|Peak Vegetation Growth
What this map layer shows:
The peak level of vegetation growth in Alaska and the conterminous
United States, for the years 1990 to 2005.
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 Peak Vegetation Growth map layers are 16 images that show the peak
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. Peak greenness
is calculated by examining all of the biweekly composites for a year,
pixel-by-pixel, and recording the maximum NDVI value for each pixel.
See the Greenness
home page for further information on USGS greenness
The National Atlas also includes Average Vegetation
Growth map layers
for the years 1990 to 2005.