Methods for Geometric Accuracy Investigations of ... · Trimble GX, Leica ScanStation 1 and 2,...

Photogrammetrie • Fernerkundung • Geoinformation 4/2009, S. 301–316

DOI: 10.1127/1432-8364/2009/0023 1432-8364/09/0023 $ 4.00© 2009 DGPF / E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart

Methods for Geometric Accuracy Investigations ofTerrestrial Laser Scanning Systems

Thomas P. KersTen, Klaus mechelKe, maren lindsTaedT & harald sTernberg,Hamburg

Keywords: Accuracy, evaluation, inclination compensation, laser beam, positioning, standards

tive single point to the exact and highly de-tailed 3D point cloud” (Staiger & Wunderlich

2006). Advanced technology and new featuresof 3D laser scanners have been developed inthe past two years, introducing additional in-strument features such as electronic levels,inclination compensation, forced-centring, onthe spot geo-referencing, and sensor fusion(e. g., digital camera and GPS). Most of theseelements are obviously equivalent to featuresthat can be seen in total stations. Several au-

1 Introduction

Terrestrial laser scanning (TLS) systems havebeen available on the market for about tenyears and within the last five years the tech-nology has become accepted as a standardmethod of 3D data acquisition, finding its po-sition on the market beside established meth-ods such as tacheometry, photogrammetryand GPS. Terrestrial laser scanning also standsfor a paradigm change “from the representa-

Summary: Currently the second, or for some man-ufacturers even the third, generation of terrestriallaser scanning systems is available on the market.Although the new generation of terrestrial 3D laserscanning systems offer several new (geodetic) fea-tures and better performance, it is still essential totest the accuracy behaviour of the new systems foroptimised use in each application. As a continua-tion of previously published investigations the De-partment Geomatics of the HafenCity UniversityHamburg (HCU Hamburg) carried out comparativeinvestigations into the accuracy behaviour of thenew generation of terrestrial laser scanning sys-tems (Trimble GX, Leica ScanStation 1 and 2, andRiegl LMS420i using time-of-flight method andLeica HDS6000, Z+F IMAGER 5006, and FaroLS880 HE using phase difference method). The re-sults of the following tests are presented and dis-cussed in this paper: test field for 3D accuracyevaluation of laser scanning systems, accuracytests of distance measurements in comparison withreference distances, accuracy tests of inclinationcompensation, and influence of the laser beam’s an-gle of incidence on 3D accuracy.

Zusammenfassung: Methoden für geometrischeGenauigkeitsuntersuchungen terrestrischer Laser-scanningsysteme. Die neueste Generation der ter-restrischen 3D-Laserscanner bietet einige neuegeodätische Eigenschaften und bessere Leistung.Dennoch ist es weiterhin sehr wichtig, das Genau-igkeitsverhalten auch neuer Systeme zu testen, umsie optimal in verschiedenen Anwendungen einset-zen zu können. Standardisierte Prüfverfahren fürterrestrische Laserscanner gibt es jedoch bisherheute noch nicht. Das Department Geomatik derHafenCity Universität Hamburg (HCU Hamburg)hat eigene Prüfverfahren entwickelt, die Aussagenüber das Genauigkeitsverhalten terrestrischer La-serscannersysteme (TLS) erlauben. In diesem Bei-trag werden Untersuchungen mit den SystemenTrimble GX, der Leica ScanStation 1 und 2, demRiegl LMS-Z420i (alle mit Impulslaufzeitverfah-ren), sowie Faro LS880, Leica HSD 6000 und dembaugleichen IMAGER 5006 von Zoller + Fröhlich(alle mit Phasendifferenzverfahren) vorgestellt.Streckenvergleiche im 3D-Testfeld und auf einerVergleichsstrecke, sowie Genauigkeitstests derNeigungssensoren und Untersuchungen zum Ein-fluss des Auftreffwinkels des Laserstrahles auf die3D-Punktgenauigkeit wurden durchgeführt. Dieerzielten Ergebnisse bestätigen weitestgehend dietechnischen Spezifikationen der Systemhersteller.

