Pacific Coastal & Marine Science Center
USGS Pacific Coral Reefs Website
AVIRIS data collected in April 2000 was used to produce hyperspectral images of the South Molokaʻi reef tract. The true color (RGB) image at top shows the location of processed images below. Pseudo-color infrared image (lower left) shows detectable variations across the reef. The yellow pixels in the middle image show the locations of live coral colonies (mostly Porites compressa), identified by a supervised classification technique from in situ ground-truth information. The image at lower right shows these results overlain on a grayscale image.
Hyperspectral imagers measure reflected sunlight from many different wavelengths, including the red, green and blue spectral windows within the visible-light spectrum as found in conventional photography. The NASA-operated AVIRIS (Airborne Visible Infrared Imaging Spectrometer) instrument collects high-resolution data from 224 narrow wavelengths ranging from ultraviolet to near infrared. Features in the shallow water have "thumbprint" reflective signatures that can be used to differentiate them from neighboring features.
This graph shows distinct differences between typical algae, coral and sand found on the Molokaʻi reef flat. These signature readings, taken with a hand-held hyperspectral radiometer in shallow water, are used to help identify specific features of interest during image processing and analysis.