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Source: Adapted from Smith, 2002.

Note: The rows in Table 7.2 are to be read in conjunction with the rows in Table 7.1.

length bands. By comparison, hyperspectral remote sensors collect image data simultaneously in tens or hundreds of narrow, adjacent spectral bands (as little as 0.01 jam in width). These measurements make it possible to derive a near-continuous spectrum for each image cell (Figure 7.6). After adjustments for sensor, atmospheric, and terrain effects are applied, these image spectra can be compared to field or laboratory reflectance spectra to identify and map surface materials such as particular types of vegetation or diagnostic minerals associated with ore deposits. Hyperspectral images contain a wealth of data and are difficult to interpret. Interpretation requires an understanding of the exact properties of the ground materials measured and how these relate to the data produced by the hyperspectral sensor. Further information on hyperspectral remote sensing is given in a tutorial at <http://www.microimages.com/>.

Plant Spectra

The spectral reflectance curves of healthy green plants also have a characteristic shape that is dictated by various plant attributes (Figure 7.7). In the visible portion of the spectrum, absorption effects from chlorophyll and other leaf pigments govern the curve shape. Chlorophyll absorbs visible light very effectively but absorbs blue and red wavelengths more strongly than green, producing a characteristic small reflectance peak within the green wavelength range. As a consequence, healthy plants appear green to the eye. Reflectance rises sharply across the boundary between red and near-infrared wavelengths (sometimes referred to as the red edge) to values of around 40 to 50 percent for most plants. This high near-infrared reflectance is primarily due to interactions with the internal cellular structure of leaves. Most of the remaining energy is transmitted and can interact with other leaves lower in the canopy. Leaf structure varies significantly among plant species and can also change as a result of plant stress.

Thus, species type, plant stress, and canopy state can all affect near-infrared reflectance measurements. Beyond 1.3 pm, reflectance decreases with increasing wavelength, except in two pronounced water absorption bands near 1.4 and 1.9 nm. At the end of the growing season leaves lose water and chlorophyll. Near-infrared reflectance decreases and red reflectance increases, creating the familiar yellow, brown, and red leaf colors of autumn.

Spectral Libraries

Several libraries of reflectance spectra of natural and man-made materials are available for public use. These libraries provide a source of reference spectra that can aid the interpretation of hyperspectral and multispectral images.

ASTER Spectral Library

This library has been made available by the National Aeronautics and Space Administration (NASA) as part of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imaging instrument program (Figure 7.8). ASTER is one of the instruments on the EOS (Earth Observing System) AM-1 satellite and records image data in 14 channels from the visible through thermal infrared wavelength regions as part of NASA's Earth Science Enterprise program. The ASTER spectral library includes spectral compilations from NASA's Jet Propulsion Laboratory, The Johns Hopkins University, and the United States Geological Survey (Reston, Virginia). The ASTER spectral library currently contains nearly 2,000 spectra, including minerals, rocks, soils, man-made materials, water, and snow. Many of the spectra cover the entire wavelength region from 0.4 to 14 ^m. The library is accessible interactively via the Internet at <http://speclib. jpl.nasa.gov>. It is possible to search for spectra by category, view a spectral plot for any of the retrieved spectra, and download the data for individual spectra as a text file. These spectra can be imported into an image processing spectral library. The ASTER spectral library can also be ordered on CD-ROM at no charge from the Web site.

USGS Spectral Library

The United States Geological Survey Spectroscopy Laboratory in Denver, Colorado, has compiled a library of about 500 reflectance spectra of minerals and a few plants over the wavelength range from 0.2 to 3.0 ^m. This library is accessible online at <http://speclab.cr.usgs.gov/spectral. lib04/ spectral-lib04.html>. Users browse individual spectra online or download the entire library. There is information at this Web site on the Airborne Visual and Infrared Imaging Spectrometer (AVIRIS) and a tutorial on imaging spectroscopy.

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