Recent Evaluations of GPS Technology in the Southeastern ...gps-test-site.uga.edu/Files/UGA_GPS_TEST_SITE...Recent Evaluations of GPS Technology in the Southeastern United States Pete

Recent Evaluations of GPS Technology in the Southeastern United States

Pete BettingerProfessorWarnell School of

Forestry and Natural Resources

University of GeorgiaAthens, GA 30602

Over the last four years several research studies concerning the accuracy of consumer- and mapping-grade GPS receivers have been completed on the Whitehall Forest GPS Test Site in Georgia.

These studies have attempted to correlate GPS accuracy with forest type, weather conditions, and times of year, and provide insight into the relative accuracy of the technology when used under a tree canopy.

University of Georgia Whitehall Forest GPS Test Site

37 control pointsSouthern hardwood and pine stands

Horizontal Position Accuracy

1. A multipath study using a choke-ring antenna2. One-year's experience with a consumer-grade receiver3. A Study of Newer mapping-grade receivers

Southeastern United States GPS Research Studies

2010 Society of American Foresters Convention

Multi-Path StudyHorizontal Accuracy (2009)

Overview

Receiver Configurations

Trimble ProXR + GPS/MSK beacon antennaTrimble ProXR + Topcon JPL Spec CR-4 antenna

Conditions

Hardwood40-90% canopy closure60-70 yr old southern hardwoods

Nine test points from those available at the Whitehall Forest GPS Test Site in Athens, GA.

1: Multipath Study (2009)

Forest Science. Volume 55, number 2, pages 109-116.


Methods

36 samples - 4 visits to eachsample point

• Test points were located in three slope positions, and represent the best choices for comparison.

• All points were visited during both leaf-on and leaf-off conditions, in random order.



Methods

• 200 position fixes were collected at each test point during each visit.

• The recording of position fixes (waypoints) was performed automatically, with 2 seconds elapsing between fixes.

• During each visit, the GPS receiver was plumbed directly over each test point (using a staff and a plumb bob), and the lead investigator stood on the north side of each as data was collected.

• The order of visit to each of the nine test points was randomized each day.

• Consistent parameter settings and environmental variables were assumed.

• WAAS was not available.

• Differential correction was applied.



Receiver

Trimble ProXRTrimble ProXR diff. corrected

Choke ringChoke ring diff. corrected

LowerSlope

6.53.1

2.80.3

Mid-Slope

8.12.8

2.00.2

UpperSlope

5.63.0

2.00.2

Results

Leaf-on(Summer)

All unitsare meters



Results

Leaf-off(Winter)

All unitsare meters


Receiver

Trimble ProXRTrimble ProXR diff. corrected

Choke ringChoke ring diff. corrected

LowerSlope

8.71.9

2.80.3

Mid-Slope

7.31.9

2.10.2

UpperSlope

5.71.9

2.40.2


Results

• Number of position fixes seems to matter - up to a point



Discussion

The choke ring antenna more effectively mitigated signal degradation.Real-time accuracy with the choke-ring configuration was about as goodas the accuracy associated with differentially-corrected ProXR data.

One might assume that 3.5 m of inaccuracy is associated with other factorsthan multipath (this is the uncorrected choke-ring data - differentiallycorrected data).

Similarly, multi-path results in 2-12 m of inaccuracy depending on slope andcanopy position and time of year.

WAAS was not available.

We would expect accuracy to be higher if WAAS was available and enabled

Post-processing (differential correction) was performed.

Accuracy would be lower if differential correction was not applied



Consumer-Grade Receiver StudyHorizontal Accuracy (2010)

Overview

Receiver

Garmin Oregon 300

Conditions

Young pine80-90% canopy closure15 yr old loblolly pine

Older pine 30-40% canopy closure60-70 yr old loblolly / shortleaf pine


2: One Year's Experience with a Consumer-Grade Unit (2010)

International Journal of Mathematical and Computational Forestry & Natural-Resource Sciences. Volume 2, number 2, pages 153-160.


Methods

Three test points (numbers 6, 31, and 37) from those available at the Whitehall Forest GPS Test Site in Athens, GA.

All three test points are located in upper slope positions, and represent the best choices for comparison among the three forest types.

• Test point 37 is located in a young loblolly pine plantation (15 years old, 130 ft2 per acre basal area, 450 trees per acre, southwest aspect, 8% slope, 700 ft elevation, 95% canopy closure).

• Test point 31 is located in an older loblolly / shortleaf pine stand (60-70 years old, 86 ft2 per acre basal area, 59 trees per acre, south aspect, 2% slope, 730 ft elevation, 50% canopy closure).

• Test point 6 is located in an older hardwood stand (60-70 years old, 88 ft2 per acre basal area, 144 trees per acre, northeast aspect, 18% slope, 690 ft elevation, 90% canopy closure in the summer, 40-50% canopy closure in the winter). The older hardwood stand around test point 6 is dominated by oak and hickory.



Methods

The three test points were visited once per test day (289) over the course of a year (September 15, 2008 to September 14, 2009).

• Fifty position fixes were collected at each test point during each visit.

• The order of visit to each of the three test points was randomized each day.

