

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 IIB2 and IIB3). 

Table IIB2: 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 IIB3: 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 IIB4). 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 IIB1).
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 IIB2). 

Table IIB4: Magellan SporTrak® vs. averaged locations as determined from postprocessed 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 IIB4: Trimble GPS and Magellan GPS generated boundaries of tall fescue field, Sauvie Island. 



Figure IIB5: Trimble GPS and Magellan GPS generated boundariesof a ponded area in a wheat field, Sauvie Island. 


