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Spectral reflectance measurements were acquired at various viewing angles at three sites along the James Riber representing both tidal and non-tidal waters. The upper James River reaches were characterized by optically clear waters and resulted in spectral measurements that represented bottom substrates. In contrast, the lower James River sites were characterized by turbid waters having high suspended sediment and algal chlorophyll. Concurrent pyranometer measurements showed the maximum downwelling radiation occurring from 1030 to 1330 local sun time. During this period, two strategies emerged for consideration when collecting water column reflectance data. In optically clear waters, statistical analysis using the variance of the 575 nm waveband (as a reference) showed a nadir viewing angle (90 degree(s)) and upsun (+30 degree(s)) off-axis viewing angle were the most effective at characterizing the bottom substrates. The conclusion drawn for optically clear waters was that (independent of sun angle), nadir position of the sensor optics is critical, but confident measurements can still be acquired up to +30 degree(s) off axis. In contrast, lower James River sites (turbid reaches) showed no correlation between the nadir and off-axis measurements using the variance of the 680 nm chlorophyll absorption line. Furthermore, the conclusions drawn from these reaches demonstrated that reflectance data are best acquired at nadir viewing angles for highly turbid waters. These measurements could have implications for both non-imaging and imaging remote sensor data.
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