302 Photogrammetrie • Fernerkundung • Geoinformation 4/2009

2 The Terrestrial Laser ScanningSystems Used

The investigations into the accuracy behav-iour of terrestrial laser scanners were carriedout by using the following laser scanning sys-tems: Trimble GX, Leica ScanStation 1, LeicaScanStation 2, Leica HDS 6000, Faro LS 880,IMAGER 5006 from Zoller & Fröhlich, andRIEGL LMS-Z420i (cf. Fig. 1).The technical specifications and the impor-

tant features of these laser scanners are sum-marised in Tab. 1. The tested scanners arepanoramic scanners, but they represent twodifferent distance measurement principles:Faro LS880, Z+F IMAGER 5006, and LeicaHDS6000, which is structurally identical withthe IMAGER 5006, use phase differencemethod, while Leica ScanStation 1/2, TrimbleGX, and Riegl LMS-Z420i scan with the time-of-flight method. In general it can be statedthat phase difference method is fast, but signalto noise ratio depends on distance range andlighting conditions. If one compares scan dis-tance and scanning speed in Tab. 1, it can beclearly seen, that scanners using the time-of-flight method measure longer distances but arerelatively slow compared to the phase differ-ence scanners. Trimble GX and both LeicaScanStation instruments scan with a green la-ser beam (532 nm), while the other three scan-ners use laser light with wavelengths at nearinfrared. The precision (internal accuracy) ofthe scanning instrument is not unitarily speci-fied in the specifications of the manufacturer,i. e., some uses the 3D position and some thedistance as a precision criterion.

Most of the presented investigations usespheres as test bodies to obtain the referencepositions. The diameters of the used spheres

thors have already reported on different ap-proaches for investigations into terrestrial la-ser scanning systems. Nevertheless, standard-ized test and calibration methods of laserscanning systems do not yet exist for the user.

Due to the huge variety of types of terres-trial laser scanners it is difficult for the user tofind comparable information about potentialand precision of the laser scanning systems inthe jungle of technical specifications and to beable to validate the technical specifications,which are provided by the system manufactur-ers. Thus, it may be difficult for users to choosethe right scanner for a specific application,which emphasises the importance of compara-tive investigations into accuracy behaviour ofterrestrial laser scanning systems.

Therefore several groups, primarily univer-sity-based, carried out geometrical investiga-tions into laser scanning systems in order toderive comparable information about the po-tential of the laser scanners and to find practi-cal testing and calibration methods (Boehler

et al. 2003, ingenSand et al. 2003, JohanSSon

2003, clark & roBSon 2004, Schulz & ingenSand 2004, lichti & Franke 2005, rietdorF 2005, neitzel 2006, reShetyuk 2006,Büttner & Staiger 2007, Schulz 2007, Wehmann et al. 2007, gordon 2008, gottWald

2008, kern 2008, gottWald et al. 2009). Thedepartment Geomatics of the HafenCity Uni-versity Hamburg (HCU Hamburg) validatesterrestrial laser scanners since 2004, in orderto develop their own testing and evaluationmethods (kerSten et al. 2004, kerSten et al.2005, SternBerg et al. 2005, mechelke et al.2007, mechelke et al. 2008), which allowstatements about the accuracy behaviour andabout the application potential of terrestriallaser scanner systems to be made.

Fig. 1: Terrestrial laser scanning systems for investigation at HafenCity University Hamburg:Trimble GX, Leica ScanStation 1 and 2, Riegl LMS-Z420i, Faro LS 880HE, IMAGER 5006 fromZoller & Fröhlich, and Leica HDS6000.

T.P. Kersten et al., Methods for Geometric Accuracy Investigations 303

positions of the spheres, each point cloud rep-resenting the sphere was manually correctedfor outliers. The fitting of the sphere geometrywas performed for each scanner station usingalgorithms in the Trimble software RealWorksSurvey and 3Dipsos, where the radius of thesphere was fixed with the known value. Thealgorithm for sphere fitting used by the Trim-

are 76.2 mm, 145 mm, and 199 mm, respec-tively. The materials in the spheres are solidplastic for the small diameter (76.2 mm) andhollow plastic with a special surface coatingand centring option for the larger spheres(145 mm & 199 mm). These spheres are ofmatt white colour and are checked for eccen-tricity and exact diameter. To obtain centre

Tab. 1: Summary of technical specifications (according to system manufacturer) of the tested laserscanning systems.

Scanner/Criterion TrimbleGX

LeicaScanSta-tion 1

LeicaScanSta-tion 2

RieglLMS-Z420i

FAROLS 880HE

Z+FIMAGER5006 /HDS 6000

Scan method Time-of-flight Phase difference

Field of view [°] 360×60 360×270 360×270 360×80 360×320 360×310

Scan distance [m] 350 300 300 2–1000 0,6–76 < 79

Wavelength [nm] 532 532 532 ~1500 785 658

Scanning speed[pts/sec]

≤ 5000 ≤ 4000 ≤ 50000 ≤ 11000 120000 ≤ 500000

Angularresolution [°]

