September 2, 2004 By Emily Montandon
To cover the costs associated with the NPDES -- which requires that municipalities implement management practices to mitigate pollution in urban runoff -- many cities started billing residents based on the amount of impervious surface area on their property, which affects their contribution to runoff since water can't soak into the ground.
The city and county of Denver is way ahead of the game.
Denver is hit with intense rainstorms in the summer, causing surges in runoff into the storm water system, said Jeff Blossom, GIS photogrammetry administrator for the Denver Wastewater Management Division (WMD). He said ongoing construction and maintenance of the storm water system are required to prevent flooding and protect public health.
In 1980, Denver passed an ordinance allowing the WMD to bill residents to cover the costs of building and maintaining infrastructure that controls the flow of rain and melting snow, which is disrupted when land is developed.
Though storm water billing brings in needed funds, Blossom said billing by impervious surfaces is often laborious.
"If you're going to map every property down to every 10 square feet -- every sidewalk, every patio, etc. -- you have to have detailed mapping, and that takes quite a few people," he said. "But that's what they committed to in 1980, and that's what we've been committed to since."
Recently the WMD worked with DigitalGlobe on a pilot that Blossom estimates would help the division generate four times the revenue per hour. The DigitalGlobe technology used multispectral satellite data -- red, blue, green and near infrared -- to map impervious surface areas in five Denver neighborhoods. The red, green and blue combine to form an image visible to human eyes, said Blossom.
"The near infrared measures the vegetative content -- or lack of it -- and it's highly sensitive, so it can discriminate between a gravel parking lot versus an asphalt parking lot," he explained.
The technology distinguishes impervious surfaces with 95 percent accuracy, according to Jeff Liedtke, DigitalGlobe's director of Civil Government Applications. He said there are two components to mapping the impervious surfaces, one of which is using the satellite imagery.
"The other is running it through an algorithm that classifies the image into impervious and pervious areas, and using our proprietary algorithm and special techniques to refine that," he said.
Liedtke said the Denver pilot helped DigitalGlobe assess the product's usefulness in a real-life setting.
"We had of lot of ideas on how this technology could be used, but we needed to deploy it in a real-life situation," he said. "We were very interested in assessing both the utility of the information, the accuracy of the information and how it would be used in day-to-day operations."
Denver's WMD will continue purchasing aerial photography -- which according to Blossom, costs $150,000 to $200,000 -- every two years because of its higher resolution, but Blossom said he hopes to purchase the satellite data in the interim years so the WMD has current data at least every year.
The WMD currently has aerial photos taken of the city/county of Denver biennially and digitizes them to map impervious surfaces for billing.
"That's mostly due to budget purposes since it's pretty expensive to acquire aerial photos for the whole city," Blossom said.
The aerial photos are taken when obstructions are least likely, but trees and other impediments block the camera's
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