June 21, 2004
Satellite Imagery for Impervious Surface Mapping
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Susan Smith - Managing Editor


by Susan Smith - Managing Editor
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Message from the Editor -


Welcome to GISWeekly!Recently, Denver's Wastewater Management Division (WMD) entered into a study with DigitalGlobe of their QuickBird imagery and impervious surface map products in order to streamline WMD's storm water billing operation.


Jeff Blossom, GIS photogrammetry administrator, Wastewater Management Division, explained that stormwater runoff transports any chemicals, wastes, or other pollutants that exist on surfaces such as parking lots, roadways, etc. into the rivers, lakes, reservoirs they drain into, thus polluting these water bodies. Also, stagnant water that does not drain properly can act as breeding grounds for disease, specifically West Nile Virus resulting from mosquitoes.
“We rely on updated imagery to perform our impervious surface mapping, which updates our storm billing database.” Read about it in this week's Industry News.


From June 21-July 1, I will be on vacation in Colorado. GISWeekly will continue to appear in your email mailbox during that time, bringing you timely industry news and topics of interest. Look for these headline stories: Interactive Decision Support for Communities and
Review: Cartographica Extraordinaire--The Historical Map Transformed in the coming weeks. During that time, please refer to
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Susan Smith, Managing Editor




Industry News

Satellite Imagery for Impervious Surface Mapping

By Susan Smith


A technical presentation given at the American Society for Photogrammetry and Remote Sensing (ASPRS) Conference May 26-28 in Denver was about a creative use for satellite imagery in the management of a wastewater division's stormwater billing procedures.


Recently, Denver's Wastewater Management Division (WMD) entered into a pilot project with DigitalGlobe of their QuickBird imagery and impervious surface map products in order to streamline WMD's storm water billing operation. In the study, two-foot resolution multispectral QuickBird satellite imagery processed using superior classification techniques produced an impervious surface map that resulted in significant revenue generation over what could be produced using traditional mapping methods.
Jeff Blossom, GIS photogrammetry administrator, Wastewater Management Division, said, “We rely on updated imagery to perform our impervious surface mapping, which updates our storm billing database. Since commercial imagery satellites pass over Denver roughly every four days, and imagery can be delivered in 2-3 weeks, this presents an excellent opportunity to acquire updated imagery on a frequent basis."


According to DigitalGlobe director of civil government applications, Jeff Liedtke, there has been much interest in the impervious surface layer from counties and municipalities. The pilot project with WMD served as a way to verify the utility and accuracy of the product. The impervious surface layer has also recently been delivered to the City of Wheatridge, Colorado.


WMD's customer service group bills all property owners in the City and County of Denver for sanitary sewer use and storm water drainage collection. According to Blossom, stormwater runoff transports any chemicals, wastes, or other pollutants that exist on surfaces such as parking lots, roadways, etc. into the rivers, lakes, reservoirs they drain into, thus polluting these water bodies. Also, stagnant water that does not drain properly can act as breeding grounds for disease, specifically West Nile Virus resulting from mosquitoes.


According to the press release, the storm water drainage bill is calculated by mapping the impervious areas - or surfaces that water does not penetrate, such as concrete and asphalt - contained within a parcel. “We produce the impervious surface map and then we overlay on top of that the customer's parcel layer, that's the customized part. We then derive the amount of impervious surface per parcel, customized for each project,” explained Liedtke.


Once the amount of impervious area contained within a parcel is determined, that property's billing rate is calculated. While many cities that bill for storm water management do so by charging property owners based on average parcel size and landuse, Denver's WMD bills property owners based on actual property size and total impervious surface area contained within the property. Essentially, property owners are billed according to their contribution to storm water runoff.


In May, 2003, WMD initiated a pilot project to determine the value of using high-resolution multispectral (blue, green, red, near infrared) satellite imagery for mapping impervious surfaces in five Denver neighborhoods, including three residential, one commercial, and one industrial neighborhood. DigitalGlobe donated QuickBird imagery collected in April 2003 for the project.


WMD was happy to discover that the QuickBird satellite imagery offered significant benefits in terms of cost and delivery time, allowing for quicker mapping, assessment, billing, and revenue generation at a lower cost to division.


The original impervious surface map was created using field techniques (planimetric wheel and tape measure), and digitizing from a light projection system using aerial photo negatives, Blossom said.


Blossom added that outdated images are their biggest limitation to the efficient mapping of impervious surfaces and maintaining their stormwater billing utility databases. The integration of QuickBird imagery into the city of Denver's GIS streamlines the stormwater billing process and keeps databases current. The method also provides a more effective and precise method of identifying properties where changes of impervious areas have occurred, and flags the area. Identifying changed properties (those where a customer may have replaced concrete with grass, for example) has up until the present been done with a visual comparison of existing impervious areas and recently collected images. Now,
using an automated algorithm from DigitalGlobe changed properties throughout the city can be identified within minutes.


WMD's customized approach to the new DigitalGlobe process is as follows: 1) WMD requests a time period for DigitalGlobe to acquire imagery. 2)Imagery is acquired, and WMD supplies DigitalGlobe with base map and DEM information. 3) DigitalGlobe automatically collects impervious from the multi-spectral characteristics of the imagery. 4) DigitalGlobe delivers the impervious map and imagery to WMD.


The two-foot resolution QuickBird data costs roughly one-tenth the amount of six-inch resolution aerial orthophotos, noted Blossom. DigitalGlobe delivered the imagery within two weeks, obviously faster than the four to six months typically required for traditional aerial photography.


WMD has used aerial orthophotos for Denver's impervious surface mapping since 1988. the aerial data is of high resolution and WMD will continue to use it, but Blossom said they plan to add updated QuickBird imagery each year now. They will use the existing aerials to refine data collected with the satellite images.


According to Liedtke, a sophisticated county [meaning one with a GIS department] could manage the impervious surface layer themselves. “A less sophisticated county or municipality could use us and our partner, Carter-Burgess, which actually converts the amount of impervious surface per parcel and then they develop a fee schedule. Then the individual parcels are assessed that fee. We would give them the impervious surface map and they could overlay their own parcels, or we could give them their parcel layer with two more fields in their database and in those fields would be the amount of impervious surface per parcel in square feet and the percentage of imperviousness on their parcel.
The largest percentage of impervious surface on your parcel, the higher the fee.”


The current solution could benefit many other city departments that rely on updated feature mapping, according to Blossom. Perhaps a collaborative, multi-agency acquisition of QuickBird imagery could prove to be more cost effective in the long run.


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