AffordableGPS

Affordable GPS for Forest Engineers & Loggers

Michael G. WingAaron Eklund

Forest Engineering DepartmentOregon State University

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Project Goals

• Test the accuracy and reliability of affordable Global Positioning Systems (GPS)– Different forest settings– Different times of satellite signal quality– Determine the expected range of accuracy– A focus on horizontal measurements, and not

on vertical (elevation)– Are affordable GPS any good?

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What is affordable GPS?

• Units roughly in the $300 price range or less

• Garmin, Magellan, Lowrance, Brunton, and others

• These units typically do not have post processing capabilities and do not allow point averaging

• Usually confined to a single hand-held piece of equipment

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Affordable GPS• A 15 to 20 m horizontal accuracy is

claimed by most manufacturers• GPS packaging now proclaims 2-3 m

accuracy with WAAS capabilities• Some allow map products to be loaded

onto the unit– Can view your position relative to major

features• Most will allow you to digitally download

coordinates that you collect– A limit of 500 points (coordinate pairs) is

standard– Hopefully you receive the cable with the unit

when you purchase it– Positions can be viewed in a mapping

program

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GPS Units• Garmin

– Map76S ($310)– GPS V ($320)– Etrex Vista ($245)– Geko ($220)

• Magellan– Meridian Platinum ($280)– SporTrak Map ($150)

• Some of these units have other versions and not all come with a download cable

• The most full featured unit is the GPS V• The Geko even features games

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Group Photo

SporTrak, Map 76s, GPS V, Etrex, Geko, Meridian

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Study Design

• McDonald Forest (12,000 acres), located just north of Corvallis

• Three test courses were created– Open canopy

• Two visits– Closed canopy– Partially closed canopy

• 12 2x2” wooden hubs were set to mark measurement stations at each test course

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Course Design• Points were set near enough so that

operators at each station could communicate with other station operators

• All stations on each course were surveyed using a Digital Total Station

• Station measurements from each course were tied to two existing survey-grade GPS benchmarks– Aluminum caps set in concrete– Both benchmarks located near the courses in

McDonald Forest– Accuracy rated at 0.033 of a foot or better– Measurements were in US Survey Feet in NAD

83/91 (State Plane)

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Typical Test Course

60°

25 feet

15 feet

Inner core stations (six): Affordable GPSOuter core stations (six): Trimble ProXR

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Data Collection Design• Four periods of PDOP were defined using

GPS mission planning software• Rising (3-7 PDOP)• Rising (3-6 PDOP)• Rising (3-7 PDOP)• Off the charts (> 15 PDOP)

• Collection Process– Since you can’t occupy the same space with

multiple units• Place a GPS unit at each station

– Verbally synchronize the collection of 25 points at each station

– Move to next station and repeat– Repeat entire process above three times for

each PDOP period at each measurement course

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Collection Details

• Operators facing south so that affect of body on satellite signals minimized

• All GPS units mounted on a wooden staff so that antenna at same height

• 4 courses * 6 stations * 3 round trips * 25 points * 6 GPS units = 10,800 points

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The Open Test Course

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Methods• After data collection, data downloaded

from GPS units using ExpertGPS• The average of the UTM easting and

northing measurements from each group of 25 GPS points were used to determine GPS coordinates at measurement stations

• GPS coordinates at all stations were compared to the Total Station-derived coordinates by computing a horizontal distance (error) to determine accuracy

• To determine reliability, standard deviations of the error were computed

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RECREATION GRADE GPS TESTING RESULTS 10-May-04 All units in metersCourse A Course B Course A (Poor PDOP)

PDOP Avg Error Std Dev Avg Error Std Dev Avg Error Std Dev Total ErrorG_Geko Good 1.9 1.4 12.2 6.6 3.6 0.8 17.7

