Assessing Point Accuracy of DGPS Under Forest Canopy Before Data Acquisition, in the Field and After Postprocessing

Abstract

A low-cost, hand-held, 10-channel, single-frequency Global Positioning System (GPS) receiver observing pseudorange and carrier phase was used to determine the positional accuracy of 35 points under tree canopies. The mean position error based on differential postprocessing ranged from 0.49 to 3.60 m for 2-20 min of observation at points with basal area <30 m 2 ha -1 . For points with basal area S 45 m 2 ha -1 the mean position error ranged from 2.15 to 5.60 m. Regression analysis revealed that basal area and observation period were the most significant factors to predict the position error that could be obtained by GPS data collection before or during data acquisition ( R 2 = 0.37). After differential postprocessing, the most significant factors to predict position error were the standard deviation reported by the postprocessing software and whether both pseudorange and carrier phase were used to compute the coordinates or pseudorange only ( R 2 = 0.57). The position error decreased with decreasing density of forest, increasing length of observation period, decreasing standard deviation, and combined use of pseudorange and carrier phase.