V 0,0018 0,0023 0,0023 0,0020 0,00900 0,0018

H 0,0018 0,0023 0,0023 0,0025 0,00076 0,0018

Spot size at 10 m 0,6 mm 4,0 mm 4,0 mm 2,5 mm 2,5 mm 3,2 mm

Preci-sion

position 12 mm/100m

6 mm/50 m

6 mm/50 m

– – 10 mm/50m

distance 7 mm/100 m

4 mm/50 m

4 mm/50 m

10 mm/50 m

3 mm/25 m

6 mm/50 m

Camera integrated add-on option

Inclination sensor compensator yes yes

Fig. 2: 3D test field at the HafenCity University Hamburg for geometrical investigations into TLSsystems.


between spheres were used for the accuracyevaluation at HCU Hamburg. However, theprecision of 3D laser scanning systems is com-posed of a combination of errors in distanceand angle measurements, and in the algorithmfor fitting the spheres/targets in the pointcloud. The influence of these errors is difficultto determine independently and this causes is-sues when the goal is the testing of the wholesystem (hard- and software). However, in me-trology, the accuracy of measurements is af-fected by the impacts of all random compo-nents and systematic errors. For the followingevaluations accuracy is defined as a measureto an independent reference.A durable established 3D test field was used

in the hall of building D at the HCU campus(cf. Fig. 2) for test campaigns in March, Octo-ber and December 2007. This was used in or-der to evaluate the 3D accuracy of distancemeasurements derived from the sphere coor-dinates and of point cloud registration regard-ing the practical acceptance and verificationmethods of VDI/VDE 2634. The volume ofthe test field is 30×20×12m3, including 53 ref-erence points, which can be set up with prisms,spheres or targets. Just 38 (in March) and 30points (in October/December) were used forthese investigations. The points are distributedover three hall levels on the floor, on walls oron concrete pillars using M8 thread holes. Thereference points were measured from four sta-tions with a Leica TCRP 1201 total station. Ina 3D network adjustment using the softwareLeica GeoOffice the station coordinates weredetermined with a standard deviation of lessthan 0.5 mm, while the standard deviation of

ble software has not been published. Theseresults of the fitting were compared to resultsof a MATLAB routine on a random basis. Theprogrammed MATLAB software uses thesphere fitting algorithm as described by drixler (1993). Since there were no differences inthe centre coordinates of the spheres, theTrimble software continued to be used for allfitting tasks due to simplified data handling.The standard deviation for sphere fitting wasin the range of 0.4–1.0mm for the test fieldinvestigations (cf. Section 3.1), although somedeviations increased to 6.0 mm dependent ondistance length and sphere diameter.

3 Geometric Investigations

3.1 3D Test Field for AccuracyEvaluation of 3D Laser ScanningSystems

Referring to the guidelines in part 2 and part 3of the VDI/VDE 2634 (VDI/VDE 2634 2002)the accuracy of 3D optical measuring systemsbased on area scanning shall be evaluated bychecking the equipment at regular intervals.This can be achieved by means of lengthstandards and artefacts, which are measuredor scanned in the same way as typical meas-urement objects. One important quality pa-rameter can be defined as sphere spacing errorsimilar to that in ISO 10 360 (INTERNA-TIONAL ORGANIZATION FOR STAND-ARDIZATION 2007). Instead of calibratedartefacts in object space reference distances

Tab. 2: Comparison of 3D distances (all in all combination (left) and seven selected distances(right)) between laser scanner and reference in the 3D test field (tests in March 2007).

Scanner # 3Dpoints

#dist.

Δlmin[mm]

Δlmax[mm]

span[mm]

syst.shift[mm]

|Δl|[mm]

#dist.

Δlmin[mm]

Δlmax[mm]

span[mm]

LeicaScanStation 1

38 703 − 9.2 2.3 11.5 −3.6 3.6 7 −5.6 1.6 7.2

Trimble GX 38 703 −27.6 16 43.6 −5.5 6.5 7 −5.9 −1.8 7.7

Z+F IMAGER5006

38 703 − 6.6 7.4 14.0 0.3 1.8 7 −2.1 3.3 5.4

Faro LS 880HE

38 703 −30.7 41.1 71.8 0.1 5.0 7 −3.5 29.9 33.4


Two results of the 3D test field investiga-tions from the March 2007 test campaign areshown in Tab. 2: (a) all differences betweenscanned and reference distances for all sta-tions (registered in one common object coor-dinate system using the sphere centre coordi-nates for transformation), and (b) the differ-ences between scanning and reference of sev-en selected well-distributed distances (samedistances for all scanners) are summarised asthe span (Δlmax – Δlmin) from minimum tomaximum deviation value as an indication ofthe accuracy of each system. The differencesbetween the distances in case (a) are highlycorrelated, while the distances in case (b) wereselected as proposed in the VDI/VDE 2634,and to avoid these correlations. Instead of sev-en distances, heiSter (2006) proposed eightspatial distances in the object for this test. Thisrange value Δl is influenced by the measure-ment precision of the instrument and by thealgorithm for the fitting of the sphere. Sincethe fitting with the Trimble software has beenchecked as previously mentioned, errors insphere fitting can be excluded. The best resultwas a range from minimum to maximum of11.5 mm for all differences, which wasachieved with the Leica ScanStation 1, whilefor the IMAGER 5006 a span of 5.4 mm wasobtained using the seven differences of dis-tances (see Tab. 2). The three scanners Scan-Station 1, GX und IMAGER 5006 show simi-lar accuracy behaviour (between 5 and 8 mm)using the same seven distances, while the Faro

