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February 11, 2014
NASA Cracks a Baffling Satellite Illusion

A NASA-led research team found that the timing and the amount of sunlight falling on the forest, combined with the angles of the satellite view, can create false “hot spots” in satellite images. Essentially, the forest looks greener in the dry season because the angles between the sun, the satellite and the forest can focus light more in certain parts of an image.

Why does Amazon rainforest vegetation appear even greener and healthier in satellite images during dry periods and drought? NASA scientists have the answer after nearly a decade of debate.

Researchers have built computer models, analyzed and processed satellite images, and conducted field studies in the Amazon forests. For a decade, they have postulated and speculated, trying to understand why a system so dependent on rainfall would appear healthier when water is scarcer.

Doug Morton and colleagues at NASA’s Goddard Space Flight Center finally have the answer: Much of that greening, Morton asserts, is a trick of the light and an artifact of satellite instruments.

“Scientists who use satellite observations to study changes in Earth’s vegetation need to account for seasonal differences in the angles of solar illumination and satellite observation,” says Morton. “The seasonal change in greenness has nothing to do with how forests are changing.”

There already are some scientific techniques to correct for reflectance by the atmosphere and land as well as for the angles of the satellite view, but none account for the changing angles of sunlight on short-to-medium time scales. Morton and colleagues have developed algorithms and tools to compensate for the lighting hot spots and shadowing effects in satellite pixels.

Image courtesy of NASA.

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