Applications of Impervious Surface Data to Stormwater Management

Communities subject to the NPDES Phase II Stormwater Program can benefit from accurate geographic data delineating their impervious surfaces as they develop stormwater management plans.

• Many communities are considering stormwater billing fees on commercial and residential properties to fund costs associated with meeting the mandates of the NPDES Phase II Stormwater Program. Developing an equitable and defensible billing system requires accurate geographic data on the area of impervious surfaces by ownership parcel.
• The development and implementation of best management practices (BMPs) may be required to mitigate the impacts of stormwater runoff. Knowing the location and extent of impervious surfaces within the jurisdiction can help target the location of BMPs.
• To effectively plan for growth, elected officials and policy makers need to be well informed. To provide the best advice to these officials, planners and resource managers need accurate and up-to-date information on impervious surfaces to conduct trends analysis, tracking the change in the extent of impervious surfaces over time.

NASA/CGIA Research Project

CGIA recently completed a four-year NASA funded project to evaluate methods for mapping impervious surfaces. The research goal was to use remotely sensed data and image processing technology to support various needs of local and state government agency partners in North Carolina. Through this project, CGIA developed a semi-automated feature extraction method for mapping the location and geographic extent of impervious surfaces.

The CGIA NASA Project final report (text only) can be downloaded here [PDF file; 307 KB]. The supporting report figures, visual images, and captions, that demonstrates CGIA's Geospatial Imaging capabilities, can be downloaded here [PDF file; 16.4 MB].

Traditional versus New Methods

The traditional method of generating impervious surface data involves manually digitizing large scale aerial photography. This method is labor intensive and therefore costly. Based on figures provided by CGIA’s NASA grant partners and other local governments in NC that have used the manually digitizing method, the average rate for manually digitizing an impervious surface layer is 12 acres an hour or 53.3 hours per square mile. Using the semi-automated feature extraction method developed by CGIA during the NASA grant, the average rate for extracting impervious surface data was 7.2 hours per square mile.

The result is significant cost savings. In most cases the cost can be as much as six to eight times less expensive than manually digitizing impervious surfaces. The other advantage is that for most communities there are no data acquisition costs. The source imagery is high resolution, true color aerial imagery, which is regularly acquired by most counties in North Carolina.

Updates Are Less Costly

Another advantage to the process is that once an initial impervious surface dataset has been extracted from aerial imagery, there are significant savings in cost and time to update the dataset when new imagery is flown. The original impervious surface dataset can be used to "mask out" areas previously confirmed as impervious surfaces. The feature extraction method can then be applied to the remaining areas in the new imagery to extract areas that have been developed and paved since the original imagery was flown.

Contact CGIA

This capacity to extract impervious surfaces form high resolution aerial imagery is now available to local and state governments on a cost-recovery basis. Municipalities and counties subject to the Phase I and Phase II Stormwater Rules are encouraged to contact CGIA for more information:

Tom Tribble, Business Development Manager, tom.tribble@its.nc.gov (ph: 828-296-4600)
Frank Obusek, Remote Sensing Program Manager, frank.obusek@its.nc.gov (ph: 828-296-4605)
Joe Sewash, Services Program Manager, joe.sewash@its.nc.gov (ph: 919-754-6590)