the coordinates of the reference points is lessthan 1 mm (local network). Specially builtadapters of the same length as those used withthe prisms guaranteed a precise, stable and re-peatable set up of spheres. Thereafter, sphereswith a diameter of 145 mm (in March 2007)and 199 mm (in October and December 2007)were installed on these reference points. Thesespheres were scanned with all tested scannersfrom five scan stations for each system, wheretwo scan stations were located on the groundfloor, two on the first floor and the fifth stationwas placed on the second floor, so that a goodgeometric configuration for point determina-tion could be guaranteed. For evaluation, allcombinations of distances between all refer-ence points were compared to those obtainedfrom the centres of the fitted spheres derivedfrom the registered point cloud. In accordancewith the guidelines of VDI/VDE 2634 part IIIall scan stations were transformed into onecommon object coordinate system for each la-ser scanner using the determined coordinatesof each sphere centre. The sphere-spacing er-ror Δl is determined by Δl = lm−lk, wheremis measured and k is the reference distance.Additionally, the mean value of all absolutevalues |Δl| (sphere spacing error) has been de-termined according to heiSter (2006). Theminimum distance is 1.5 m and the maximumdistance is 33.1m in the test field, which iswithin the scanning range of each tested scan-ner.

0

50

100

150

200

250

-30 -20 -10 0 10 20 30 40

Difference [mm]

Frequency

Faro LS880

Z+F Imager 5006

Trimble GX

Leica ScanStation1

Sphere 145mm

Fig. 3: Distribution of differences (2 mm interval) between scanned distances and reference dis-tances for four tested terrestrial laser scanner (test campaign in March 2007).


plained. As demonstrated in Tab. 2 the spherespacing error (|Δl|) is very good for IMAGER5006 with 1.8mm, while this error is worse bya factor 3–4 for Faro LS880 (5.0 mm) andTrimble GX (6.5 mm).The results of the subsequent 3D test field

investigations in October and December 2007for the five scanners Leica ScanStation 1 and2, Leica HSD6000 and IMAGER 5006, andRiegl LMS-Z420i are summarised in Tab. 3.In these test field investigations spheres with adiameter of 199mm were used since signifi-cantly more measured points were achievedon each sphere over longer distances whencompared to the smaller spheres. These resultsconfirm the previous results fromMarch 2007,where the span (Δlmax – Δlmin), which was ob-

scanner is significantly worse. In contrast tothese good results the spans of the Trimble GXand Faro scanners show huge values of43.6 mm and 71.8 mm, respectively (cf. Tab. 2),using all differences, which demonstrates thatthese scanners obviously have problems withsome 3D distances. In an earlier investigation,which is not published, a significantly betterresult (span min/max = 17.3mm) was achievedwith the Trimble GS100, the predecessor mod-el of the GX. The average value of all differ-ences was less than +1 mm for Faro and Z+Fscanner, while this value was -3.6 mm for Lei-ca ScanStation 1 and -5.5 mm for Trimble GXscanner, which indicates a systematic shiftand which is clearly illustrated in Fig. 3. Cur-rently, these systematic shifts cannot be ex-

Tab. 3: Comparison of 3D distances (all in all combination (left) and seven selected distances(right)) between laser scanner and reference in the 3D test field (Oct./Dec. 2007).

Scanner # 3Dpoints

#dist.

Δlmin[mm]

Δlmax[mm]

span[mm]

syst.shift[mm]

|Δl|[mm]

#dist.

Δlmin[mm]

Δlmax[mm]

span[mm]

LeicaScanStation 1

27 351 −6.4 5.4 11.8 −0.7 1.8 7 −3.1 1.6 4.8

LeicaScanStation 2

28 378 −8.6 4.8 13.4 −2.2 2.6 7 −4.3 −2.3 6.6

Riegl LMS420i 27 351 −6.5 19.8 26.3 6.3 6.5 7 2.5 12.8 10.3

LeicaHDS6000

29 406 −6.3 6.7 13.0 0.2 2.0 7 −2.4 2.4 4.8

Z+F IMAGER5006

29 406 −7.7 5.7 13.4 −0.4 2.1 7 −4.4 1.6 6.0

0

20

40

60

80

100

120

140

-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20

Difference [mm]

Frequency

Leica ScanStation1

Leica ScanStation2

Riegl LMS-Z420i

Z+F Imager 5006

Leica HDS6000

Sphere 199mm

Fig. 4: Distribution of differences (2 mm interval) between scanned distances and reference dis-tances for five tested terrestrial laser scanner (test campaign in October and in December 2007).


