 per square mile, has not yet been sold to any jurisdiction, but he said the NPDES -- a permitting process that regulates water pollution and is overseen by the U.S. Environmental Protection Agency -- will drive demand for the technology.

"It's an unfunded mandate," Liedtke said. "So one way of generating the funds to comply with this mandate is to develop user fees."

NPDES regulations, which came out of the Clean Water Act, are being implemented in two phases. Phase I, which is already complete, required municipalities with populations of more than 100,000 to implement best management practices to minimize storm water pollution. Phase II, which started in 2003, requires the remaining municipalities -- with a few exceptions -- to implement the same practices.

Wheat Ridge, Colo., which is considering implementing a storm water billing utility, may be the first to purchase DigitalGlobe's technology for mapping impervious surfaces. Many Colorado communities are facing Phase II requirements, according to Jon Reynolds, project supervisor in Wheat Ridge.

"Almost one or two dozen in this Colorado area

Emily Montandon  |  Staff Writer/Copy Editor