Vertical Measurement Accuracy of Mapping-Grade Global Positioning Systems Receivers in Three Forest Settings

Elevation or height differences are necessary measurements for many forest operation activities. We rigorously examined the vertical measurement performance of five mapping-grade GPS receivers in three forest settings representing open-sky, young-forest, and closed-canopy conditions. The mapping-grade GPS receivers collected data simultaneously at each of the three forest settings and had different hardware and data-collection configurations, including internal and external antennas, and real-time differential corrections. We evaluated the influence of forest setting and postprocessed differential corrections on all GPS receiver measurements, including those that were collected with real-time differential corrections. We also compared the effect of 1-, 30-, and 60-point averaging intervals on vertical measurement accuracy. We found average vertical accuracies for unprocessed GPS receiver measurements of 0.9, 1.7, and 2.8 m in the open-sky, young-forest, and closed-canopy settings, respectively. The influence of data postprocessing was inconsistent under closed canopy and resulted in average vertical GPS accuracies of 0.2, 0.4, 3.3 m in open-canopy, young-forest, and closed-canopy settings, respectively. Different point averaging intervals did not result in statistically significant differences in vertical accuracies for either unprocessed or postprocessed GPS data.