Balancing horizontal accuracy and data collection efficiency with mapping-grade GPS receivers

Abstract

Weused mapping-gradeglobal positioning system(GPS) receivers to examinepost-processed horizontalmeasurement accuracies during nine separate visits to six sites that each contained 10 test points within a second-growth Douglas-fir forest. Our primary objectivewas to compare GPS accuracies resulting fromtwo different receiver configurations that vary in productivity and accuracy. A secondary objective was to determine how accuracies would change with recording interval (whether we averaged 1, 5, 10, 30 or 60 positions).We also examined which environment- and GPS-related factors most influenced GPS accuracy. Root mean square error (RMSE) at the 95 per cent confidence level (RMSE95) using the 'default' GPS setting (position dilution of precision (PDOP): 6; SNR 39; minimum elevation 158) was 6.0 m, while RMSE 95 using 'maximum' settings (PDOP: 20; SNR 33; minimum elevation: 58) was 8.0 macross all sites and recording intervals. Using default settings,RMSE95 decreased from7.3 mwhenaveraging one position to 4.4 m when averaging 60. For maximum settings, RMSE95 decreased from 9.1 to 6.5 m. Our best models selected basal area, dilution of precision and weather, but not terrain. When using default settings the averagewait timewas 55 sec when collecting one position and 236 sec when averaging 60. Using maximum settings, averagewait timewas8 secwhencollectingoneposition,and74 secwhenaveraging60positions.Theerrors that we report are acceptable for many forestry applications. These results should help users to balance productivity with accuracy when using mapping-grade GPS receivers in second-growth Douglas-fir forests. © 2014 © Institute of Chartered Foresters, 2014. All rights reserved.