Recent Evaluations of GPS Technology in the Southeastern United States Pete Bettinger Professor Warnell School of Forestry and Natural Resources University of Georgia Athens, 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 points Southern hardwood and pine stands Horizontal Position Accuracy 1. A multipath study using a choke-ring antenna 2. One-year's experience with a consumer-grade receiver 3. A Study of Newer mapping-grade receivers Southeastern United States GPS Research Studies 2010 Society of American Foresters Convention

Multi-Path Study Horizontal Accuracy (2009)

Overview Receiver Configurations Trimble ProXR + GPS/MSK beacon antenna Trimble ProXR + Topcon JPL Spec CR-4 antenna Conditions Hardwood 40-90% canopy closure 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 Society of American Foresters Convention

Methods 36 samples - 4 visits to each sample 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. 1: Multipath Study (2009) 2010 Society of American Foresters Convention

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. 1: Multipath Study (2009) 2010 Society of American Foresters Convention

Receiver Trimble ProXR Trimble ProXR diff. corrected Choke ring Choke ring diff. corrected Lower Slope Mid- Slope Upper Slope Results Leaf-on (Summer) All units are meters 1: Multipath Study (2009) 2010 Society of American Foresters Convention

Results Leaf-off (Winter) All units are meters 1: Multipath Study (2009) Receiver Trimble ProXR Trimble ProXR diff. corrected Choke ring Choke ring diff. corrected Lower Slope Mid- Slope Upper Slope Society of American Foresters Convention

Results Number of position fixes seems to matter - up to a point 1: Multipath Study (2009) 2010 Society of American Foresters Convention

Discussion The choke ring antenna more effectively mitigated signal degradation. Real-time accuracy with the choke-ring configuration was about as good as the accuracy associated with differentially-corrected ProXR data. One might assume that 3.5 m of inaccuracy is associated with other factors than multipath (this is the uncorrected choke-ring data - differentially corrected data). Similarly, multi-path results in 2-12 m of inaccuracy depending on slope and canopy 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 1: Multipath Study (2009) 2010 Society of American Foresters Convention

Consumer-Grade Receiver Study Horizontal Accuracy (2010)

Overview Receiver Garmin Oregon 300 Conditions Young pine 80-90% canopy closure 15 yr old loblolly pine Older pine 30-40% canopy closure yr old loblolly / shortleaf pine Hardwood 40-90% canopy closure yr old southern hardwoods 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 Society of American Foresters Convention

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 ft 2 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 ft 2 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 ft 2 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. 2: One Year's Experience with a Consumer-Grade Unit (2010) 2010 Society of American Foresters Convention

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. 2: One Year's Experience with a Consumer-Grade Unit (2010) 2010 Society of American Foresters Convention

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.  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. 2: One Year's Experience with a Consumer-Grade Unit (2010) 2010 Society of American Foresters Convention

Worst RMSE Mean RMSE Forest type Young pine Older pine Hardwood Best RMSE Season Full year Results All units are meters 2: One Year's Experience with a Consumer-Grade Unit (2010) 2010 Society of American Foresters Convention

Worst RMSE Mean RMSE Forest type Young pine Older pine Hardwood Young pine Older pine Hardwood Young pine Older pine Hardwood Young pine Older pine Hardwood Best RMSE Season Fall Winter Spring Summer Results All units are meters 2: One Year's Experience with a Consumer-Grade Unit (2010) 2010 Society of American Foresters Convention

Results 2: One Year's Experience with a Consumer-Grade Unit (2010) 2010 Society of American Foresters Convention

Results 2: One Year's Experience with a Consumer-Grade Unit (2010) 2010 Society of American Foresters Convention

Results 2: One Year's Experience with a Consumer-Grade Unit (2010) 2010 Society of American Foresters Convention

Results 2: One Year's Experience with a Consumer-Grade Unit (2010) 2010 Society of American Foresters Convention

Results 2: One Year's Experience with a Consumer-Grade Unit (2010) 2010 Society of American Foresters Convention

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 temperature Relative humidity Atmospheric pressure Solar wind speed Planned PDOP Season of the year 2: One Year's Experience with a Consumer-Grade Unit (2010) 2010 Society of American Foresters Convention

Results There seemed to be some interaction between the placement of the test points and the surrounding trees. 2: One Year's Experience with a Consumer-Grade Unit (2010) 2010 Society of American Foresters Convention

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 2: One Year's Experience with a Consumer-Grade Unit (2010) 2010 Society of American Foresters Convention

Mapping-Grade Receiver Study Horizontal Accuracy (2010)

Overview Receiver Configurations Trimble GeoXT (2008) TDS Ranger / Crescent A100 antenna Conditions Older pine 30-40% canopy closure yr old loblolly / shortleaf pine Hardwood 40-90% canopy closure yr old southern hardwoods 3: A Study of Newer Mapping-Grade Units (2010) RURALS: Review of Undergraduate Research in Agricultural and Life Sciences. Volume 5, number 1, article Society of American Foresters Convention

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: years old, 88 ft 2 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: 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. 3: A Study of Newer Mapping-Grade Units (2010) 2010 Society of American Foresters Convention

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. 3: A Study of Newer Mapping-Grade Units (2010) 2010 Society of American Foresters Convention

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.  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. 3: A Study of Newer Mapping-Grade Units (2010) 2010 Society of American Foresters Convention

Avg. PDOP Avg. SNR Forest type Older pine Hardwood Older pine Hardwood Avg. RMSE Receiver GeoXT Crescent A100 Results All units are meters 3: A Study of Newer Mapping-Grade Units (2010) 2010 Society of American Foresters Convention

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. 3: A Study of Newer Mapping-Grade Units (2010) 2010 Society of American Foresters Convention

Results Within a forest type, there was no significant relationship between GPS accuracy and: Relative humidity Atmospheric pressure Actual PDOP Signal-to-noise ratio There seemed to be some correlation between air temperature and the Crescent A100 results, but only in the hardwood stand: As air temperature increased from around 40 , accuracy increased. 3: A Study of Newer Mapping-Grade Units (2010) 2010 Society of American Foresters Convention

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 3: A Study of Newer Mapping-Grade Units (2010) 2010 Society of American Foresters Convention

Comments and Discussion Southeastern United States GPS Research Studies 2010 Society of American Foresters Convention