performance comparison of a low- cost mapping grade...

Performance Comparison of a Low-Cost Mapping Grade GlobalPositioning Systems (GPS) Receiverand Consumer Grade GPSReceiver under Dense ForestCanopy

Michael G. Wing and Aaron Eklund

We compared the measurement accuracy and reliability of a low-cost mapping grade global positioningsystems (GPS) receiver to a consumer grade GPS receiver while operating under a very dense forestcanopy. The mapping grade GPS receiver collected both autonomous (uncorrected) GPS measurementsand measurements that were differentially corrected in real time through the Wide Area AugmentationSystem (WAAS). Although we found average measurement accuracies of 7.2 m for uncorrected data and7.8 m for differentially corrected data for the mapping grade GPS, these differences were notstatistically significant. The overall average positional error for the consumer grade GPS was larger (8.9m) than that of the mapping grade GPS but the consumer grade GPS collected data more efficientlyand, for about half of all points collected, with smaller measurement errors.

Keywords: spatial, accuracy, forestry, measurements

G lobal positioning systems (GPS)use has spread throughout manydisciplines but still faces challenges

in forested environments. Effective GPS usewithin forests can be hindered by canopycover and terrain blocking satellite signals

from reaching a GPS receiver (Liu and Bran-tigan 1996) and environmental conditionsthat require operator care and rugged equip-ment (Wing and Kellogg 2004). GPS receiv-ers are available in a number of configura-tions and price ranges. Typically, GPS

receivers are separated into one of threebroad categories based on measurement ac-curacy and capabilities: survey, consumer,and mapping grade. Survey grade GPS re-ceivers typically cost in excess of $8,000 butoften will include many features. These fea-tures include the capability to store numer-ous coordinate pairs, a robust software suitethat includes mission planning software tobest-schedule field visits, measurementpoint averaging, and the ability to differen-tially correct measurements. Measurementpoint averaging involves taking multiple co-ordinate readings at a single location with agoal of creating an average coordinate pairthat is closer to the true location. Differen-tial measurement correction involves com-municating with a GPS base station, down-loading location corrections, and applying

Received February 14, 2006; accepted June 29, 2006.

Michael G. Wing ([email protected]) is assistant professor, Forest Engineering Department, Oregon State University, Corvallis, OR 97331. AaronEklund ([email protected]) is graduate student, Forest Engineering Department, Oregon State University, Corvallis, OR 97331.

Copyright © 2007 by the Society of American Foresters.

Journal of Forestry • January/February 2007 9

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corrections to collected data (Leick 2004).The locations of GPS base stations are estab-lished using very accurate measurementsand base stations continually compare theirknown locations to positions generated bysatellite signals. Any difference between theknown and satellite-derived location of thebase station is regarded as a positional errorand can be used to estimate a correction fac-tor for field-collected GPS data. Throughdifferential correction techniques, surveygrade GPS receivers are capable of generat-ing location measurements that are within 1cm of true position, provided that satellitesignal geometry is consistently strong andavailable over periods of time. However, for-est environments feature canopy cover, veg-etation, and topography that often precludethe efficient use of survey grade GPS receiv-ers (Wing and Kellogg 2004). Operator skillalso is necessary for both hardware and soft-ware applications of survey grade GPS re-ceivers and forest conditions often are notsuitable for the delicate nature of the equip-ment. In addition, many measurement ap-plications do not require accuracies that arewithin 1 cm of true position.

