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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 60s intervals) and calculated the elevation error of all GPS receiver measurements for both unprocessed and differentially corrected (processed) measurements. The average error was 2.7m (1.8 SD) for unprocessed and 0.4m (0.3 SD) for processed measurements when all GPS receiver data and point intervals (1, 30, and 60s) were combined at the open sky course. Average measurement error was 5.0m (4.6 SD) for unprocessed and 3.0m (2.8 SD) for processed measurements when grouping all GPS receiver data and point intervals (1, 30, and 60s) 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. [Copyright &y& Elsevier]

We collected measurements using five identical high-quality mapping-grade GPS receivers that were configured the same and collected data simultaneously in two distinctly different settings within a forest. Our primary objective was to determine whether measurement accuracies were different among the mapping-grade GPS receivers. A secondary objective was to determine whether measurement accuracies were different depending on whether receivers established their locations by taking a single 1-second measurement or by averaging 30 or 60 measurements. In the open-sky setting, where receivers had few obstructions overhead, we found that all five receivers recorded measurements with similar positional accuracies. Errors were lower when measurements were differentially corrected(postprocessed). We found an average error of 1.6 m for unprocessed data and an average error of 0.2 m for postprocessed data. Our results indicate that in open-sky conditions, all five receivers performed similarly when measurements were postprocessed. In addition, there was no significant difference in accuracy whether 1, 30, or 60 points were averaged, regardless of whether data were postprocessed. In the young-forest test course, examination of errors between receivers revealed that one receiver had significantly different errors compared with other receivers, which was likely the result of environmental influences on satellite signal strength and availability. We also found that measurement errors for all five receivers were significantly lower when measurements were postprocessed. On average, measurement errors were 5.9 m for unprocessed data and 1.4 m for postprocessed data. In analyzing individual receiver errors, no receiver had significantly different measurement errors whether 1, 30, or 60 measurements were recorded. [ABSTRACT FROM AUTHOR]