The Vegetation Condition images are
created in two types, NDVI and Ratio.
NDVI (Normalized Difference Vegetation Index) images are directly based
on values created by the USGS's EROS Data Center in their biweekly
composite [3]
of AVHRR sensor data from one of the NOAA weather satellites. Prior to
1995,
the NOAA-11 satellite was the source for AVHRR data. From 1995 to 1999,
the
NOAA-14 satellite provided AVHRR sensor coverage, but this
satellite/sensor
failed in 2000. Starting in 2001, the NOAA-16 satellite began providing
AVHRR data for this data series. NOAA-16 started malfunctioning in
2004, with a scan motor problem. Images from NOAA-17 are used beginning
with the 2004 season.
The NDVI measures vegetation vigor
caused by chlorophyll activity; this is sometimes called "greenness".
These data have proven valuable to USDA policy officials in providing
geographic location and monitoring information for vegetation condition
in crop areas. NDVI values can theoretically range from -1 to +1; high
values represent healthy, vigorous vegetation while low values
typically depict bare soil and water.
Ratio images compare current NDVI
values to NDVI values from some previous period; these comparisons are
expressed at NASS in the form of a percent change. There are two
comparison ratios that can be created, a percent change from closest
corresponding dates of the previous year plus a percent change from the
median value of the corresponding periods from the previous three years
(when 3 comparable years are available).
The unsampled US NDVI image is
available for download as a GeoTiff image for input into Geographics
Information
Systems software. For quick viewing online, several standard graphics
products are created at NASS, based directly on NDVI data or on ratios
of NDVI from two time periods:
- An Overview image showing
thumbnails
of all images created
for the current period
- Entire US-level images showing
ASD
(Agricultural Statistics
District) and state boundaries
- Thumbnails showing up to 9
periods
at one time
- Side-by-side zoom images with
county
boundaries, showing current 'hot spots' and
comparable previous year values
- Zoom images with county
boundaries,
highlighting 'hot spot' areas
CAUTION: Comparing images from year to
year requires a knowledge of crop phenology for both years. These
images are not used in a quantitative way by the Agricultural
Statistics Board in setting official USDA estimates. USDA/NASS and
USDA/ARS have been studying the properties of NDVI based images for
several years through a cooperative research agreement[1,2,4,5].
References:
- Doraiswamy, P., Hart, G., Craig,
M.E.,
Cook, P.W. (1994) "The Anomalous '93 Growing Season -- How USDA Used
AVHRR
Data," 1994 ASPRS/ACSM Convention Technical Papers, Volume One, pp.
144-151.
- Doraiswamy, P., Cook, P.W.
(1995)
"The
Spring Wheat Yield Assessment Using NOAA AVHRR Data," 1995 Canadian
Journal
of Remote Sensing, Ottawa, Ontario, Volume 21, No.1, March 1995, pp.
43-51.
- "The 1990 Conterminous
U. S. AVHRR Data Set", Photogrammetric Engineering & Remote
Sensing,
Volume 58, No.6, June 1992, pp 809-813.
- Mueller, R.W., Wade, L.G., Cook,
P.W.
(1996) "Agricultural Statistics Board AVHRR and GIS Product
Development",
1996 ASPRS/ACSM Annual Convention Technical Papers, Baltimore, Md.
- Wade, L.G., Mueller, R.W., Cook,
P.W.,
Doraiswamy, P. (1994) "AVHRR Map Products for Crop Condition
Assessment:
A Geographic Information Systems Approach," Photogrammetric Engineering
& Remote Sensing, Volume 60, No.9, September 1994, pp 1145-1150.
For more information about this topic
or about the NASS Spatial Analysis Research Section (SARS), contact the
GeoSpatial Information Branch at HQ_RDD_GIB@nass.usda.gov.
Growing Season
Animations | National Crop Progress
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