A hand-held Garmin eTrex Venture global positioning system (GPS) receiver operated in autonomous mode was used to test the precision and accuracy of autonomous GPS in location determination in the absence of the selective availability (S/A) error. In addition, the potential of utilizing the GPS to measure distances in plain agricultural fields was explored. GPS precision in location determination was tested by placing the GPS receiver at specific points in an agricultural field, where the actual distances between these points were known, and collecting position data (latitude and longitude) for a specific period of time. For each field point, values collected were averaged and data scatter around the average was measured. Autonomous GPS accuracy in location determination was investigated by placing the GPS receiver at a point (reference point) where latitude and longitude (coordinates) values were precisely defined and collecting position data for a specific time period. Values of data collected were compared to the values of reference point coordinates and positional errors were calculated. To explore the potential of utilizing the tested GPS in distance measurement, distances between field point averages were calculated by converting the differences between these points in latitude and longitude values measured in degrees and minutes into differences in meters. The GPS accuracy in distance measurement was revealed by comparing the calculated distances to the actual ones.
For over 1500 data points, precision tests revealed that the maximum difference in longitude between the average and a data point was 1.65 m, where the maximum difference in latitude was 1.74 m. Results of the accuracy test showed that, on the average, the system maintained an accuracy of less than 2.10 m. Values of circular error probable (CEP) and two-distance root mean square (2DRMS) were 2.36 m and 2.76 m, respectively. For an actual distance of 270 m, the maximum difference between calculated and actual distance was 1.90 m (0.70 m/100 m) with an average of 1.42 m (0.53 m/100 m). However, the maximum difference reached 3.20 m (0.59 m/100 m) for an actual distance of 540 m with an average of 2.89 m (0.54 m/100 m).