Medium 2.7 1.7 11.8 5.1 2.7 1.4 17.2Poor 3.6 1.8 11.9 5.9 4.6 0.9 20.1

G_Etrex Good 1.0 0.5 7.0 2.7 2.0 0.5 10.0Medium 1.4 1.0 6.6 3.8 2.2 0.9 10.2Poor 1.4 0.9 6.6 2.1 1.8 1.0 9.9

G_76 Good 1.5 0.8 4.5 2.0 2.7 0.4 8.7Medium 1.8 0.4 3.7 2.0 1.8 0.5 7.4Poor 1.1 0.8 5.0 2.8 2.7 0.6 8.7

M_Merid Good 11.1 1.6 6.6 1.7 2.2 0.8 19.9Medium 19.3 10.0 5.5 2.8 2.3 0.5 27.0Poor 26.9 1.2 4.0 1.2 2.0 0.9 32.9

M_STrak Good 1.2 0.5 6.6 1.8 2.3 0.8 10.2Medium 1.7 0.9 4.6 0.2 3.2 1.1 9.5Poor 2.4 1.8 6.8 2.4 2.7 0.7 11.9

G_V Good 1.1 0.8 5.5 2.7 3.0 0.7 9.6Medium 1.1 0.5 7.3 3.0 2.5 0.7 10.9Poor 1.1 0.5 7.7 2.5 2.3 0.7 11.1

AVG 4.6 6.9 2.6

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Course A- Open Canopy

• Average Accuracy: 4.6 m• Standard Deviation: 0.5 – 10 m• Most Accurate: GPS V (3.3 m total

error)– This is the sum of the average errors from

the three rounds made at Course A• Most Reliable: GPS V (1.8 m total

standard deviation)– This is the sum of the average standard

deviations from the three rounds made at Course A

• Least Accurate: Meridian (57.3 m total error)

• Least Reliable: Meridian (12.9 m total standard deviation)

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Course B- Closed Canopy

• Average Accuracy: 6.9 m• Standard Deviation: 0.2 – 3.8 m• Most Accurate: Map 76s (13.2 m total

error)• Most Reliable: SporTrak (4.4 m total

standard deviation)• Least Accurate: Geko (35.9 m total

error)• Least Reliable: Geko (17.6 m total

standard deviation)

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Course A Revisited- Open Canopy (Poor PDOP)• Average Accuracy: 2.6 m !• Standard Deviation: 0.5 – 1.1 m !• Most Accurate: Etrex (6.0 m total error)• Most Reliable: Map 76s (1.5 m total

standard deviation)• Least Accurate: Geko (10.9 m total

error)• Least Reliable: Geko (3.1 m total

standard deviation)

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GPS Overall Summary• Average Error

– Map 76s (4.1 m)– Etrex (5.0 m)– GPS V (5.3 m)– SporTrak (5.3 m)– Geko (9.2 m)– Meridian (13.3 m)

• Total Deviation– SporTrak (10.2 m)– Map 76s (10.3 m)– GPS V (12.1 m)– Etrex (13.4 m)– Meridian (20.7 m)– Geko (25.6 m)

• Remember: accuracies were based on 25 points being collected at each station

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Tips for your GPS work

• Realize that you’ll need to pay for higher accuracy and precision

• Use reliable mission planning software to choose the best times– Number of satellites– PDOP values– http://www.trimble.com/planningsoftware.s

html (free mission planning software)• Will need data manipulation software

(GIS for basic measurements)• Get some practice with your GPS

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Value to consumers is increasing… Garmin eTrex Legend

• Color screen– Visible in sun or rain

• USB download• MapSource data

(cities, roads, streams) preloaded

• Removable 32 mbSD card

• 36 hour battery life (AA)

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Consumer GPS Training

• GPS and GIS Applications– Friday, April 6 2007– Oregon State University– GPS fundamentals– Collect, download, and map data from a

Garmin Legend GPS receiver

• Registration and information:– http://www.cof.orst.edu/cof/fe/research

/gis/workshop/index.htm

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Some final

advice:

Be careful

what you take to

the field!