Comparison of radiometric quantities measured in water, above water and derived from seaWiFS imagery in the South Atlantic Bight, North Carolina, USA

This paper reports on an ongoing study to better understand the bio-optical properties of a portion of the South Atlantic Bight-the Cape Fear River (CFR) plume area and Onslow Bay (OB). Sampling mid-Onslow Bay provides a contrasting coastal system relatively un-impacted by the high dissolved organic matter-waters of the CFR. Data were obtained during regular research-cruise observations, from October 2001 to September 2003, using two different measurement systems: above-water and in-water radiometers. Measurements were performed in marine and estuarine waters optically classified as Cases 1 and 2, respectively, and under variable atmospheric conditions. A statistical comparison of both approaches was conducted in support of the validation of remote sensing data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and local algorithm development. Remote sensing reflectance was calculated at four wavelengths 412, 443, 490, and 555 nm, and results from the two in situ approaches and SeaWiFS were compared. The spectrally averaged unbiased percent difference between remote sensing reflectance derived from the two field instruments was greater than, 25%, in the best case. Radiometric quantities derived from field measurements (e.g. diffuse attenuation coefficient at 490 nm and spectral remote sensing reflectance) were compared to available estimates from SeaWiFS images. The random mean square root error (RMSE) between field measurements and SeaWiFS estimates of the remote sensing reflectance ranged from 26.3% (at 555 nm) to 52.9% (at 412 nm). The RMSE between field measurement and SeaWiFS estimates of Kd 490 was 34.3%. Because the spatial scale of in situ measurements (meters) differ greatly from that of SeaWiFS (kilometers), sub-pixel variability in field measurements was investigated. Our results suggest that factors other than sub-pixel variability are responsible for observed discrepancies between in situ and satellite-based remote sensing reflectance.