Other developed technologies such as sampling, soil analysis and others are also playing a role. Alberta Agriculture, Food and Rural Development has been in the Precision Farming business since 1993. Transfer this technology to private industry. Before then, positioning via radio was possible, but not very accessible to the researcher, or ultimately the farmer. The Navstar system is capable of better positioning than the Glonass system.
Agricultural applications require positioning at sub-meter precisions, most often in real time.
Some enterprising person realized if a GPS receiver were put over some known position, a correction to the position as determined by GPS could be calculated. How good the correction is depends on how close the roving receiver is to the fixed known station, and how long ago the correction was calculated. The best precision I am familiar with is typically on the order of 10cm. For most people, it is not practical to own two GPS receivers plus radio-modems to gather positioning information. Fortunately, it is possible for many people to obtain usable differential corrections without going to the trouble of owning a differential base station.