Assessing point accuracy of DGPS under forest canopy before data acquisition, in the field and after postprocessing

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 post processing ranged from 0.49 to 3.60 m for 2-20 min of observation at points wit h basal area <30 m(2) ha(-1). For points wit h basal area >= 45 m(2) ha(-1) the mean posit ion 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 post processing, the most significant factors to predict position error were the standard deviation reported by the post processing soft ware and whet her both pseudorange and carrier phase were used to compute the coordinates or pseudorange only (R-2=0.57). The posit ion error decreased wit h decreasing density of forest, increasing length of observation period, decreasing standard deviation, and combined use of pseudorange and carrier phase.