Data collected in the field are recorded by sensors which are mounted on the tractor and scan the field as the tractor moves forward, collecting data sequentially (in rows).
In order to assign geographic co-ordinates to each spectral measurement, the co-ordinates of the tractor are frequently recorded through the Global Positioning System (GPS).
For more information on the GPS
GPS receivers have variable accuracies.
Expensive receivers are equipped with software and algorithms that help correct the received signal and minimize the positioning error.
In general the errors range from 10-20 meters for low cost receivers, to up to a few centimeters for very expensive receivers.
Since Precision Agriculture is dealing with individual small sized portions of the field, very high spatial accuracy is required.