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  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].
- 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.
Last Modified: 04/09/2012