Vertical measurement accuracy and reliability of mapping-grade GPS receivers

Abstract

Elevation measurements are essential for many natural resource activities including hydrologic modeling, viewshed analysis, and forest road and trail engineering design. We collected elevation measurements with five identically configured mapping-grade global positioning system (GPS) receivers at two distinctly different test courses within a forest. Our primary objectives were to quantify the accuracy and reliability of GPS elevation measurements within two distinctly different environmental settings and to examine the influence of different data collection approaches. One test course was established within a forest clearing and had a generally unobstructed view of the overhead sky while the other course was mostly covered by forest canopy. We collected measurements using several different data groupings (1, 30, and 60 s intervals) and calculated the elevation error of all GPS receiver measurements for both unprocessed and differentially corrected (processed) measurements. The average error was 2.7 m (1.8 SD) for unprocessed and 0.4 m (0.3 SD) for processed measurements when all GPS receiver data and point intervals (1, 30, and 60 s) were combined at the open sky course. Average measurement error was 5.0 m (4.6 SD) for unprocessed and 3.0 m (2.8 SD) for processed measurements when grouping all GPS receiver data and point intervals (1, 30, and 60 s) at the forest canopy course. Although the GPS receivers were identical in model and data collection parameters, we found statistically significant differences in elevation measurements between the receivers that we tested. We determined no statistical influence of 1, 30, and 60 point intervals on elevation measurement accuracy within each individual GPS receiver, regardless of measurement course or data processing. For many basic elevation measurement requirements, mapping-grade GPS receivers may provide an acceptable level of accuracy and reliability. Projects that require a high degree of elevation measurement accuracy may have to consider alternative methods, such as electronic digital measurement tools or manual means, of capturing elevation change across a forested landscape.