The global positioning system (GPS) has come into widespread use for many forestry applications. However, there is still some uncertainty as to how receivers perform in dense forest, where trees interfere with the GPS signal and increase error. These two manuscripts examine GPS accuracy and overall effectiveness within second growth Douglas-fir forest across variable terrain and stand densities. This research was specifically concerned with how GPS filters (settings) affect accuracies and wait times. Two settings were examined: maximum (PDOP 20, SNR 33, and minimum elevation 5 degrees) and default (PDOP 6, SNR 39, and minimum elevation 15 degrees). The first maximizes productivity with a corresponding loss in accuracy. To examine accuracy we surveyed six 0.4-hectare test courses and compared GPS measurements to surveyed measurements.
The first manuscript (chapter 2) examines the ability of mapping-grade GPS receivers to locate a horizontal position. It was found that the number of averaged positions (recording interval), weather, basal area, and satellite geometry or dilution of precision (DOP) significantly influenced GPS accuracy, while terrain and receiver settings did not. Average errors were 2.9 m and horizontal accuracy at the 95% confidence level was 6.8 m when using maximum settings and averaging 30 positions. These results suggest that mapping-grade GPS receivers are acceptable for many forestry applications under canopy, but not for high order surveying work.
The second manuscript (chapter 3) quantifies area and perimeter errors, traverse times, and overall time and cost effectiveness for area measurement. It was determined that averaging 30 positions and using maximum settings was the most time effective combination of recording interval and settings. This combination of recording interval and settings proved slightly more cost effective than other traditional surveying methods such as a laser and digital compass. Average absolute percent area errors when averaging 30 positions and using maximum settings were 2.6%. Average absolute percent perimeter errors were 2.0%.