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Impervious Surface Mapping in Coastal New Hampshire, 2005
Lead Agency: UNH Complex Systems Research Center
Primary Contact: David Justice
Sponsor(s): NH Estuaries Project
Start Date - End Date 09/01/2005 - 04/30/2006
Related Project(s): Impervious Surface Mapping in Southern Maine Communities, Impervious Surface Mapping in the Coastal Watershed of NH and Maine, 2010
Description: Estimates of impervious surface acreage in 2005 were generated and compared to prior estimates for 1990 and 2000 for a 48-town region in coastal New Hampshire, including the 42 towns within Zones A and B of the New Hampshire Estuaries Project (NHEP) area. The estimates were based on applying both traditional and subpixel image classification techniques to 30-meter Landsat 5 Thematic Mapper (TM) satellite data, acquired 3 October 2005. The classifications indicated that impervious surface acreage increased from 4.3% (31,233 acres) in 1990, to 6.3% (45,445 acres) in 2000, to 7.4% (53,408 acres) in 2005. At the subwatershed level, the Portsmouth Harbor subwatershed recorded the highest percentage of impervious surface acreage in 1990 with 19.8% coverage (2,310 acres) and in 2000 with 25.5% coverage (2,975 acres), and this finding continued in 2005 with 28.9% (3,364 acres) of the watershed mapped as impervious.
An accuracy assessment was applied to the regional data, and indicated an accuracy of 98.3% for the 2005 data, which compared favorably with the assessment of the 1990 effort (98.6% correct) as well as the 2000 data (93.1% correct). These figures reflect the overall presence/absence of impervious surfaces within the randomly selected pixels. The accuracy was further evaluated against April, 2003 Emerge 1-ft. resolution aerial photography to estimate the validity of the predicted range of imperviousness for a second set of randomly selected pixels. This assessment proved disappointing, as only 7% of the pixels sampled predicted the correct impervious percentage range.
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