Mapping grade GPS receivers representthe middle ground between survey and con-sumer grade receivers, although prices stillmay be prohibitive to potential consumers.Costs of mapping grade GPS receivers varyfrom $2,000 to $12,000 depending on themanufacturer and model. Previous studieshave found mapping grade GPS receivers tobe capable of acceptable measurement accu-racies when working under forest canopies.Bolstad et al. (2005) tested several mappinggrade GPS receivers below hardwood cano-pies in Minnesota and found average errorsbetween 3.0 and 4.8 m for uncorrected mea-surements, 2.9 m for real-time differentiallycorrected measurements, and average errorsbetween 2.5 and 4.0 m for postprocesseddifferentially corrected data. Johnson andBarton (2004) reported nondifferentiallycorrected mapping grade errors of 20–30 munder a partial hardwood forest canopy inNew Hampshire. Naesset and Jonmeister(2002) found positional errors between 2.2(20-minute observation time) and 5.6 m (2-minute observation time) in dense spruceforests in Norway for differentially correctedGPS measurements. Liu (2002) determinedan average positional error of 4.0 m underthick hardwood canopies using uncorrectedmapping grade GPS data but did not reporta study location. The 4.0-m average was

based on 17 observations with the average of180 readings being used to create a locationfor each observation. Sigrist et al. (1999) de-termined a differentially corrected rootmean square error of 5.1 m under a whitepine (Pinus strobus) canopy in north centralIndiana based on a 3-hour acquisition timefor a single point.

Consumer grade GPS receivers areavailable for several hundred dollars or lessand have been found to collect measure-ments with accuracies that are acceptable formany forestry applications. Wing et al.(2005) found that consumer grade GPS re-ceivers were capable of accuracies within10 m under closed canopies and 7 m underyoung forest canopies in western Oregon.Bolstad et al. (2005) tested a consumer GPSreceiver under heavy forest canopy (morethan 70% sky obstruction) in Minnesotaand found average errors of 6.5 and 7.1 m.Karsky at al. (2001) reported average errorsof 3–24 m under medium canopy in Mon-tana. Although consumer grade GPS receiv-ers are affordable for many users, they arelimited in a number of characteristics. Lim-itations typical of consumer grade GPS re-ceivers include not being able to set mini-mum standards for satellite geometry fordata collection, a data storage limitation of500 coordinate pairs, and an inability to dif-ferentially correct data after field data collec-tion without third-party software. Differen-tial correction allows errors caused byatmospheric conditions to be addressed andreduced; atmospheric interference is ex-pected to increase in future years. In addi-tion, many consumer grade GPS receiversdo not allow users to automatically conductpoint averaging and the software that ac-companies most consumer GPS is limited inscope.

A mapping grade GPS receiver calledthe SXBlue (GENEQ, Montreal, Quebec,Canada) has become available for about$2,000 and makes use of Bluetooth wirelesstechnology to communicate with a digitaldata logger. The SXBlue is intended to takeadvantage of Space-Based AugmentationSystems (SBAS) that are capable of provid-ing conventional real-time differential cor-rections to GPS receivers as they collect data.Conventional real-time differential uses themore accessible coarse/acquisition satellitesignals rather than phase code signals. Al-though phase code signals have a greater po-tential for more accurate GPS measure-ments, continuous and uninterruptedsatellite signals are required, conditions

which often are not attainable under forestcanopy. A SBAS derives separate measure-ment correction factors (rather than a singlefactor) for several potential sources of GPSerror including atmospheric interference ofsignals, timing intervals used to estimate sat-ellite signal range (distance), and the track-ing of satellite orbital patterns. In the UnitedSates, there is currently one operationalSBAS. This is the US Federal Aviation ad-ministration’s Wide Area AugmentationSystem (WAAS), which featured two opera-tional WAAS satellites during the study pe-riod. The satellites operate in geosynchro-nous orbits with equatorial locations overthe Pacific Ocean and northern Brazil. A lineof sight between a GPS receiver and a WAASsatellite is necessary for satellite signal recep-tion. Forest canopy, structures, and land-forms can effectively block signal reception.In addition, GPS measurement reliabilitydecreases as distance between a GPS receiverand the WAAS satellites increases. Only asingle WAAS satellite signal is necessary for aGPS receiver to apply real-time correctionfactors but reception from two WAAS satel-lite signals is preferred because a second pro-vides a backup should reception from onesatellite become unavailable. In the UnitedStates, only western states had the potentialto receive signals from both operationalWAAS satellites during the study period.Two additional WAAS satellites are antici-pated in 2006. Other SBAS include the Eu-ropean Geostationary Navigation OverlaySystem and the Japanese Multi-FunctionalTransport Satellite-based AugmentationSystem. The SXBlue GPS receiver configu-ration is intended to be compatible withthese international SBAS in addition toWAAS.

