Overall, the shape and magnitude of the curve indicates many things about the vegetation in the area. The greater the slope in the red-edge region (to be described later), the "healthier" the vegetation is and the "flatter" or more "relaxed" the curve is, the less healthy/more sparse the vegetation is. The magnitude is an indicator of the relative brightness of the vegetation. If the reflectance values are high, then the vegetation is reflecting a large amount of light, while low reflectance values indicate dim or dark-coloured vegetation or shadows. All of the features are also subject to error and artefact from changing surface and light conditions.

Some of the key features in this include the chlorophyll absorption region, red-edge, and the water absorption feature. In addition to some of the key features, there are also noise and other artefacts included in this sample. Of note are the oxygen absorption peak at 763-765 nm region and noise in the water absorption region. This page will go through each of the key features, how they are measured, and what they represent.

The next feature is the red-edge region, where there is a sharp increase in reflectance from about 680nm to about 800nm. This is the transition from the red to the near-infrared region. This is a commonly used region for plant ecophysiological studies using remote sensing because it is an indicator of vegetation/canopy health and cover. A common way of measuring this region is NDVI, or normalised difference vegetation index. The generic NDVI calculation is (R_NIR - R_Red)/(R_NIR + R_Red), and unless indicated, this unit will use 800 nm as the NIR wavelength and 680 as the red wavelength. This normalised index has the advantage of correcting for particularly bright or dim vegetation, although this region is sensitive to effects such as shadow, snow, and sun angle. Unlike either PRI or WBI (next one), this one can also be used on non-hyperspectral instruments because of the wider range of wavelengths involved.

The third region is the water absorption feature region, which is located from about 900 nm to about 970 nm. This region has been found to be a strong indicator of vegetation water content. However, this region is prone to noise, especially from snow and water droplets on the sensor, but it is still a useful region to analyse for water. The larger the "dip" between about 900 nm to about 970 nm is, the greater the vegetation (or other target) water content is, while a flat or nearly-flat region indicates a dry area. A common method of measuring this feature is the WBI, or water band index, which is calculated as R900/R970.

In the figure above, the spectra to the left were taken during a wet season, while the spectra to the right were taken during a drought. The symbol Af indicates the species Adenostoma fasciculatum (chamise), Ap is Arctostaphylos pungens (manzanita), and As is Adenostoma sparsifolium (redshank). Note the change in the shape of the spectra for the three species between wet and water-stressed conditions -- the curve "relaxes" and shrinks in steepness and magnitude, and both the PRI and WBI features shrink with decreasing water. The manzanita (Ap) is especially affected by drought while chamise (Af) still retains a marked curve about the 700-800 nm region.

Remember the spectra from above? The leaf to the left is from a non-water stressed plant while the one to the right is from a water-stressed plant. Note the more yellowish shading on the leaves to the right and the greener shading on the leaves to the left. This shows with more reflectance in the green (500-600nm) region for the greener leaf.

Again, remember the spectra from above? The leaf to the left is from a non-water stressed plant while the one to the right is from a water-stressed plant. Note the more yellowish shading on the leaves to the right and the greener shading on the leaves to the left. This shows with more reflectance in the green (500-600nm) region for the greener leaf.

To see an alternative view of the same data set, click here