The Utility of Digital Globe's WorldView-2 Satellite Data in Mapping Seagrass in North Carolina Estuaries

Abstract

Submerged aquatic vegetation (SAV) is a valuable natural resource in North Carolina estuaries. The State's Coastal Habitat Protection Plan (CHPP) has stated a need to monitor SAV coverage over time. Thus, the Albemarle-Pamlico National Estuarine Program (APNEP) SAV Partners has a project underway developing a mapping methodology combining remote sensing and boat-based methods to map SAV. As a partner in the APNEP mapping program, this research investigated the utility of satellite remote sensing in the mapping of SAV in NC estuaries. In particular, the data of DigitalGlobe's WorldView-2 (WV-2) satellite launched October 2009 were studied. The WV-2 data are of high spatial resolution (~2x2 m) and 5 visible multi-spectral bands, including a "coastal" band (400-450 nm). One WV-2 image per site was acquired. Three sites were, Jarrett Bay, Blounts Bay, and Sandy Point. Land and deep water (>2 m) pixels were eliminated from each image and subjected to a principal component analysis (PCA), where the first two components were input into the Iterative Self-Organizing Data Analysis Techniques (ISODATA) unsupervised classification. Ground reference points were used to perform an accuracy assessment. At Jarrett Bay, where a continuous SAV bed covered 40%-70% of the study site, results showed an 86.4% classification accuracy in water depths < 0.8 m and 40.9% accuracy in water depths > 0.8m. At Blounts Bay, where SAV coverage was sparse (0%-10%), classification accuracy was 50% in water depths < 0.8 m and remained at 50% in depths > 0.8m. The Sandy Point image was deemed unusable due to extensive sun glint. Most misclassifications were due to dark sediment and the sensor's difficulty in detecting small SAV patches (< 1x1 m). Additionally, according to the environmental conditions present in the images, a water depth threshold where WV-2 can accurately detect SAV was determined at 0.8 m in NC estuaries. With improved water clarity, this 0.8 m threshold would increase. Finally, the unique coastal band was highly susceptible to scattering and absorption due to suspended sediment and colored dissolved organic matter (CDOM) present in the water column of the study area.