Images & Animations
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Credit
The most common way scientists measure Earth's vegetation from space is by using a vegetation index that describes the relative "greenness" Earth's vegetation on a scale of minus one (-1) to plus one (+1). In mathematical terms, greenness is a comparison of amounts of visible and near-infrared sunlight that are absorbed and reflected by the plants. In physical terms, it's a way to describe the net result of canopy coverage, leaf area, and canopy architecture in a vegetated area—in others words, the number and size of leaves and how they are arranged horizontally and vertically. Low values on the index mean less vegetation, and values near 1 mean maximum vegetation.
The vegetation index has been around for decades, and it has been the workhorse of space-based vegetation studies since those kinds of measurements were first collected. The images above show an Enhanced Vegetation Index (EVI) that is based on data collected by the Terra satellite beginning in March 2002.
In addition to the traditional visible light-infrared light comparison common to previous vegetation indices, this product includes calculations that help to reduce common problems with vegetation indices. In tropical areas, where biomass burning often creates thick smoke, the EVI should reduce smoke's interference. It also reduced the effect of saturation. Saturation occurs when the all values of greenness above a certain threshold appear as the highest possible number on the index's scale; it’s common in areas of extremely high vegetation like rainforests. In semi-arid places on Earth where the vegetation is thin and widely spaced, the EVI minimizes the influence of background interference caused by bare soil reflecting off the ground.
Visualizations by Reto Stockli, Earth Observatory, based on data provided by the MODIS Science Team, NASA Goddard Space Flight Center
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Terra/MODIS, Aqua/MODIS