• To ensure consistent parameter settings and environmental variables throughout the year, the Wide Area Augmentation System (WAAS) was disabled, since it typically is not available 100% of the time, and the GPS receiver was unable to record whether the system was being used at any one point in time.



Methods

At the beginning of each day's visit, some warm-up time (up to five minutes on occasion) was required to ensure that a sufficient number of satellites were available to provide a reasonable position fix.

• The recording of position fixes (waypoints) was performed manually, with 2-3 seconds elapsing between fixes.

• We were unable to use a standard period of time between position fixes, because the GPS receiver did not have the ability to automate data collection.




WorstRMSE

38.246.228.7

MeanRMSE

11.96.67.9

Forest type

Young pineOlder pineHardwood

BestRMSE

0.50.20.8

Season

Full year

Results

All unitsare meters



WorstRMSE

28.646.218.3

30.119.720.5

38.219.528.7

20.322.518.1

MeanRMSE

11.36.27.1

12.46.56.3

11.85.58.3

11.15.07.4

Forest type





BestRMSE

5.30.81.7

0.50.20.9

2.00.50.8

2.51.00.9

Season

Fall

Winter

Spring

Summer

Results

All unitsare meters



Results



Results



Results



Results



Results



Results

The only significant relationship was between GPS accuracy and forest type.

Within a forest type, there was no significant relationship between GPS accuracy and:

Air temperatureRelative humidityAtmospheric pressureSolar wind speedPlanned PDOPSeason of the year



Results

There seemed to be some interaction between the placement of the test points and the surrounding trees.



Discussion

This was a year-long study.

We sampled a variety of times of day, and weather conditions

WAAS was not enabled.

We would expect accuracy to be higher if WAAS was enabled

Post-processing (differential correction) was not performed.

Accuracy may be higher when differential correction is applied

Only one GPS receiver was studied.

Time, effort, funding considerations



Mapping-Grade Receiver StudyHorizontal Accuracy (2010)

Overview

Receiver Configurations

Trimble GeoXT (2008)TDS Ranger / Crescent A100 antenna

Conditions

Older pine 30-40% canopy closure60-70 yr old loblolly / shortleaf pine


3: A Study of Newer Mapping-Grade Units (2010)

RURALS: Review of Undergraduate Research in Agricultural and Life Sciences. Volume 5, number 1, article 2.


Methods

Six test points (numbers 3, 6, 11, 28, 31, and 32) from those available at the Whitehall Forest GPS Test Site in Athens, GA.

• All six test points are located in upper slope positions, and represent the best choices for comparison among the three forest types.

• Hardwoods: 60-70 years old, 88 ft2 per acre, 144 trees per acre, northeast aspect, 18% slope, 690 ft elevation, 90% canopy closure in the summer, 40-50% canopy closure in the winter. The stand is dominated by oak and hickory.

• Older pine: 60-70 years old, 86 ft2 per acre, 59 trees per acre, south aspect, 2% slope, 730 ft elevation, 50% canopy closure. The stand is dominated by oak and hickory.



Methods

The test points were visited once per test day (10) over the course of one month(February 2009).

• Fifty position fixes were collected at each test point during each visit.

• The order of visit to each of the three test points was randomized each day.

• To ensure consistent parameter settings and environmental variables throughout the year, the Wide Area Augmentation System (WAAS) was enabled.



Methods

At the beginning of each day's visit, some warm-up time (up to five minutes on occasion) was required to ensure that a sufficient number of satellites were available to provide a reasonable position fix.

• The recording of position fixes (waypoints) was performed automatically, with 1 second elapsing between fixes.




Avg.PDOP

3.33.9

3.64.4

Avg.SNR

44.143.6

41.740.9

Forest type

Older pineHardwood

Older pineHardwood

Avg.RMSE

1.81.6

2.12.0

Receiver

GeoXT

Crescent A100

Results

All unitsare meters



Results

From the average of 50 position fixes:

• The best estimated position was 0.2 m.• The worst estimated position was 9.9 m.

When examining the complete set of individual position fixes:

• The single worst position fix was 16 m off.

No significant differences between forest type when using the same equipment.

No significant differences within a forest type when comparing the two systems.



Results

Within a forest type, there was no significant relationship between GPS accuracy and:

Relative humidityAtmospheric pressureActual PDOPSignal-to-noise ratio

There seemed to be some correlation between air temperature and the Crescent A100results, but only in the hardwood stand:

As air temperature increased from around 40°, accuracy increased.



Discussion

This was a winter-time (leaf-off) study.

Accuracy may be higher in winter than in summer

WAAS was enabled, but not necessarily used 100% of the time.

We would expect accuracy to be lower if WAAS was disabled

Post-processing (differential correction) was not performed.

Accuracy may be higher when differential correction is applied



Comments and Discussion

Southeastern United States GPS Research Studies


http://warnell.forestry.uga.edu/Warnell/Bettinger/GPS/UGA_GPS.htm

Documents

Recent Evaluations of GPS Technology in the Southeastern ...gps-test-site.uga.edu/Files/UGA_GPS_TEST_SITE...Recent Evaluations of GPS Technology in the Southeastern United States Pete