The SXBlue GPS receiver also features anavigation system (COAST Technology)that is designed to allow accurate GPS mea-surements during times when satellite recep-tion becomes degraded or lost, such as whatmight occur if data are being collected undera dense forest canopy. The navigation sys-tem uses algorithms that are intended tocontinue applying differential correctionseven if real-time connectivity to WAAS islost. More specifically, errors that would beexpected from atmospheric, satellite, andephemeris conditions are predicted and re-moved from recorded measurements. Forthe navigation feature to work, successful re-ception of differentially corrected satellitesignals (WAAS) for up to 5 minutes mustoccur first. After successful reception, the

10 Journal of Forestry • January/February 2007

GPS receiver is supposed to allow up to30–40 minutes of data collection before ad-ditional signal reception is necessary.

Our objectives were to compare the ac-curacy and reliability of a relatively low-costmapping grade GPS receiver operating intwo different data collection modes with aconsumer GPS receiver collecting measure-ments under a dense forest canopy. Themapping grade GPS receiver measurementswere collected in autonomous mode andalso through real-time differential correc-tions as supplied by WAAS for comparison.We synchronized measurement times forboth GPS receivers and used a point averag-ing benchmark (the average of 60 points col-lected at one-second intervals) that shouldpresent field-collection efficiencies for thosewho collect measurements under canopycover. In addition, we were interested in de-

termining whether the SXBlue’s navigationsystem capabilities would enable us to col-lect accurate GPS measurements when satel-lite signal reception was not initially avail-able.

MethodsWe conducted our testing at the

Clackamas GPS test course in western Ore-gon. The Clackamas GPS test course is lo-cated in the western portion of the CascadeMountain range at an elevation of approxi-mately 2,200 m. The course is situated onrelatively level terrain in a dense Douglas-fir(Pseudotsuga menziesii) and western hem-lock (Tsuga heterophylla) forest in an approx-imate 55- to 65-year age class with dbh val-ues ranging from 60 to 100 cm. Canopyclosure within the Clackamas test course isnearly 100%. Eight measurement test points

are marked by vertically planted 1.6-cm re-bar with a plastic cap. All test points on theClackamas test course were established byfirst-order GPS methods with a stated accu-racy of 5 cm (Karsky et al. 2001).

We visited each of the eight measure-ment points during seven rotations aroundthe test course with a mapping grade and aconsumer grade GPS receiver and collected60 coordinate pairs during each visit to a testpoint. The GPS receivers were placed on ad-jacent points during data collection andmeasurements were synchronized so thatboth GPS receivers began collecting data atthe same time. The average of the 60 coor-dinate pairs was used to create a GPS mea-surement for analysis purposes. A horizontaldistance error was calculated between theGPS measurements and the known coordi-nates of the test points by deriving thestraight-line distance between coordinate lo-cations.

We attempted to collect data with theSXBlue in differential GPS (DGPS) mode atthe beginning of each rotation around themeasurement course. If we could not receivedifferential GPS during the beginning of ameasurement course rotation, we collecteddata in autonomous mode throughout theend of the rotation. In cases where differen-tial GPS was available at the beginning of acourse rotation, we took point measure-ments through the entire rotation until welost differential GPS capability. Remainingpoint measurements were collected in au-tonomous mode after losing differentialGPS. If we were unable to collect 60 pointsat a measurement point during a 5-minuteperiod once we began a measurement courserotation, we moved to the next measure-ment point. During the sixth and seventhcourse rotations, we were initially not able toestablish sufficient satellite reception to be-gin recording measurements with the SX-Blue. To examine COAST Technology, wetraveled a short distance (50 m) to a clearing,established connectivity with WAAS, andattempted to complete the course rotation.

