English: This image is comprised of data from the Advanced Very High Resolution Radiometer (AVHRR) aboard the National Oceanic and Atmospheric Administration's (NOAA) Polar Orbiting Environmental Satellite (POES) series to map global vegetation cover. The vegetation covering the continents was recorded by the AVHRR aboard NOAA-7, NOAA-8, and NOAA-11. Dense vegetation is represented by shades of purple and green; sparse vegetation by shades of brown. The vegetation "index," an indicator of vegetation cover, is calculated by comparing reflected infrared light to reflected visible light for a specific area of land. NOAA's two operational polar orbiting satellites scan the entire earth once every six hours from altitudes of about 850km (529 miles). The data collected by the AVHRR sensor are held in the archives of the United States Geological Survey's EROS Data Center. The objective of the AVHRR instrument was to provide radiance data for investigation of clouds, land-water boundaries, snow and ice extent, ice or snow melt inception, day and night cloud distribution, temperatures of radiating surfaces, and sea surface temperature.