3.2 Accuracy Tests of DistanceMeasurements in Comparison toReference Distances

Accuracy tests of distance measurements us-ing reference distances derived from a precisetotal station were performed in an outdoor en-vironment for distance ranges from 10 m to100 m in steps of 10 m (targets on a tripod) forTrimble GX, Leica ScanStation 1, Faro LS880HE and for Z+F IMAGER 5006 in March2007. Reference distances were measured witha Leica TCRP1201 10 times before and 10times after the scanning using averaging dis-tance measurement mode. The differences be-tween the first and second measurement se-quences were less than 0.3 mm. A standarddeviation of 0.1 mm was achieved for the ref-erence distances. Since all tested scanners useWild-type forced-centring, it was possible toexchange prisms for scanner targets. By usingspecial adaptors the centre of the scanner tar-get could be placed in the same position as theprism centre.

All scanning distances for Faro LS880 andIMAGER 5006 were derived from scannedspheres with a diameter of 145 mm, while forLeica ScanStation HDS targets and for Trim-ble GX green flat targets were used. For re-peatability and reliability reasons each dis-tance to sphere or target was scanned threetimes in the sequence forward-backward-for-ward with each scanner from the same posi-tion. Due to the limitation of scanning rangeFaro LS880 scans were checked to the dis-

tained with the Riegl scanner, is slightlyworse, but better than the span for GX andFaro LS880. Again, two scanners (Leica Scan-Station 2 and Riegl) show a systematic shift inthe deviation from the reference (cf. Tab. 3),which is also illustrated in Fig. 4. On the otherhand the systematic shift, which was comput-ed for the Leica ScanStation 1 in March 2007(cf. Tab. 2), could not be confirmed with a dif-ferent Leica ScanStation 1 in the investigationof October 2007 (see Tab. 3). As shown inTab. 3 the sphere spacing error |Δl| is verygood for both ScanStations (1.8 mm/2.6 mm),for HDS6000 (2.0 mm) and for IMAGER 5006(2.1 mm), while this error is worse by a factor3 for Riegl LMS420 (6.5 mm). However, ingeneral the results of the IMAGER 5006 (andHDS6000) are very similar for both independ-ent test campaigns in March and October2007, which is also a confirmation of the reli-ability of the approach used.Better results of the span (Δlmax – Δlmin) and

of the sphere spacing error have been achievedfor Leica HDS6000 (3.5 mm/1.8 mm) and FaroLS880 (8.9 mm/2.0 mm) by kern & huxhagen

(2008) using a test field with short distancesbetween 0.9 m and 3.5 m for spheres with a di-ameter of 76.2 mm. The number of referencedistances used for this test is not published.gordon (2008) used a test field with the di-mensions of 12.5 m×5 m×3.5 m with 37spheres for the IMAGER 5003. In this testslightly worse results have been obtained forthe span: 17.8 mm using 666 distances with 37spheres and 15.6 mm for 8 sphere distances.

-6

-4

-2

0

2

4

6

10 20 30 40 50 60 70 75 80 90 100

Distance [m]

Difference [mm]

Leica ScanStation 1 / HDS Target

Trimble GX / Flat Target

Z+F 5006 / Sphere 145mm

Faro LS880 / Sphere 145mm

Fig. 5: Comparison of the differences between scanning and reference distances (campaignMarch 2007).


for the distance range up to 75 m. All referencedistances were measured by a precision totalstation Leica TCA2003. These determinedreference distances deviated on average by ±0.5 mm from distances which were measuredwith a high precision Kern Mekometer 5000before these investigations.

The scans of the ScanStation 2 to differenttargets (HDS flat blue target, HDS black/whitetarget as well as spheres with a diameter of199 mm) were controlled using the softwareLeica Cyclone 5.8. The spheres used are plas-tic hollow balls with a special surface coatingand centring option, which were developed atthe HCU. All scans were executed with activeinclination compensation and distance correc-tions for atmospheric pressure and tempera-ture, whereby each target was scanned fourtimes. The respective sphere centre coordi-nates were computed automatically in Cycloneand averaged afterwards in order to comparethe scanned horizontal distances with the ref-erence distances (cf. Fig. 6).

