A farming system which identifies the boundaries of agricultural fields and performs functions based upon whether a vehicle is located within a particular field is disclosed. The system includes a location signal generation circuit which receives positioning signals and generates location signals representative of the location of the vehicle. The system further includes a memory circuit which stores geo-referenced digital maps of the fields and may include boundary data or application maps for each field. The system further includes a control circuit which compares the location signals to the data stored in the memory circuit to determine when the vehicle is located in a particular field. In one embodiment, the control circuit selects a variable-rate application map for the particular field and generates variable-rate control signals from the map data which are applied to a variable-rate applicator. In another embodiment, the control circuit stores characteristic data sensed by a sensing circuit with correlated location signals in a geo-referenced digital map for the particular field. In another embodiment, the control circuit generates height control signals to raise and lower an agricultural tool such as a tractor plow or a combine header based upon relationships between the location of the tool in a field and the location of attributes in the field such as field boundaries or obstructions.