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Leaf-off accuracy of mapping-grade GPS receivers. Pete Bettinger Michael Ransom James Rhynold University of Georgia Warnell School of Forestry and Natural Resources Athens, GA. 7th Southern Forestry and Natural Resources GIS Conference. Overview

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slide1

Leaf-off accuracy of mapping-grade GPS receivers

Pete Bettinger

Michael Ransom

James Rhynold

University of Georgia

Warnell School of Forestry and Natural Resources

Athens, GA

7th Southern Forestry and Natural Resources GIS Conference

slide2

Overview

This study represents an assessment of two recently-available GPS receiver

configurations used in mature southern pine and hardwood forests in the

Piedmont of Georgia.

Recent studies of GPS technology in the Piedmont of Georgia indicate that

mapping-grade GPS receivers, when used in forested conditions, can have

horizontal position error of 5 m or greater.

However, these studies were conducted with equipment 2-3 years old at the

time of data collection, and results were published almost 2 years after data

collection.

Given recent advances in GPS technology, we decided to enact a study of

two recently-introduced mapping-grade GPS receiver configurations in

winter-time to determine how much better current technology may be with

regard to horizontal position accuracy.

7th Southern Forestry and Natural Resources GIS Conference

slide3

Objectives

With the advent of the Wide Area Augmentation System (WAAS), it is

questionable whether post-collection differential correction is now necessary,

therefore it was our desire to report real-time accuracy levels that might be

obtained with the latest equipment.

As a result, our objective was to design a study that would allow us to

understand the current state of real-time mapping-grade GPS receiver

accuracy in leaf-off forested conditions.

7th Southern Forestry and Natural Resources GIS Conference

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Methods

GPS point positions were collected with two mapping-grade GPS

receivers during leaf-off (winter) conditions at the Whitehall Forest GPS

Test Course in Athens, GA.

The systems were (1) a Trimble GeoXT receiver with a self-contained antenna,

and (2) a Tripod Data Systems Ranger data collector equipped with a

Crescent A100 antenna.

During each sample, fifty position fixes were recorded.

7th Southern Forestry and Natural Resources GIS Conference

slide5

Methods

Hardwood stand:

60-70 years old

88 ft2 per acre basal area

144 trees per acre

Pine stand:

60-70 years old

86 ft2 per acre basal area

59 trees per acre

7th Southern Forestry and Natural Resources GIS Conference

slide6

Methods

7th Southern Forestry and Natural Resources GIS Conference

slide7

Methods

A 1-second interval separated the position fixes that were collected at each

visit to each control point.

Our intent was to visit each control point during periods of time when the

planned position dilution of precision (PDOP) was adequate; pre-planning

was thus accomplished using GPS planning software.

The WAAS signal was enabled for real-time augmentation of positions,

although the service was not necessarily available 100% of the time.

Since the method of data collection was randomized, and since visits to

control points were made within minutes of each other, the effect of WAAS

was assumed the same for the conditions tested.

Post-collection differential correction was not applied, to determine whether

the current accepted theory (differential correction is necessary) could be

challenged.

7th Southern Forestry and Natural Resources GIS Conference

slide8

Methods

Accuracy of the horizontal positions collected with each GPS receiver

configuration was evaluated by using the root mean square error (RMSE).

RMSE is the raw difference between collected measurements and the control

points, and places greater weight on larger errors since the error term is

squared.

Using raw RMSE to evaluate the differences between GPS receiver

configurations may make more sense to land managers than what the federal

government suggests, which is to report accuracy in RMSE ground distances

at the 95% confidence level.

Here, one would take the raw RMSE, and multiply it by a factor (1.7308) to

arrive at a value which suggests that one would be 95% confident that the

true accuracy is this resulting value (raw RMSE x 1.7308), or lower.

7th Southern Forestry and Natural Resources GIS Conference

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Methods

There are two distinct ways to evaluate RMSE:

(1) Determine the error for each of the 50 position fixes collected on each visit

to a control point, and then average the error (RMSE1).

