**4.1 Before the 1980s**

Early in the twentieth century, research focused on topics such as field heterogeneity, spatial variability, and site-specific agriculture was concentrated mainly on crop nutrient management [49–51]. Soil sampling at 15 cm depth on a 0.4 ha grid was reported as the first known recommendation to address these concerns [50], including works focused on developing statistical tools and methods [52]. The mechanization of agriculture, including tractors and fertilizer applicators in the 1930s, increased food production and farm efficiency [53]. Melsted and Peck [54] considered visionaries, helped build the foundation for successful variable-rate fertilizer application using a soil sampling on a 24.3-m grid pattern around 1961 [51]. The Green Revolution that occurred during the 1950's and 1960's increased agricultural production worldwide, particularly in the developing world [55], and saved over a billion people from starvation. The main components of the "Green

Revolution" included high-yielding cereals, fertilizers and agro-chemical application, irrigation, and improved management practices and mechanization. The U.S. Department of Defense first developed a satellite-based radio-navigation system, the global position system (GPS), in the 1970s, but it was confined for military uses until 1980, when encryption was partially eliminated, thereby encouraging civilian use [56]. However, the GPS usage was prevalent in agriculture did not occur until 1993, given the lack of available correction to improve the horizontal accuracy of coordinates (< 2.0 m) [57]. GPS served as a model for other countries to develop similar radio-navigation systems today, and when combined, is now known as GNSS.