As a result, indicated in Fig. 6, a scale factorof approx. +65 ppm can be derived for theScanStation 2. In the scan range under 100mmeasurements to the HDS flat targets showthe smallest residuals (< 5 mm), while over100 m distance the measurements to thespheres indicate the best results (residual ofmax. 12.5 mm to a distance of 287 m). It can be

tance of 60 m and IMAGER 5006 scans to75 m. All major results of this accuracy testare illustrated in Fig. 5. This figure clearly in-dicates that the differences between the LeicaScanStation and IMAGER 5006 and the refer-ences distances are always less than 2 mm,while for the Trimble GX the differences arealso less than 2 mm between 10–60m, butfrom 70 to 100 m distance the differences in-creased to a systematic effect of 3–5 mm. Thedifferences between the Faro LS880 scans andthe reference were in the range of 1–5 mm. Al-though Faro LS880 and Z+F IMAGER 5006are capable of measuring up to 80 m, it mustbe stated that even with the highest resolutionthe number of ‘hits’ on the 145mm sphere isnot high enough for distances beyond 50 m toallow precise fitting of sphere geometry. Ad-ditionally, in several practical outdoor tests itwas notable that signal to noise ratio rises de-pending on daylight conditions for longer dis-tances.

Due to the long range of the Leica ScanSta-tion 2 and the Riegl LMS-Z420i the investiga-tions into the accuracy of distance measure-ments were carried out with a different setupon the official baseline of the city of Hamburgin Ohlsdorf, which consists of seven granitecolumns and covers a distance range up to 430m. For these investigations additional pointsin 10 m intervals were integrated on a tripod

-5

0

5

10

15

20

10 30 41 50 61 70 76 82 102 123 164 184 205 226 246 287

Distance [m]

Difference[mm]

HCU-Kugel HDS-Target HDS B/W-Target

Fig. 6: Comparison of the differences between scanning and reference distances for the LeicaScanStation 2 (test campaign in October 2007).


3.3 Accuracy Tests of InclinationCompensation

All scanners in the test programme areequipped with an inclination sensor (see alsoTab. 1), making it possible to level the scannerduring measurements. Leica ScanStation 1/2and Trimble GX are able to compensate forchanges of main axis inclination during meas-urement, while Faro LS 880 uses correctionsonly for post-processing (in the registration ofscans). The Z+F IMAGER 5006 uses the incli-nation sensor for gross error detection to indi-cate changes during the scanning, and for cor-rections of the scanned data in the postprocessing. If the inclination sensor is switchedon during the scanning process, it is assumedfor the time-of-flight scanners that the XY-plane of the scanner coordinate system is hori-zontal.

In order to check the accuracy of inclinationcompensation of each scanner, an outdoor testfield was established using 12 spheres in stepsof 30° on the circumference of a circle with aradius of 50 m. Each sphere was set up on apole and was adjusted to the same height byusing a Wild N3 high-precision level instru-ment, while the tested scanners were set up inthe centre of the circle on a heavy-duty tripod(cf. Fig. 8). While scanning the spheres, it isassumed that the centre coordinates of the fit-ted geometries (spheres) lie in-plane and thatthis plane is horizontal (Z = constant). Tocheck for movements of the scanner tripodduring scanning, a Leica Nivel20 inclinationsensor was fixed to the tripod, recording incli-nation in x and y direction in intervals of 5

assumed that the fitting algorithm is positiveaffected by the larger sphere surface comparedto the HDS flat blue target. For the measure-ments to black/white targets the fitting algo-rithm of Cyclone could only supply a result upto a distance of 205 m. This scale factor is af-fected by the targets/spheres being too smallin relation to the long scan distances (over200 m). Larger targets/spheres would probablyyield better results. Therefore other ScanSta-tions 2 should be tested for the presence of thesame problems.

The results of the investigations into thescanning accuracy of the Riegl LMS-Z420iscanner using the reflective target (size50 mm), which was scanned three times foreach position, are illustrated in Fig. 7. The dif-ferences between scanning distance and refer-ence are in the range of ±5 mm for distancesup to 205 m, but for distances over 205 m thetarget used was too small to derive reliable re-sults from these scanned distances. Thus, thetarget size must be adapted to a larger size(e. g., 100 mm) for scanning longer distancesin future tests. tauBer (2005) could achievesimilar results on the baseline of the LeibnizUniversity Hanover using the Riegl LMS360i.

The accuracy investigations into the testedlaser scanning systems clearly demonstratedthat the systems meet the technical specifica-tions of the manufactures for distances up to200 m.

