way. The limitation with some commercial software is that spatial relationships among data layers often cannot be rigorously quantified; only visual relationships can be made. This situation is rapidly changing as several vendors are developing fully functional GIS programs intended for use on PCs. This should lead to software and hardware systems that are more user friendly and less expensive. In addition, firms are emerging in the marketplace that can provide GIS services or software tools to growers and field consultants. There is an urgent need to make fully functional GIS easier for nonspecialists to learn and use in order to transfer this technology to the agricultural community.

GIS can be used with a spatially distributed process model as the basis for subsequent decisions on precision agricultural practices such as variable-rate applications. Several classes of models should be considered as part of the suite of tools for precision agriculture.

Yield Mapping Systems

Yield mapping systems record the relative spatial distribution of yield while the crop is being harvested. These systems collect georeferenced data on crop yield and characteristics such as moisture content. The resulting maps can dramatically illustrate the areas of yield variability from either natural processes or agricultural practices. Because yield is a primary factor in most management decisions, precise yield maps are desired to confirm spatial treatment decisions.

Yield monitors have been developed for only a few crops, primarily cereal grains. Reliable monitors for vegetables, fruits, cotton, and other high-value crops are currently under development but are not yet widely available. Yield is more difficult to monitor for fruit or vegetable crops that are harvested manually or repeatedly. Use of machine-mounted yield monitors currently is limited to crops that are mechanically harvested in a single pass, such as potatoes, sugar beets, and processing tomatoes. Other techniques such as remote sensing may provide alternatives to yield monitors. The use of precision agriculture techniques in nongrain crops may be limited by the lack of appropriate yield monitoring systems.

Since 1992, grain yield mapping has been done by using mass flow and moisture sensors to determine grain mass and using GPS receivers to record position. Yield monitors measure wet grain flow, grain moisture, and area harvested to determine moisture-corrected yield per acre. Because the mass-flow measurements are made in the combine's clean-grain conveying system, there is a shift in harvester position between the point where the grain is actually cut and the location of the machine where it is measured. This shift results in dynamic inaccuracies that currently cannot be completely removed by subsequent data processing. Field totals (with recommended calibrations) are considered more accurate than are small subfield yield measurements. Although yield monitors have been promoted widely, further yield monitor refinements are needed to improve their accuracy for precision agriculture applications.

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