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Map Layer Info

Average Vegetation Growth

What this map layer shows:

The average level of vegetation growth in Alaska and the conterminous United States, for the years 1990 to 2005.

opens the U.S. Geological Survey home page
Background Information
Sample map Sample Map
The staff at the U.S. Geological Survey (USGS), Center for Earth Resources Observation and Science (EROS) produces vegetation growth images for the United States from Advanced Very High Resolution Radiometer (AVHRR) data. AVHRR sensors are carried in Earth orbit onboard several National Oceanic and Atmospheric Administration (NOAA) satellites. AVHRR data have a ground resolution of 1.1 kilometer, which means that the satellite records discrete information for areas on the ground that are 1.1 by 1.1 kilometers. This smallest recorded unit is called a pixel. The swath width, the width of the area on the Earth's surface that the satellite can "see", is over 2000 kilometers.

AVHRR imagery that covers the whole Earth is recorded every day, but the most commonly used AVHRR products are composite images that are created from AVHRR data every two weeks. Compositing two weeks of images means that data obscured by cloud cover on any particular day can be replaced by data recorded on a clear day during the same period. Biweekly AVHRR composites are the basis for Normalized Difference Vegetation Index (NDVI) calculations, which measure vegetation cover and plant growth. Because they indicate vegetation vigor, NDVI values are commonly called "greenness data." Generally, healthy vegetation is considered an indicator of favorable climatic and environmental conditions, while poor vegetation condition is indicative of droughts and diminished productivity.

Greenness composites have been produced for the conterminous United States every two weeks since 1989, and for Alaska, during the growing season, since 1990. Greenness maps are used by government agencies and private industry for a wide variety of applications, including agricultural assessment, environmental monitoring and global climate change studies.

The Average Vegetation Growth map layers are 16 images that show the average level of vegetation growth in Alaska and the conterminous United States for the years 1990 to 2005. They were produced by the National Atlas of the United States® with data provided by EROS. Average greenness is derived by examining the composites for a year and calculating the arithmetic average of all the biweekly NDVI values for each pixel. See the Greenness home page for further information on USGS greenness mapping.

The National Atlas also includes Peak Vegetation Growth map layers for the years 1990 to 2005.