Using "Landsat 5" images of Tucson, the slider compares the NDVI values of the foothill area (about 80 square miles) in 1988 and 2008. The color photos from the sites were taken in spring 2013. The satellite images indicate that the increase of human activities in the foothill area caused the decrease of natural vegetation cover.

WTF is NDVI?

NDVI, or normalized difference vegetation index, is developed for studying the healthiness of vegetation.

NDVI measures vegetation's absorbance of light in red band and its reflectance in near infrared band. Typically, green and healthy vegetation has high NDVI values, with low absorption in red and high reflection in near infrared. Thus, green and healthy vegetation appears bright on NDVI images.

Since NDVI produces ratio values, the sun angle won't affect the output. But, the accuracy is affected by atmospheric scattering and soil moisture. NDVI's descendant EVI, or enhanced vegetation index, adds atmospheric and soil corrections in the algorithm to produce more accurate measures. But NDVI is still dominant in remote sensing because it provides good comparison with historical data.

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