AffordableGPS
Affordable GPS for Forest Engineers & Loggers
Michael G. WingAaron Eklund
Forest Engineering DepartmentOregon State University
AffordableGPS
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?
AffordableGPS
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
AffordableGPS
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
AffordableGPS
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
AffordableGPS
Group Photo
SporTrak, Map 76s, GPS V, Etrex, Geko, Meridian
AffordableGPS
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
AffordableGPS
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)
AffordableGPS
Typical Test Course
60°
25 feet
15 feet
Inner core stations (six): Affordable GPSOuter core stations (six): Trimble ProXR
AffordableGPS
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
AffordableGPS
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
AffordableGPS
The Open Test Course
AffordableGPS
AffordableGPS
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
AffordableGPS
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
AffordableGPS
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)
AffordableGPS
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)
AffordableGPS
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)
AffordableGPS
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
AffordableGPS
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
AffordableGPS
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)
AffordableGPS
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
AffordableGPS
Some final
advice:
Be careful
what you take to
the field!