Vegetation Condition images at this site are based on NDVI (Normalized Difference Vegetation Index) values created by the US Geological Survey's EROS Data Center using AVHRR (Advanced Very High Resolution Radiometer) sensor data from one of the NOAA weather satellites. A full description of the AVHRR sensor is available at the EROS Web Site. The NDVI measures vegetation vigor caused by chlorophyll activity; this is sometimes called "greenness". NDVI values can theoretically range from -1 to +1; high values represent healthy, vigorous vegetation while low values typically depict bare soil and water. These data have proven valuable to USDA policy officials in providing geographic location and monitoring information for vegetation condition in crop areas. NDVI images are usually delivered in the form of a biweekly composite to remove cloud cover problems found in single date images. 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.
In late 2000, the USGS EROS Data Center informed NASS that the orbital characteristics of the NOAA-14 AVHRR satellite were deteriorating. NASS decided to pull the 2000 AVHRR/NDVI images from our web site.
The NOAA-16 satellite was launched to begin AVHRR coverage in 2001. NASS has monitored the NOAA-16 AVHRR progress throughout the 2001 and the beginning of the 2002 growing season to research how to best utilize the data supplied from the NOAA-16 satellite. The main issues were if and how to utilize the data that has been adjusted for the effects of atmospheric water vapor. NASS studied the difference this adjustment makes. The resultant differences are not consistent across the country or throughout the growing season. Therefore, in the interest of consistency with NASS'S data series, we used the non water vapor corrected data for 2001-2003.
The NOAA-16 satellite developed a scan motor problem in early 2004. This caused one side of the image to look like the Landsat 7 SLC-off issue. Beginning with late April 2004, imagery from the NOAA-17 (AM) satellite was used to monitor vegetation condition. Since the new sensor's data may not be directly comparable to that of NOAA-16, we did not process any ratio based images. We also plan to continue using non-water vapor corrected data.
In 2010 AVHRR image production was delayed due to a scan motor problem in the AVHRR NOAA-17 sensor that affected data quality. AVHRR NOAA-18 (PM) was being utilized starting in March 2010, with no direct comparisons to prior years, based on the different observation times. In 2011 direct comparisons between years will resume.
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