gps accuracy testing using: “rtk” (real time kinematic) & “one step transformations”...

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


Err

or

in F

eet

EAST

NORTH

ELEV

Poly.(NORTH)

Poly.(EAST)

Poly.(ELEV)


-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


Err

or

in F

ee

t

EAST

NORTH

ELEV

Poly.(NORTH)

Poly.(EAST)

Poly.(ELEV)


-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)


-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


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


• 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.


• 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.


• When:

Inside transformation 3 to 6 mi. to base GDOP less than 6 VDOP less than 4

• 4 out 0.10 ft. in height


• Measurements exceeding 0.10 ft.

Vertical Error in ft. Miles From Base0.13 3.00.11 5.20.14 5.70.11 5.8


• 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:


• Ok for Confidence Points and Hard Surfaces

• Distance from base and outside transformation degrades accuracy.

• Expect “Fliers”


• GDOP over 6 or VDOP over 4 will generate poor results.

• Keep One Step transformation size to 6 miles square or less.


• 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

gps accuracy testing using: “rtk” (real time kinematic) & “one step transformations”...

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