Horizontal Measurement Performance of Five Mapping-Grade Global Positioning System Receiver Configurations in Several Forested Settings

We examined the horizontal measurement performance of five mapping-grade GPS receiver configurations operating simultaneously at three measurement test sites established in open sky, young forest, and closed canopy conditions. Two of the GPS receivers had external antennas, and two receivers were configured to collect data with real-time differential corrections through the Wide Area Augmentation System (WAAS). The GPS receivers collected data using 1-, 30-, and 60-point recording intervals to test the influence of the number of point recordings on position determination. We also postprocessed all data to examine the influence of differential corrections. We found statistically significant differences in measurement accuracy between GPS receiver configurations that had an external antenna and receivers that did not. The top performer for unprocessed data collected measurements with real-time differential corrections and had average measurement errors of 0.4, 0.8, and 2.2 m, in open sky, young forest, and closed canopy conditions, respectively. The top performer for postprocessed data had average measurement errors of 0.2, 0.1, and 1.2 m, in open sky, young forest, and closed canopy conditions, respectively. The influence of number of points on measurement accuracy was observed between the 1- and 30-point intervals, with no statistically significant differences between the 30- and 60-point intervals. No statistically significant difference resulted in WAAS measurements that were postprocessed. The measurement accuracies we report are acceptable for many natural resource measurement applications. These findings encourage the use of external antennas when using GPS receivers under forest canopy. In addition, point recording intervals of 30 appear to be efficient for accurate measurements with mapping-grade GPS receivers.