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GPS Accuracy TestingGPS Accuracy Testing
Using:Using:““RTK” (Real Time Kinematic)RTK” (Real Time Kinematic)
&&““One Step Transformations” (Site One Step Transformations” (Site
Calibrations)Calibrations)
GPS Accuracy TestingGPS Accuracy Testing
• Procedure
• Results
• Summary and conclusions
ProcedureProcedure
•Found a suitable test site
•Established “True” positions
NGS 1st Order Vertical Control
10+ Miles
NGS Cooperative
Base Network Control Station
For Horiz.
Measured 3 to 4 points in each area using Rapid Static
•Set points at 1\2 Mi. increments
•Used new control
•Tied with Rapid Static
•Transformed to North Zone
•Calculated one combined scale
factor
•Converted all to LDP
•Ran Digital Levels
•“True” positions established
•Collected RTK data to compare against “true”
positions
•Set Base at north end
•3 to 4 Transformation points in each area
•All the rest measured as topo
Repeated with base at the south end
Used:
•RTK WGS 84 data collected with base
at north end
•LDP coordinates from rapid static
Created the following
transformations:
3/4 Mile Square
3/4 x 6 Mile
3/4 x 10 1/2 Mile
Used data from base at south
end
3/4 Mile Square
3/4 x 5 Mile
3/4 x 10 1/2 Mile
After comparison decided to test East-
West
Established “True” positions in an East-
West orientation
Used Rapid Static GPS only for x, y, & z
Used primary control
4 Transformation
points in 3 areas Points at 1\2 Mi.
increments using Rapid
Static
Converted to LDP
RTK with base at West end
Measured topo and
transformation points
Repeated with RTK base at
east end
Transformation at West end
1 Mile Square
Same pattern of transformations
as the North-South course
1 x 5 Mile
1 x 11 1/2 Mile
Transformation at East end
1 Mile Square
1 by 6 Mile Square
1 by 11 1/2 Mile Square
Each set of data was converted with 3 transformations
Then compared to “True” Values
ResultsResults
3/4 Mile Transformation
-1.00
-0.80
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
0 2 4 6 8 10
Distance From North Base in Miles
Err
or
in F
ee
t
EAST
NORTH
ELEV
Poly.(NORTH)
Poly.(EAST)
Poly.(ELEV)
5 Mile Transformation
-1.00
-0.80
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
0 2 4 6 8 10
Distance From North Base in Miles
Err
or
in F
eet
EAST
NORTH
ELEV
Poly.(NORTH)
Poly.(EAST)
Poly.(ELEV)
10 Mile Transformation
-1.00
-0.80
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
0 2 4 6 8 10
Distance From North Base in Miles
Err
or
in F
ee
t
EAST
NORTH
ELEV
Poly.(NORTH)
Poly.(EAST)
Poly.(ELEV)
1 Mile Transformation
-1.00
-0.80
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
0 2 4 6 8 10
Distance From West Base in Miles
Err
or
in F
ee
t
EAST
NORTH
ELEV
Poly.(NORTH)
Poly.(EAST)
Poly.(ELEV)
5 Mile Transformation
-1.00
-0.80
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
0 2 4 6 8 10
Distance From West Base in Miles
Err
or
in F
ee
t
EAST
NORTH
ELEV
Poly.(NORTH)
Poly.(EAST)
Poly.(ELEV)
11 Mile Transformation
-1.00
-0.80
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
0 2 4 6 8 10
Distance From West Base in Miles
Err
or
in F
ee
t
EAST
NORTH
ELEV
Poly.(NORTH)
Poly.(EAST)
Poly.(ELEV)
Summary & ConclusionsSummary & Conclusions
Goal was to answer the Goal was to answer the following:following:
• Verify the accuracy of RTK.
• Can hard surfaces be collected with enough accuracy to pass confidence point standards?
• Can confidence points be collected with GPS single observation?
Goal was to answer the Goal was to answer the following:following:
• How are elevations affected as you go outside of a transformation (site calibration) set?
• How is accuracy affected by distance from the base?
• What is the break point?
What is the accuracy of RTK?What is the accuracy of RTK?
Horizontal =10mm (0.033 ft.) + 1 ppm
Vertical =20mm (0.066 ft.) + 1 ppm.
Recommends One Step Transformations less than 6 mi. sq.
• Leica states that 99.9% System 1200
What is the accuracy of RTK?What is the accuracy of RTK?
• One ppm for 6 mi. = 0.032 ft.
• Plus 0.066 ft. vertical spec.
• Equals 0.098 ft.
• 99.9% should equal or be less than 0.10 ft.
What is the accuracy of RTK?What is the accuracy of RTK?
• 180 measurements & many transformations
• When: Inside transformation 0 to 3 mi. to base GDOP less than 6 VDOP less than 4
• All less than 0.10 ft.
What is the accuracy of RTK?What is the accuracy of RTK?
• When:
Inside transformation 3 to 6 mi. to base GDOP less than 6 VDOP less than 4
• 4 out 0.10 ft. in height
What is the accuracy of RTK?What is the accuracy of RTK?
• Measurements exceeding 0.10 ft.
Vertical Error in ft. Miles From Base0.13 3.00.11 5.20.14 5.70.11 5.8
What is the accuracy of RTK?What is the accuracy of RTK?
• There will be some “Fliers”.
• Good conditions at test site.
Can hard surfaces be collected Can hard surfaces be collected with enough accuracy to pass with enough accuracy to pass confidence point standards?confidence point standards?
• Yes
• Be aware of the pitfalls
Can confidence points be Can confidence points be collected with GPS single collected with GPS single
observation?observation?
• Yes
• Single GPS observation O.K. for topo
• Single GPS observation O.K. for confidence points
How are elevations affected as How are elevations affected as you go outside of a you go outside of a transformation set?transformation set?
• Elevations and Northings adversely affected
• Eastings to a lesser degree
• Affect not always immediate
How is accuracy affected by How is accuracy affected by distance from the base?distance from the base?
• Not as well defined
• Inconsistent beyond 5 to 6 mi.
What is the break point?What is the break point?
• 5 to 6 miles from base
• GDOP over 6
• VDOP over 4
What is the break point?What is the break point?
• This testing indicates 98% better than 0.10 ft. at 0 to 6 miles
• 0 to 5 miles from base 99.5%
• 0 to 3 miles 100%
ConclusionConclusion
The following conclusions are based on the
test results:
ConclusionConclusion
• Ok for Confidence Points and Hard Surfaces
• Distance from base and outside transformation degrades accuracy.
• Expect “Fliers”
ConclusionConclusion
• GDOP over 6 or VDOP over 4 will generate poor results.
• Keep One Step transformation size to 6 miles square or less.
ConclusionConclusion
• Stay close to the base, the closer the better within reason, no farther than 3 miles if possible.
• Stay within the transformation boundary as close as possible.
Any Questions?
We’re all ears