The essence of a global positioning system is the ability to define a location anywhere on Earth in terms of three coordinates. The calculation is performed in terms of Cartesian axes including one through the poles, but the result is transformed into local coordinates that include height. This chapter introduces the use of global positioning systems in agriculture, explaining how the system works and focussing on ways in which accuracy can be improved. The chapter examines the use of a peer-differential system and looks ahead to future developments in this area, providing detailed guidance on where to find further reading on the subject.