(2) Generate an average position using the set of 50 position fixes collected

from each visit to each control point, then determine the error from this value

(RMSE2).

7th Southern Forestry and Natural Resources GIS Conference

slide10

Methods

A correlation analysis was performed between the RMSE values and the

average PDOP, SNR, air temperature, relative humidity, and atmospheric

pressure experienced with each visit.

Goodness of fit tests for a normal distribution of data were testing using

BestFit software.

These tests were necessary, since a Student's t-test was ultimately applied

to determine whether significant differences existed between stand types

(using the same receiver configuration) and between receiver configurations

(within the same stand type).

7th Southern Forestry and Natural Resources GIS Conference

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Results

Leaf-off RMSE

RMSE2

1.8

1.6

2.1

2.0

RMSE1

2.4

1.9

2.5

2.7

PDOP

3.25

3.93

3.59

4.44

SNR

44.1

43.6

41.7

40.9

Receiver / stand

GeoXT - Pine

- Hardwood

Crescent A100 - Pine

- Hardwood

Range of error:

0.2 m to 16 m (RMSE1).

0.2 m to 10 m (RMSE2).

7th Southern Forestry and Natural Resources GIS Conference

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Results

RMSE2

No significant differences in accuracy between stand types, when using the

same GPS receiver configuration.

No significant differences in accuracy between receiver configurations,

within the same stand type.

RMSE1

Similar story, except a significant difference between pine and hardwood

stand types when using the Crescent A100.

And, there were significant differences between the two GPS receiver

configurations in the hardwood stand.

7th Southern Forestry and Natural Resources GIS Conference

slide13

Results

Correlation Analysis

Little correlation between:

Planned PDOP and accuracy levels

SNR and accuracy levels

Relative humidity and accuracy levels

Atmospheric pressure and accuracy levels

The only interesting correlations were between air temperature and

RMSE1 (-0.517) and RMSE2 (-0.608), when using the Crescent A100 antenna

in the hardwood stand.

As air temperature increased in the hardwood stand, accuracy

of the Crescent A100 increased.

7th Southern Forestry and Natural Resources GIS Conference

slide14

Discussion

Recent studies have shown that under both leaf-on and leaf-off conditions,

enabling WAAS can increase the horizontal position accuracy of both

mapping-grade and consumer-grade GPS receivers.

Although in our study the WAAS signal was not available 100% of the time,

we assumed it was enabled sufficiently to simulate common data collection

practices.

Our work suggests that the current technology may be able to provide forest

managers with a level of quality that is acceptable for most forest management

purposes, although leaf-on conditions and other forest types should also

subsequently be assessed.

7th Southern Forestry and Natural Resources GIS Conference

slide15

Discussion

We found that horizontal position accuracy was on the order of 2 m when

WAAS is enabled in these receivers and post-collection differential correction

is not employed.

This is significant in that the accuracy of the receivers evaluated is greater

than recent studies suggest even without the use of differential correction,

revising our notion of how well GPS receivers can perform in real time in

forested conditions.

While post-processing was not performed here, since real-time horizontal

position accuracy was of interest to us, one might assume that horizontal

position accuracy might be improved about 1.0 m, assuming the results from

other studies are transferable to our results.

7th Southern Forestry and Natural Resources GIS Conference

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Discussion

In general, there was no significant difference in horizontal position accuracy

between the two receiver configurations when the error of an average position

(from a set of position fixes) was analyzed.

These results are therefore applicable to forest managers who desire real-time

data for assisting with forest management decisions.

7th Southern Forestry and Natural Resources GIS Conference

slide17

Discussion

In addition, in general there was no correlation between horizontal position

accuracy and PDOP (Positional Dilution of Precision), signal-to-noise ratio,

relative humidity, air temperature, and atmospheric pressure values at the

time of data collection.

However, one of the two receiver configurations seemed sensitive to air

temperature.

7th Southern Forestry and Natural Resources GIS Conference

slide18

Questions and Comments are Welcome

The assistance of LandMark Systems is greatly appreciated.

7th Southern Forestry and Natural Resources GIS Conference