GNSS Location Accuracy

Kristen Biondo, Environmental Studies

Global Navigation Satellite Systems (GNSS)-based surveying methods are becoming more common. The GNSS methods of data collection have an inherent range of errors for determining geographic position. We used a Trimble Juno 3B unit to locate trees at a local level. This type of unit has a stated location accuracy of 3-5 meters after differential correction, but in difficult environments such as landscapes with dense canopies or tall buildings the accuracy decreases significantly because of multi-path trajectories of the signal. The Trimble Juno 3B collects global positioning system (GPS) data using code signals, which are less accurate at determining location than carrier signals. Code signals calculate the distance between a set base location and the receiver, using the difference between the signal from the base and the signal from the receiver. Carrier signals have variable reference points, reducing the amount of error in the location measurement. The study area is located north of Discovery Hall on the University of Washington Bothell campus. This area is characterized by high tree density, close proximity to buildings, and variable topography. To examine the amount in the variation between the Trimble Juno 3B and the correct geographic location, GPS and survey measurements of bearing and distance were taken from individual trees and then mapped to compare locations and quantify errors using a root mean square error approach. The results of these comparisons were visually assessed using Light Detection and Ranging (LiDAR) data and summarized using location and distance error measurements. These data indicate the distances that should be accounted for when using this type of device to collect location information in areas that have many difficult characteristics.