United States Department of Agriculture Forest Service
		Technology & Development Program
		February 1997	9771-2305-MTDC
		2200/2300/2400/2600/3400/ 5100/5300/5400/6700/7100

A test of two common GPS traverse methods was made under a typical West Coast forest canopy at the Clackamas GPS Test Network in Oregon. The test compared the accuracy of acreage measurements obtained using the "walk method" and "point method" of logging traverse data.

Methods

The Clackamas GPS Test Network on the Mt. Hood National Forest includes 13 stations (turning points) located on gentle terrain, with a geographic posi-tional accuracy of 5 cm or less. The network is under a moderate-to-heavy Douglas-fir and hemlock (approx. 24 to 40 inches d.b.h.) overstory with a vine maple and red alder understory, typical of a West Coast forest canopy. By selecting various station combinations, the area of the resulting traverse can be varied from 2.19 to 7.20 acres.

The GPS data used in this test were collected with a Trimble Navigation ProXL eight-channel receiver (Figure 1). It was set to log data in the manual 3-D mode with an elevation mask of 15° and with the PDOP (Position Dilution of Precision) and SNR (Signal-to-Noise Ratio) settings shown in the tables. All GPS data were differentially corrected using the MCORR400 utility in the Trimble Pathfinder PFINDER software. The reference station was the Portland CBS, which is approximately 50 miles from the site. All tests used the 12 points of Loop D of the test network with an area of 7.20 acres and a perimeter of 875 meters (0.54 miles).

Figure 1--Trimble ProXL GPS receiver.

The "walk method" traverse files were made by logging GPS data while walking the perimeter of Loop D using a generic data dictionary "area feature." The data logging rate was set for 5 seconds and an effort was made to log continuous data with no large gaps in the perimeter. This was accomplished by walking slower when the signal was intermittent. The average time required to walk the traverse and collect the data for each repetition using this method was about 25 minutes. Areas were computed by querying the corrected area feature in the PFINDER software. The results are shown in Table 1.

The "point method" traverse files were made by logging GPS data at each of the 12 "turning points" of Loop D using a generic data dictionary "point feature." The data logging rate was set for 1 per second with a minimum of 180 positions per point. The time required to collect the data for each repetition using this method varied from 60 to 145 minutes. Areas were computed using the "compute area" function of the PFINDER software. The results are shown in Table 2.

Table 2--Results of a GPS test at the Clackamas GPS Test Network using the "point method" of traversing under a dry canopy and under a wet canopy.

PDOP SNR Observed acres Actual acres Difference in acres Percent error Dry canopy (percent error) Dry Canopy (September 1996) Traverse 1 6 6 7.16 7.2 0.04 0.56 0.56 Traverse 3 6 6 6.99 7.2 0.21 2.92 2.92 Traverse 4 6 6 7.04 7.2 0.16 2.22 2.22 Wet Canopy (November 1996) Traverse 2 6 6 7.45 7.2 0.25 3.47 ---- Traverse 5 6 6 7.08 7.2 0.12 1.67 ----   Average 1.9%

Conclusions

Both traverse methods measured within 3.5% of the true area for the 7.20-acre traverse. The "walk method" appears to give slightly better results with an average error of 1.5% compared to 1.9% for the "point method" under the same test conditions. Also, the data collection was about twice as fast, 25 minutes compared to 60 to 145 minutes for the "point method." Based on these results, the "walk method" is the most efficient way to collect data to deter-mine the area of a traverse and is just as accurate as the "point method." Accuracy expressed as a percent of the area would be expected to increase with larger areas.

For further technical information, contact either MTDC at:

USDA Forest Service, MTDC
Building 1, Fort Missoula
Missoula, MT 59804-7294
Phone: (406) 329-3922
Fax: (406) 329-3719

or Ken Chamberlain at:

USDA Forest Service
Pacific Northwest Region
333 SW 1st Avenue, P.O. Box 3623
Portland, OR 97208
Phone: (503) 808-2417

Hard copies of this document may be ordered from:

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USDA Forest Service
Missoula Technology & Development Center
Building 1, Fort Missoula
Missoula, MT 59804-7294 Phone: (406) 329-3900
Fax: (406) 329-3719