GNSS Accuracy

GNSS DATA ANALYSIS Averaged XY plotting demonstrates that all three of the four units will yield relatively accurate results when compared to the baseline. The accuracy of the iPhone 5 demonstrates that it is not suitable even as a check against more accurate/precise units. Table 1 demonstrates that when compared to the baseline in the forested environment the Garmin eTrex 30 was more accurate and precise. Table 2 shows that in an open environment the Garmin Oregon 600 was the most accurate and precise. Lastly in the urban environment the Garmin eTrex is once again the most accurate and precise.

Accuracy Evaluation of Hand Held GNSS Units in Applied Urban Forestry Raymond Whitlock, Randy Carmical, Jon Erhart, Dr. Yanli Zhang, Dr. Daniel Unger

Arthur Temple College of Forestry and Agriculture, Stephen F. Austin State University, Nacogdoches, TX 75962

BACKGROUND Undergraduate students were asked to identify a problem with the application of GNSS units for Geographic Information course 301 (GIS Applications). Given the resources of Arthur Temple College of Forestry and Agriculture it was decided to test the accuracy of four GNSS units while inducing error. These were tested in three environments meant to encompass the wide use of GNSS units by an urban foresters and the general populace. The main errors that were trying to be induced were Ionosphere/Troposphere, Ephemeris, Receiver clock, and Multipath error. It was expected that the more expensive units were going to be more accurate and more precise.

Study Site

Table 1. Accuracy assessment of GNSS units in Forested Environment.

WHY GNSS? Global Navigation Satellite Systems is an all encompassing term that includes GLONASS (Russian), the European Union (Galileo), and Beidou (Chinese) navigation satellite information. GPS which is a particular satellite system to the United States can only achieve optimum angles for signals at given periods of time but with the addition of several other compatible satellites, accuracy is increased due to a more complete web of signal transmission, reception, and response.

RESULTS Based on the data presented it can be concluded that the most accurate and precise unit was the Garmin eTrex 30, this is also the cheapest unit. This unit was followed by the Garmin Oregon 600, then the Trimble Juno SB, and lastly the iPhone 5. The iPhone 5 data was accurate in locating itself in a consistent location, but, was incredibly imprecise because it did not align with the baseline. The Garmin eTrex 30, Garmin Oregon 600, and Trimble Juno SB are all suitable GNSS units for non specific location determination. When used by the general populace it is suitable for area determination and general location referencing. None of these units are suitable to be applied in urban forestry due to the fact that none of them could precisely pin point a location within all three environments. The most suitable use of the Garmin eTrex 30, Garmin Oregon 600, and the Trimble Juno SB would be in an open field environment which would imply the least amount of errors effecting the unit. For the urban environment multipath error was expected to induce a larger error as well as in the forested environment. Despite inducing errors three of the four units performed extremely well.

Table 2. Accuracy assessment of GNSS units in Open Environment.

Table 3. Accuracy assessment of GNSS units in Urban Environment

Figure 3. Forested GNSS data plotted via ArcMap 10.1.

Figure 1. Open GNSS data plotted via ArcMap 10.1.

Stephen F. Austin State University Figure 2. Urban GNSS data plotted via ArcMap 10.1.

Figure 4. Averaged GNSS data plotted via ArcMap 10.1.

Ranking Unit Avg. Results

1 Garmin Oregon 600 4.782 Feet

2 Garmin eTrex 30 8.676 Feet

3 Trimble Juno SB 16.081 Feet

4 iPhone 5 699.713 Feet






Environment Latitude Longitude

Forest 0°0’0.05328” 0°0’1.7532”

Open 0°0’8.8704” 0°0’2.4696”

Urban 0°8’14.856” 0°0’0.0858456”

Table 5. Standard Deviation of Four GNSS Units Compared to Baseline

Unit Average error

Garmin eTrex 30 11.90 Feet

Garmin Oregon 600 13.373 Feet

Trimble Juno SB 27.371 Feet

iPhone 5 471.886 Feet

Table 4. Averages of Four GNSS Units Compared to Baseline






INTRODUCTION The implementation of hand held Global Navigation Satellite Systems (GNSS) units for practical use in urban forestry is drastically increasing. Knowledge that the accuracy of a particular unit’s reliability can only be attributed to what technology companies list as the unit’s specifications. Often enough these companies exaggerate the accuracy of their products for marketing purposes based on ideal conditions without making that statement. The purpose of this research was to: 1) identify the accuracy of four units in three different environments (urban, open field, and forested); and, 2) address accuracy with real world conditions that induce errors. The research was conducted over a period with set control points that were hammered into the ground. These control points (baseline) were to give reference so spatial location could be marked on Earth’s surface. After all data was collected from the field true data points and baselines were mapped and an average was established. From here a standard deviation and variance from the baseline was calculated to determine GNSS accuracy.

STEPHEN F. AUSTIN STATE UNIVERSITY This research was conducted within the property limits of Stephen F. Austin State University (SFASU) located 140 miles Northeast of Houston and 180 miles Southeast of Dallas within the city limits of Nacogdoches, Texas. With a developed infrastructure, a large intramural field section, and a ten acre arboretum the environment was ideal for the constraints set for the research. There is a total of eighty one buildings located on SFASU campus and the main campus size is 417.6 acres. SFASU has listed 12,954 enrolled for the fall semester of 2010. The location of the three environments were as follows: Open: Northeast corner of the women’s soccer field. Urban: Between Lumber Jack Landing Dormitory, Steen library and parking garage IV. Forested: Ruby Maze Azalea Gardens (SFASU Arboretum).

Baseline location was made exact by using a total station/compass and measuring from landmarks. The landmarks were as follows: Open: Measured from a stadium light to peg located on Northeast corner of SFASU women’s soccer field. Urban: Measured from corner to peg located near second oak tree between Lumber Jack Landing Dormitory, Steen library and parking garage IV. Forested: Measured from ROTC repelling tower to peg located underneath Creep Myrtle Tree in Ruby Maze Azalea Garden (SFASU Arboretum). The data was compiled into Microsoft Excel in degrees, minutes, seconds and converted to decimal degrees, this was when they were averaged as well. Following this the decimal degrees where added to ArcMap 10.1 via XY Plotter (Datum: GCS North American 1983).

INTERACTIVE FIELD-BASED METHODOLOGY Collecting a Trimble SB Juno, Garmin Oregon 600, Garmin eTrex 30, and an iPhone 5 the units were WAAS enabled in order to display real world results. An iPhone 5 was selected due to it being the first generation of Apple mobile technology that contains GPS and GLONASS capabilities paired with WIFI location services that is standard in most previous generations. Starting with designating baselines via a wooden peg hammered into the three environments previously stated the units were then individually placed on the peg and oriented North. Readings were started at 07:00 and finished by 07:45 in all three environments. This was then repeated at 19:00 and finished by 19:45. This process was repeated from 2/17/2014 through 2/21/2014. Background image compliments of: http://www.gsc-europa.eu/gnss-markets/user-applications

GNSS Accuracy

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