Vegetation Indices

EO-1 Hyperion Vegetation Indices Tutorial

Vegetation Indices (VIs) are combinations of surface reflectance at two or more wavelengths designed to highlight a particular property of vegetation. They are derived using the reflectance properties of vegetation. Each of the VIs is designed to accentuate a particular vegetation property.

More than 150 VIs have been published in scientific literature, but only a small subset have substantial biophysical basis or have been systematically tested. ENVI provides 27 vegetation indices to use to detect the presence and relative abundance of pigments, water, and carbon as expressed in the solar-reflected optical spectrum (400 nm to 2500 nm).

Selection of the most important vegetation categories and the best representative indices within each category was performed by Dr. Gregory P. Asner of the Carnegie Institution of Washington, Department of Global Ecology. The selections were based upon robustness, scientific basis, and general applicability. Many of these indices are currently unknown or under-used in the commercial, government, and scientific communities.

The indices are grouped into categories that calculate similar properties. The categories and indices are as follows:

Each category of indices typically provides multiple techniques to estimate the absence or presence of a single vegetation property. For different properties and field conditions, some indices within a category provide results with higher validity than others. By comparing the results of different VIs in a category, and correlating these to field conditions measured on site, you can assess which indices in a particular category do the best job of modelling the variability in your scene. By using the VI in any category that best models the measured field conditions for a few measurements, you can significantly increase the quality of the results from any further processing.

Note: The VIs provided in ENVI are not designed to quantify the exact concentration or abundance of any given vegetation component. Instead, they are intended for use in geographically mapping relative amounts of vegetation components, which can then be interpreted in terms of ecosystem conditions.

All VIs require high-quality reflectance measurements from either multispectral or hyperspectral sensors. Measurements in radiance units that have not been atmospherically corrected are unsuitable, and typically provide poor results.

The VIs that can be calculated on a specific dataset are determined by the spectral bands sampled in the input dataset. If all spectral bands required for a specific index are available, that VI is available for the dataset. For example, an input dataset from a sensor that matches only the near-infrared and red spectral bands (such as AVHRR, TM, and others) is only able to calculate two of the indices: the NDVI (Normalized Difference Vegetation Index) and SR (Simple Ratio). In contrast, for a high spectral resolution input dataset, such as AVIRIS, 25 of the indices will be available.

The next sections describe the VIs available in ENVI, grouped by category. Information about each index includes the formulation, expected range, usage, limitations, and references for scientific studies performed with that index. Except for the broadband greenness VI descriptions, the wavelength (in nanometers) is represented as a subscript in the reflectance identifier for all inputs to the VI formulae (for example, r₆₈₀ means the reflectance at 680 nm). To indicate that the broadband greenness indices require less precise wavelength bands, the formulae use the subscripts NIR, RED, and BLUE to indicate the spectral region of the reflectance input used to calculate the VI.

Vegetation Indices

See Also