The SXBlue GPS receiver comes in acompact case (16.5 � 3.75 � 1.75 cm) thatcan be fastened to a belt and worn comfort-ably (Figure 1). Thin wire cabling connectsthe receiver to a small antenna (4.6 � 3.9 �1.3 cm) that is mounted on a magnetic sur-face. The antenna can be placed on a hat,staff, or simply held. We placed the antennaon the top of a 2-m staff for our testing tomaintain a consistent data collection meth-odology. The SXBlue GPS receiver does not

Figure 1. The SXBlue GPS receiver (worn on hip) and external antenna mounted on top ofa range pole.


feature onboard data storage. We chose apersonal digital assistant (PDA) as our datalogger and established a wireless connectionto the GPS receiver during our field datacollection. The GPS receiver software wasEnvironmental System Research Institute’sArcPad (version 6.0), which features a user-friendly interface for managing, collecting,and displaying GPS measurements. ArcPadalso allows users to consult a current sky plotof satellite availability and potential positiondilution of precision (PDOP). PDOP is anestimate of the potential satellite signal ge-ometry for a point on the earth’s surface. Aminimum of four satellite signals are neces-sary for a GPS to reliably calculate a posi-tion. Satellite signals that come from a sim-ilar portion of the sky will have a weakergeometric resolution than those that comefrom diffuse positions. Lower PDOP valuessignify a better arrangement of satellite sig-nals with a general guideline being thatPDOP values in the 6–8 range are preferredfor GPS data collection (Stewart and Rizos2002). We configured ArcPad to only acceptPDOP values of 6 or less. All coordinate datapoints collected by the SXBlue GPS receiverwere saved in the PDA and later were down-loaded for analysis.

We chose the Garmin V consumer GPSreceiver for measurement comparisons withthe SXBlue because of its ability to automat-ically conduct point averaging. The GarminV allows users to store an average coordinatepoint based on a 1-second point collectioninterval. Up to 500 averaged points can bestored directly on the handheld unit. TheGarmin V comes in a compact case (12.7 �5.9 � 4.1 cm) and features a short externalantenna (Figure 2). The Garmin V wasmounted on a 1.4-m high wooden staff, alevel at which the unit could be comfortablyoperated, during data collection to standard-ize measurements.

ResultsThe average measurement error for the

SXBlue was 7.4 m and the average SD was3.9 for all measurement points (Table 1).The average of all the measurement errorsfor data collected in autonomous mode was7.2 m (3.9 SD) and the average error for alldata collected in differential GPS was 7.8 m(4.0 SD). This difference of less than a meterbetween autonomous and differential GPSmeasurement accuracy does not carry prac-tical significance for most forestry applica-tions. In addition, we found no statisticalsignificance between autonomous and dif-

ferential GPS measurement accuracies forthe SXBlue (Wilcoxon rank-sum test, P �0.59). The Garmin V had an average mea-surement error of 8.9 m and an SD of 6.9.

An average error and SD were calcu-lated for each of the seven rotations aroundthe GPS test course (Table 1). Average hor-izontal distance errors were smallest for thethird rotation (6.0 m; 3.2 SD) and largest forthe fifth rotation (9.4 m; 2.4 SD) for theSXBlue. We found no statistical significance(analysis of variance [ANOVA], P � 0.30)in considering the effect of course rotationon measurement error.