-8

-6

-4

-2

0

2

4

6

10 20 30 40 42 50 52 60 61 70 72 82 82 102 123 164 184 205

Distance [m]

Difference

[m

m]

Fig. 7: Comparison of the differences between scanning and reference distances for the RieglLMS 420i using the reflective target (test campaign in December 2007).


and in October 2007 with spheres with a di-ameter of 199 mm, is almost identical and isvery similar to the Faro LS880. These effectsare influenced by a slight inclination of the ro-tation axis. In Fig. 9 (bottom) differences froman average plane fitted through the centre co-ordinates of the spheres are shown. Since allspheres were positioned on a plane, differenc-es should be zero. The resulting differencesmay be interpreted as effects of a tumbling er-ror of the trunnion axis, but especially for theFaro and Z+F scanners the results are influ-enced by the sphere fitting error due to thescanning noise on the longer distances. Fur-ther investigations have to be performed withbigger targets and/or smaller radius of circle toguarantee sufficient numbers of scannedpoints on the spheres for reliable and precisesphere fitting, especially for phase differencescanners with limited scan distances.

Fig. 10 (left) shows a sine oscillation result-ing from an inclined vertical axis when theinclination compensation of the Leica Scan-Station 2 is switched off. The magnitudes ofthe amplitude following the 360° rotation de-pend on the inclination angle. When inclina-tion compensation is switched on, the graphshows very minor deviations of better than1mm for the z coordinate vs. the horizontalplane (cf. Fig. 10 right). Since these results arevery similar to the previous tests using LeicaScanStation 1 and Trimble GX (cf. Fig. 9 top),

seconds. The recordings of the Nivel20 showedno significant movements of the tripod duringscanning (cf. Fig. 8).

Each sphere was scanned consecutivelythree times (March 2007) and five times (Oc-tober and December 2007) with the highestpossible resolution settings. The fitting ofsphere geometries was performed using Trim-ble RealWorks Survey 5.1. Before sphere fit-ting, outliers were removed manually from thepoint cloud. The derived average Z-coordi-nates of all fitted spheres were compared tothe reference horizontal plane for each scan-ner. Differences in Z vs. the reference planewere obtained from the average Z-coordinateof each position in the circle and are shown inFig. 9. This is a clear indication that the com-pensation of inclination works almost perfect-ly for all tested time-of-flight scanners, whilefor the phase difference scanner it can be seenthat scanning has been conducted in an in-clined plane.

Leica and Trimble scanners show maximumdeviations of 2 mm with a very minor sine os-cillation, probably resulting from calibrationerror of the inclination sensor (cf. Fig. 9 top).Faro LS880 shows huge differences up to15mm, which may be influenced by the com-parably low resolution (8 mm / 50 m) and thelarge signal to noise ratio of this scanner. Thebehaviour of the IMAGER 5006, tested inMarch with spheres with a diameter of 145 mm

Fig. 8: Test field for inclination compensator of the terrestrial laser scanner: scanner on solid tripod(left), schematic test configuration for scanner and spheres (centre), inclination sensor LeicaNivel20 fixed at the scanner tripod (right) and illustration of tripod movement derived from Nivel20measurements over the test period of more than 871 minutes (bottom).


the laser beam. Reasons for this effect are thespot size and shape of the laser beam and thereflectivity of the object. The shape and itscentre position influences the reflectance ofthe laser beam, which affects the precision ofthe scanned distance, and the 3D position of ascanned point within the point cloud. To eval-uate the influence of the laser beam’s angle ofincidence on 3D accuracy of the point cloud aplanar white stone slab with a dimension of75×79 cm2 (cf. Fig. 11 centre) was mounted ina metal frame and could be swivelled in this

it can be stated that the dual axis (tilt) compen-sator of the scanners with the time-of-flightmethod almost perfectly adjusts for changes ofinclination during scanning.

3.4 Influence of the Laser Beam’sAngle of Incidence on 3DAccuracy

Among other effects the accuracy of a pointcloud is dependent on the angle of incidence of

Position on circle [ ° ]

Differen

ce

[m

m]

Trimble GX Leica ScanStation 1

Leica ScanStation 2 Faro LS880

Z+F IMAGER 5006 (March2007) Z+F IMAGER 5006 (Oct2007)

Position on circle [ ° ]

Differen

ce

[m

m]

Fig. 9: Test of inclination sensor in comparison: Differences between scanned spheres and hori-zontal XY plane (top), and average XY plane (bottom).

-1,5

-1,0

-0,5

0,0

0,5

1,0

0° 30° 60° 90° 120° 150° 180° 210° 240° 270° 300° 360°

Position on the circle [°]

Diffe

re

nc

e[m

m]

Fig. 10: Leica ScanStation 2: Differences vs. horizontal plane (z-coordinate) for switched off com-pensator (left) and active inclination compensator (right). Note the difference in y-scale betweenthe two graphs.


the stone slab, was compared to referencepoints.

