A series of images shows the extent of vegetation in North-central Africa, including the countries of Nigeria (bottom left), Cameroon (bottom center), Chad (bottom right and right), Niger (left), Algeria (top left), and Libya (top). The dark green patch towards the bottom of the image is Lake Chad. The first image, when you first open the page, is a true color composite—exactly how the landscape looks to the human eye. If you place your mouse over the image, it will “rollover” to a new image; the second image is produced using the Normalized Difference Vegetation Index, or NDVI. The index is a ratio of red and near infrared reflectance designed to estimate the amount of photosynthetic activity by plants. In this image, dark green is dense vegetation, such as a forest; medium green is vegetated land, perhaps partially forested; light green is grassland or crop areas; yellow is lightly vegetated or non-vegetated land; blue is water; white is cloud. The index has several distinct advantages: because it is a ratio of two bands, its calculation does not require highly complex and time-consuming atmospheric corrections. The index is robust, that is, it can be used anywhere on the planet regardless of vegetation or soil type, and thus images from all over the globe can be directly compared to others. It is used to measure changes in vegetation on a daily, seasonal, annual, and long term basis at scales ranging from landscapes, regions, continents, to the entire globe. The index has proven extremely useful as a famine early warning system and as an indicator of the spread of pests and disease.