For the Garmin V, statistically signifi-cant differences (ANOVA, P � 0.004) werefound among the average rotation measure-ment errors of the Garmin V. The third ro-tation had the lowest average error (4.1 m;2.5 SD) and the first rotation had a slightlylarger average error (5.1 m; 4.6 SD). Thesetwo average errors were lower than the aver-age rotation errors collected by the SXBlue.The seventh rotation had the largest averageerror (15.3 m; 11.0 SD) and featured threepoints that had measurement errors in excessof 22 m. Rotation five (10.4 m; 4.3 SD), six(12.6 m; 6.2 SD), and seven for the GarminV had average measurement errors that werelarger than any recorded by the SXBlue. Amultiple comparison test of the measure-

ment errors by rotation found that rotationthree was significantly different from rota-tions six and seven (Tukey multiple compar-ison test; P � 0.05).

Average measurement errors also werecalculated and summed for each coursepoint (Table 1) to detect any patterns relatedto individual point characteristics (e.g., can-opy density and surrounding landforms)that might influence accurate measure-ments. For the SXBlue, the lowest averageerror (5.2 m) was recorded at the first mea-surement point visited on each course rota-tion and the highest average error (9.2 m)was recorded at the second to last measure-ment point. We detected no statistically sig-nificant effects of the influence of measure-ment point on average error (ANOVA, P �0.27) for the SXBlue. For the Garmin V, thelowest average error (4.9 m) was for the thirdmeasurement point and this error wassmaller than any collected by the SXBlue.We found no statistically significant differ-ences of average measurement error betweenmeasurement points (ANOVA, P � 0.26).

ConclusionThe SXBlue was more consistent than

the Garmin GPS V in terms of measurementaccuracy and precision as evidenced by thedifferent ANOVA results for course rotation

Figure 2. The Garmin V GPS receiver mounted on a wooden staff.


influence on measurement error. The SX-Blue, however, was not able to consistentlycollect data given the PDOP limits (less than6) that were established. The Garmin V hadmeasurement accuracies that were both bet-ter and worse than the SXBlue, dependingon the course rotation and measurementpoint. The average accuracy of the GarminV (8.9 m) was slightly worse than that re-ported by two recent consumer GPS studies(Bolstad et al. 2005, Wing et al. 2005). Theaccuracy and reliability of measurementscollected by the Garmin V may be unaccept-able for some measurement objectives andreflect the inability of many consumer GPSreceivers to take satellite signal quality stan-dards into account while collecting data. Be-cause of the lack of data collection qualitycontrol, the Garmin V is able to collect mea-surements expediently given the availabilityof four satellite signals and can be more effi-cient in the field than the SXBlue. Typicalcollection times for 60 points rarely requiredmore than 60 seconds for the Garmin V. Itshould be noted that although we found nostatistically significant difference betweenthe measurement errors of the SXBlue andGarmin V, data collection was synchronizedbetween the two receivers so that that mea-surements were collected only in tandem.

We set a maximum PDOP value of 6 for theSXBlue in order for data collection to begin.This maximum PDOP setting likely influ-enced the measurement errors of theGarmin V by making them smaller in com-parison with those that would have been col-lected without synchronization.

We found average measurement accu-racies of 7.4 m for all GPS measurementscollected underneath heavy canopy with theSXBlue. These measurement errors werehigher than those reported by some previousmapping grade GPS studies (Liu 2002,Naesset and Jonmeister 2002, Bolstad et al.2005) but also smaller than errors reportedby other research (Johnson and Barton2004). There was no statistically significantdifference between the accuracy of autono-mous and differential GPS measurementsbut measurement accuracies of autonomousmeasurements (7.2 m) were slightly im-proved in comparison with differential (7.8m). Testing results indicated that averageSXBlue measurement accuracies were rela-tively consistent regardless of whether it op-erated in autonomous or differentially cor-rected mode. Presently, these findingssuggest that WAAS may not offer significantadditional utility to users operating in staticdata collection mode within heavily forested

environments beyond that provided by ex-isting satellites in the western US. As addi-tional WAAS satellites become available, in-creased accuracy of GPS measurementsshould be possible.

