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Infrared (IR) satellite imagery is one of the most useful crop-assessment tools. Satellite sensors measure the reflectance within specific wavelengths and store these reflectance values as digital data. After processing, the data can be displayed as "false-color" imagery. Typically, the IR reflectance value is assigned to the color red, and higher IR values translate into a brighter red color on the image display, indicating more vigorous plant growth. The advantage of digital data, as compared with aerial photography, is that the amount of vegetative biomass -- and potential crop yields -- can be quantified.
One of the properties of a satellite is its spatial resolution: the smallest unit that the satellite is able to "see." A satellite's ability to discern land features such as field boundaries improves as resolution increases, but typically at the expense of area coverage. PECAD utilizes both high and low resolution imagery for its analysis, namely SPOT 3 (20 meter) and NOAA AVHRR (1.1 km).
SPOT imagery is an excellent tool for the crop analyst to obtain "field-level" information concerning vegetative development and condition. It is also an important data source for agriculture classification analyses. This imagery is especially valuable when compared with several previous years, where known data elements such as crop yield are available. The channels utilized are the green, red, and near infra-red bands, or 0.50-0.59, 0.61-0.68, and 0.79-0.89 micrometers. Green vegetation appears red on these images and is easily distinguishable from other landuse patterns or conditions.
AVHRR (Advanced Very High Resolution Radiometer) resolution is approximately one kilometer -- actually fairly low resolution compared to other satellites used for crop-assessment purposes. The relatively low resolution, however, enables the satellite to capture a 600-mile-wide swath, making it very well suited for large-area assessment of general crop conditions.
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