Relationships can be examined and new data sets produced by combining a number of overlays.
These data sets can be combined with models and decision support systems to construct a
powerful management tool. For example, we could assess how far a field was from roads or non-
agricultural crops. This information could be important in pest infestation or in planning
chemical application. We could also examine crop yield relationship to soil type or other factors
as show in the following figure.. A number of GIS software packages are now commercially
available. Spatial data for the GIS is often collected using GPS equipment but another source of
spatial information is aerial and satellite imagery.