The SXBlue COAST Technology didappear to allow us to collect data when sat-ellite reception was initially lost. We startedthe sixth revolution of our course measure-ments but were unable to continue collect-ing data points after our first measurement.We took the GPS receiver to a nearby clear-ing and waited 10 minutes, during whichtime a position fix was possible and WAASsignals were received for 5 minutes. We re-started the sixth revolution of the course im-mediately and were able to finish the coursewith minor interruptions including not be-ing able to record 60 coordinate pairs at onemeasurement point. The start of the seventhcourse was very similar to the sixth; after wecollected an initial measurement, we couldnot continue and once again took the GPSreceiver to the clearing. Approximately 1hour had elapsed since the previous visit tothe clearing. We restarted the seventh courserevolution after spending 10 minutes in theclearing and were able again to complete thecourse with only minor interruptions, in-cluding not being able to record 60 measure-

Table 1. Average positional accuracy (m) and SD for GPS receiver measurements under heavy canopy.

Rotation 1 2 3 4 5 6 7 Average SD

Point

SX Blue GPS receiver (bold font indicates DGPS)1 7.2 * 2.6 0.9 3.5 5.6 11.3 5.2 3.72 2.4 * 3.2 16.4 9.6 9.2 11.6 8.7 5.33 3.3 4.6 4.7 8.7 10.5 2.6 8.6 6.1 3.14 14.7 5.4 7.3 1.6 9.7 6.5 * 7.5 4.45 3.5 15.5 10.3 3.4 10.5 * 6.9 8.3 4.76 5.8 1.4 9.2 13.3 9.6 9.4 7.7 8.1 3.77 11.2 10.1 8.6 5.8 11.4 7.5 9.9 9.2 2.08 2.5 2.4 2.2 6.7 10.2 12.4 7.0 6.2 4.1Average 6.3 6.6 6.0 7.1 9.4 7.6 9.0SD 4.5 5.3 3.2 5.5 2.4 3.1 2.0All SXBlue measurements 7.4 3.9Autonomous measurements 7.2 3.9DGPS measurements 7.8 4.0Garmin GPS V Receiver1 3.3 8.8 0.7 4.7 5.8 20.8 7.8 7.4 6.52 † 7.3 2.2 9.4 6.8 6.3 22.9 9.2 7.13 3.7 0.8 1.8 5.2 4.4 8.2 10.4 4.9 3.44 7.2 5.0 6.8 8.5 10.9 8.2 3.8 7.2 2.35 14.7 14.8 5.7 2.2 15.4 10.1 † 10.5 5.56 2.6 16.8 5.5 20.0 15.3 8.0 5.8 10.6 6.77 2.0 1.3 3.0 4.6 13.1 18.6 24.7 9.6 9.38 2.0 2.5 7.1 6.8 11.4 20.4 31.9 11.7 10.9Average 5.1 7.2 4.1 7.7 10.4 12.6 15.3SD 4.6 6.0 2.5 5.5 4.3 6.2 11.0All Garmin GPS V measurements 8.9 6.9

*Unable to collect 60 points within a 5-minute period.†Data unavailableDGPS, differential GPS.


ments at one course point. We were not ableto maintain a DGPS position during the sev-enth revolution.

The SXBlue is competitively priced incomparison with other mapping gradeGPS receivers and has some desirable at-tributes. The wireless connection betweenthe SXBlue GPS receiver and data collec-tor was convenient in that one fewer cordwas needed during our field data collec-tion. Several times, however, the connec-tion abruptly terminated and restartingthe PDA was necessary to regain commu-nication. Readers also must take cautionin assessing the actual operational costs ofthe mapping grade GPS that we tested.Although the receiver itself retails for ap-proximately $2,000, prospective users alsowill need access to a digital data collectorand data collection software. Purchasingthese products likely will add at least

$1,000 to the costs associated with oper-ating this mapping grade GPS receiver.

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