Abstract: A prescription farming control system includes a navigation controller and a product delivery controller for controlling the rate of operation of a number of agricultural product delivery mechanisms mounted on an applicator vehicle as a function of the global position of the vehicle in an agricultural field. Information is stored in computer memory on board the vehicle to define a number of layers corresponding to each of the delivery mechanisms, each layer including a number of zones representing different levels of activity of the corresponding mechanism. Each zone is defined by a plurality of vertices, rather than on a pixel-by-pixel basis. The navigation controller receives satellite positioning data to combine with pseudorange correction data received from a fixed ground station to determine a corrected accurate global position of the vehicle. A graphics coprocessor includes active invisible graphics memory page onto which each layer is sequentially drawn and interrogated.