One year's experience with a recreation-grade GPS receiver

Transcription

1 One year's experience with a recreation-grade GPS receiver Pete Bettinger Warnell School of Forestry and Natural Resources University of Georgia Athens, GA Songlin Fei Department of Forestry University of Kentucky Lexington, KY

2 Overview There are three general classes of GPS receivers used in natural resource management: Consumer- or recreation-grade Mapping-grade Survey grade Recreation-grade receivers generally provide the least accurate positional information - between 3 m and 10 m accuracy under optimal conditions. However, recreation-grade receivers have become popular among many outdoors enthusiasts, and this popularity has likely influenced the wide variety of inexpensive GPS receivers available on the market today.

3 Overview Which type of receiver to choose? It depends on your application, and your cost considerations. If single-fix errors of 5-20 m are acceptable, then a recreation-grade receiver would suffice. Also, recreation-grade may be acceptable for locating field points when you know you will be in one location for some time (allowing the capture of multiple fixes). When mapping large areas, where the error concerning the perimeter is small relative to the size of the area, then a recreation-grade receiver may also be acceptable. Ultimately, the cost of data collection and the desired accuracy of referenced positions should be balanced.

4 Objectives The objectives of this research were to compare the accuracy of horizontal position locations in different timber types, and across different seasons and months of the year. In addition, the research was aimed at examining the influence of air temperature, relative humidity, atmospheric pressure, and planned positional dilution of precision on the accuracy of captured position locations.

5 Methods Three GPS test points were selected from the 40 available at the Whitehall Forest GPS Test Course in Athens, GA. The test points are located in a young pine plantation (15 years old), an older, natural pine stand (over 60 years old on average), and a mature hardwood stand (over 60 years on average). The three test points were visited once per day over the course of a year, and 50 position fixes (waypoints) were collected during each visit, with each position fix captured within 2-3 seconds of the previous. Young pine stand: 15 years old 130 ft 2 per acre basal area 450 trees per acre Hardwood stand: years old 88 ft 2 per acre basal area 144 trees per acre Older pine stand: years old 86 ft 2 per acre basal area 59 trees per acre

6 Methods

7 Methods A Garmin Oregon 300 was selected for use in this study. WAAS was disabled for this study to ensure that the data were collected under consistent circumstances. Planned PDOP was collected from Trimble GPS planning software. Data were downloaded to a computer using Minnesota DNR Garmin software. Some warm-up time was necessary to ensure that a sufficient number of satellites were available to provide a reasonable position fix. Visits were randomly selected, and each test point was visited within about 2-3 minutes of the previous point. During each visit, the GPS receiver was plumbed over each control point, and the operator stood on the north side of each as data was collected.

8 Methods Accuracy of the horizontal positions collected 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. A correlation analysis was performed between the RMSE values and the following: Forest type Planned PDOP Air temperature Relative humidity Atmospheric pressure Solar wind speed Weather data were derived from the Weather Channel and Weather Underground. Solar wind speed data were acquired from the Advanced Composition Explorer (ACE) Science Center at Cal Tech.

9 Results Samples were taken on 289 days between September 15, 2008 and September 14, Range of data accuracy (RMSE) Best (m) Worst (m) Mean (m) Median (m) Young pine stand Older pine stand Hardwood stand

10 Results Scatter-plot of position fixes by stand type. Accuracy is influenced by forest type.

11 Results 5-day moving average RMSE per. Mov. Avg. (OP Error) 5 per. Mov. Avg. (YP Error) 5 per. Mov. Avg. (HWD Error) Daily mean error (m) Survey days

12 Results RMSE by relative humidity - no correlation

13 Results RMSE by atmospheric pressure - no correlation

14 Results RMSE by air temperature - no correlation

15 Results RMSE by solar wind speed - no correlation

16 Results RMSE by planned PDOP - no correlation

17 Results Significant differences Several significant differences between stand types. Fall / Winter 1. Error in hardwood stand vs. error in the younger pine stand. 2. Error in older pine stand vs. error in the younger pine stand. Spring / Summer 1. Error in hardwood stand vs. error in the older pine stand. 2. Error in hardwood stand vs. error in the younger pine stand. 3. Error in older pine stand vs. error in the younger pine stand. Entire year 1. Error in hardwood stand vs. error in the older pine stand. 2. Error in hardwood stand vs. error in the younger pine stand. 3. Error in older pine stand vs. error in the younger pine stand.

18 Results Significant differences No significant difference, for data collected within a single stand type, between seasons of the year.

19 Discussion Results are preliminary, and more analysis is forthcoming.

20 Questions and Comments Welcome One year's experience with a recreation-grade GPS receiver Garmin Oregon 300 tested on the UGA GPS Test Course