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Global
Vegetation Index Products
The third
generation polar Global Vegetation Index (GVI) products are used for monitoring
the density and vigor of green vegetation. Useful applications of GVI products
include classifying land cover, estimating crop acreage, and detecting
plant stress.
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The GVI
products now online are weekly composites, and have a resolution of about
16 km. The Normalized Difference Vegetation Index,
Fractional
Vegetation, and the Precipitable Water Index products
are now available.
A comprehensive
description of GVI products (my primary information source) can be found
at the National Climatic Data Center's NOAA
GVI Guide.
Normalized
Difference Vegetation Index
The
basic index for measuring the 'greeness' of the earth's surface is the
Normalized Difference Vegetation Index (NDVI), which is basically a calculation
of the differences between AVHRR channels 1 and 2. A reasonable estimation
of the density and coverage of green vegetation can be determined by measuring
how green the earth's surface is.
Click
on the region you would like to see at full resolution.
Regions
on display include North
America, South
America, Alaska,
Europe,
Asia,
British
Isles, Africa,
and Oceania.
NDVI
values range from -.1 to .703 and are unitless. Values greater than .1
generally denote increasing degrees in the greeness and intensity of vegetation.
Values between 0 and .1 are commonly characteristic of rocks and bare soil,
and values less than 0 sometimes indicate clouds, rain, and snow.
NOTE:
Low values of NDVI do not necessarily denote lack of vegetation. For example,
during the winter months deciduous forests may appear more orange than
green.
See the
Normalized
Difference Vegetation Index archive.
Fractional
Vegetation
Fractional
vegetation is essentially NDVI displayed as a fraction (or a percentage
if the fractional vegetation values are multiplied by 100%). NDVI values
less than or equal to .07 are set to 0.0 and NDVI values greater or equal
to .57 are set to 1.0 (NDVI values between .07 and .57 increase linearly
from 0.0 to 1.0 as fractional vegetation).
Click
on the region you would like to see at full resolution.
Regions
on display include North
America, South
America, Alaska,
Europe,
Asia,
British
Isles, Africa,
and Oceania.
See the
Fractional
Vegetation archive.
Precipitable
Water Index
The
Precipitable Water Index (PWI), otherwise known as the brightness temperature
difference, is the difference between AVHRR channels 4 and 5. PWI is a
measurement of how much water vapor is in the atmosphere (lower K values
denote higher concentrations of water vapor). Since AVHRR channels 4 and
5 are volume sampled from the top of the atmosphere to the surface, PWI,
to an extent, measures the total column precipitable water in the atmosphere
(Steve Olson, 1997).
Click
on the region you would like to see at full resolution.
Regions
on display include North
America, South
America, Alaska,
Europe,
Asia,
British
Isles, Africa,
and Oceania.
Please
note that PWI is not the same as relative humidity, which measures the
percentage of water vaper over the amount of water vapor needed for saturation
at a given temperature.
See the
Precipitable
Water Index archive.
Go back
to the Image Products Home Page.
Other
Polar Imagery Products:
Composite Mapped Mosaic Imagery
Coastwatch Imagery
WEFAX Imagery
2 km Continental U.S. Imagery
AMSU
Derived Composite Imagery
SSM/I
Derived Composite Imagery
Questions
or comments?
Contact
Joseph Askew (Joseph.Askew@noaa.gov)
Last
revised: April 17, 2003 (RR)
