CAUTION: Comparing images from year to year requires a knowledge of crop phenology for both years. These images are not yet used in a quantitative way by the Agricultural Statistics Board in setting official USDA estimates. A white paper titled 'Limited Use of Remotely Sensed Data for Crop Condition Monitoring and Crop Yield Forecasting in NASS' describes this in more detail.
USDA/NASS and USDA/ARS have been studying the properties of NDVI based images for several years through a cooperative research agreement. The user will sometimes see anomalies caused by atmospheric effects when reviewing the same area across several periods. When a large area is cloud covered, many of the surrounding data values may also be affected, especially when looking at ratio based products. Scene edges can also cause visual differences within composite scenes that are not really there.
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