A comparison of autonomous, WAAS, real-time, and post-processed global positioning systems (GPS) accuracies in northern forests

We report on accuracy comparisons among a range of global positioning system (GPS) receivers and configurations when collecting data in the open and below northern forest canopies. We compared recreational receivers in Wide Area Augmentation System (WAAS) mode, and expensive receivers optimized for spatial data collection (GIS receivers) in autonomous, WAAS, real-time differential, and post-processed differential modes. Data were collected over accurately surveyed open and subcanopy locations. Individual position fixes were logged for varying time periods, and corrected using appropriate methods. Euclidian distance errors were calculated, and analysis of variance (ANOVA), Tukey's tests, and linear regression were used to identify significant factors and differences. There were significant differences linear regression were used to identify significant differences in the mean positional error due to receiver type under forest canopies, but no statistically significant differences under open locations. There was no difference between differentially corrected and uncorrected data when using the GIS receivers. Recreational receiver accuracies were much less consistent than GIS receivers, with higher frequencies of large errors. Subcanopy tests indicate WAAS signals were available between 8 (moving) and 23 (stationary) % of the time for the recreational receivers, and between 22 (moving) and 33 (stationary) % of the time when using GIS receivers.