Performance of Mapping-Grade GPS Receivers in Southeastern Forest Conditions

This study represents an assessment of two recently-available GPS receiver configurations used in mature southern pine and hardwood forests in the Piedmont of Georgia. Six control points were visited ten times, with WAAS (Wide Area Augmentation System) enabled, and 50 position fixes were recorded during each visit to each control point. We found that horizontal position accuracy was on the order of 2 m when WAAS was enabled in those receivers and post-collection differential correction was not employed. This was significant in that the accuracy of the receivers evaluated was greater than recent studies suggested even without the use of differential correction, revising our notion of how well GPS receivers perform in real time in forested conditions. In general, there was no significant difference in horizontal position accuracy between the two receiver configurations when the error of an average position (from a set of position fixes) was analyzed. However, when the error was assessed for each position fix, and then averaged, there was a significant difference with one of the receiver configurations when used in the pine and hardwood stands, and there were significant differences between the two receiver configurations when used in the hardwood stand. In addition, in general there was no correlation between horizontal position error and PDOP (Positional Dilution of Precision), signal-to-noise ratio, relative humidity, air temperature, and atmospheric pressure values at the time of data collection. However, one of the two receiver configurations seemed sensitive to air temperature. These results illustrate the real-time horizontal position accuracy that can be obtained with current technology in similar forest conditions throughout the Piedmont of the United States during leaf-off (winter) conditions.