Part II: Section B Using GPS Technology
 4.3 Test Results
Area Estimation

The surface area of the inside of each field was estimated by connecting the points for each data set. Straight lines were used to complete the polygon and areas calculated using the formula for surface area by coordinate points. The Trimble Pathfinder Pro XR unit can collect polygon information directly by walking the perimeter. We recorded positions every 5 seconds to create a traverse perimeter. The difference in area estimation between the performances of each unit was 1% for the ponded area and 6% for the tall fescue field (Tables IIB-2 and IIB-3).

Table IIB-2: Comparing Area (acres) of a Tall Fescue Field.

 GPS Unit Tall Fescue Magellan from point features 21.5 Trimble from point features 20.2 Trimble from traverse polygon 20.3

Table IIB-3: Comparing Area (acres) of a Ponded Area in a Wheat Field.

 GPS Unit Wheat (ponded area) Magellan from point features 11.5 Trimble from point features 11.4

Measured Distance Between Points

On average, the difference for all positions between the two units was 5 meters (Table IIB-4). For the tall fescue field, there was a positive shift of 1 meter in the easting (X axis), while a negative shift of -2 meters was recorded in the northing (Y axis) (Figure IIB-1).

For the wheat field, there was a negative shift of 2 meters in the easting (X axis), while a positive shift of 4.34 meters was recorded in the northing (Y axis) (Figure IIB-2).

Table IIB-4: Magellan SporTrak® vs. averaged locations as determined from post-processed differentially corrected locations obtained with a Trimble Pro XR ® GPS unit (unit: meter).

Boundary - Tall Fescue Field
 0 Easting Difference X Northing Difference Y Change in Measured Distance PDOP Mean 0.89 -2 5 2.40 STDV 3.72 3.94 2.92 0.59 Min 7.90 6.73 12.75 4.80 Max -7.26 -11.36 0.66 1.70

Boundary - Ponded Area: Wheat Field
 0 Easting Difference X Northing Difference Y Change in Measured Distance PDOP Mean -2.09 4.34 4.9 2.91 STDV 0.74 2.30 2.0 0.54 Min -0.78 7.78 8.2 4.00 Max -3.72 0.23 1.7 2.10

Figure IIB-4: Trimble GPS and Magellan GPS generated boundaries of tall fescue field, Sauvie Island.

Figure IIB-5: Trimble GPS and Magellan GPS generated boundariesof a ponded area in a wheat field, Sauvie Island.

 << previous page next page >>
 1. What is GPS? 2.How Does GPS Work? 3. Instuctions for Using SporTrak GPS Recievers 4. Performance Testing - 4.1 Equipment - 4.2 Field Test - 4.3 Test Results - 4.4 Conclusions 5. Glossary