A couple points about GPS. A single receiver is only accurate to a few feet. The specs are 30 meters, about 33 feet, but it often is better than that. If the gps antenna is obstructed, such as trees, the quality of your position goes down hill in a hurry.
There are corrections available, the quality of this varies to a few feet to an inch. Some of these are ag based and often require a fee. They can be surprisingly accurate.
I am a retired land surveyor and when we first started using gps we did have to setup a second gps unit on a known position. Then a radio linked it to the rover which was the unit we carried around. Most of the time now we can get this data from a cell phone link. This data is from a number of gps receivers setup around the state. In some states this is free, in Illinois we have to pay for it.
There is also multiple constellations available now. Gps (USA) and Glonass (Russian) being the oldest. There is also Bediou(sp? Chinese), Galileo (European) and a Japanese system. Land surveyors with newer equipment can use these other signals but I don’t know if the ag systems do or not. All these satellites really makes for a noticeable improvement and helps with obstructions such as trees and buildings.