Since the angular position of the stone slabhas no effect on the point cloud of the spheres,the centres of the spheres were selected as ref-erence points for each position. Thus, the dis-tance between the centre of the sphere and anaverage plane fitted through the point cloudrepresenting the stone slab should be constantin an ideal case for each angular position ofthe stone slab. Nevertheless, it can be observed

frame. The frame was equipped with a read-ing device to set the stone slab at defined an-gular positions with a precision of 5’. Addi-tionally, four spheres (radius 38.1 mm) werefixed on the stone slab, thus swivelling togeth-er with the stone slab. The stone slab and thespheres were scanned with a resolution of3 mm at an object distance of 10 m. In total,ten scans were acquired in angular positionsof the stone slab from 90° to 5°. Each plane,which was fitted in the resulting point cloud of

Fig. 11: Scanning set up for the investigations into the laser beams angle of incidence on 3D ac-curacy with swivelling planar white stone slab (centre).

-2

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 85

Angle of incidence [ ° ]

Difference[mm]

Faro LS880 Leica ScanStation 1 Trimble GX

Z+F 5006 Leica ScanStation 2

Spot size of laser beam at 10m

Faro LS880: 2.5mm

Leica ScanStation: 4.0mm

Trimble GX: 0.6mm (autofocus enabled)

Z+F IMAGER 5006: 3.2mm

Fig. 12: Influence of angle of incidence on 3D accuracy in comparison.


The accuracy tests of distance measure-ments in comparison to reference distancesshowed clearly that the results of most of thescanners met the accuracy specification of themanufacturer, although the accuracy (definedas measured versus reference distance) isslightly different for each instrument. Asshown in Tab. 5 only the Faro scanner hasslight problems meeting the accuracy specifi-cation. Furthermore, the accuracy is decreas-ing significantly for increasing distanceslonger than 200 m. It can be assumed that thetargets/spheres used for these longer distanceswere too small. Consequentely, the target/sphere size must be adapted to the scanningdistance. However, it could be seen in severalpractical outdoor tests that signal to noise ratiorises in daylight conditions for longer distanc-es.

The accuracy tests of the inclination com-pensation show that the inclination of thetime-of-flight scanners is successfully com-pensated, while the phase difference scannersshow effects (not errors) resulting from incli-nation of the vertical axis. A trunnion axis er-ror could not be proven. The influence of angleof incidence on 3D accuracy can be neglectedfor time-of-flight scanners, while phase differ-ence scanners show significant deviations, ifthe angle of incidence is more than 45°. Theaccuracy is also not influenced by the spot sizeof the laser with respect to the angle of inci-dence. Nevertheless, previous investigationsinto the influence of object colour on the qual-ity of laser distance measurements showedthat for the Faro and Trimble scanners someobject colours cause significant effects on theaccuracy of the scanning distance (mechelke

et al. 2007).All investigations showed clearly that the

tested scanners are still influenced by instru-mental errors, which might be reduced by in-strument calibration. Therefore, it is necessaryto define standards for investigations and testsof laser scanning systems to derive simplecalibration methods for the scanners as is usu-al for total stations and which can be appliedby the user. These presented test proceduresmay be taken into consideration for future dis-cussions on the implementation of standard-ized test procedures. A valuable proposal forthe definition of standardised quality parame-

in Fig. 12 that the distance between the centresof the spheres and the computed plane increas-es with an increasing angle of incidence. Thetime-of-flight scanners show minor effects ofup to 3 mm for an angle of incidence of 80°–85°, while the phase difference scannersachieve difference values of up to 12 mm forthe same angle. But generally, it can be statedthat if the angle of incidence is more than 45°,significant influence on the accuracy of thepoint cloud can be expected. The conditions inthe test environment were the same for allscanning systems. But to achieve results forcomparison with other test environments, thespectral reflectance of the stone slab shouldbe determined in relation to the wavelength ofthe laser beam. Additionally, further investi-gations are still necessary to check the influ-ence of angle of incidence for longer objectdistances.

4 Conclusions and Outlook

The major results of different tests using thecurrent instruments of the new generation ofterrestrial laser scanners are summarised inthis paper. The investigations in the 3D testfield showed that the range value (span), whichis influenced by the measurement precision ofthe instrument and by the algorithm for the fit-ting of the sphere, varied from 11.5 mm to71.8 mm for the tested scanners. It must bestated that these results are derived from high-ly correlated differences between scanned andreference distances. According to the proposalof heiSter (2006) and the VDI/VDE 2634(2002) a span from 4.8–10.3 mm (exceptionalcase 33.4 mm for Faro) has been achieved inthe test field using just seven selected well-distributed distances for comparison. Howev-er, the influence of errors in distance and anglemeasurements have not been determined sep-arately due to the purpose of testing the com-plete laser scanning system (hard- and soft-ware). In this test it could be demonstrated thatonly the time-of-flight scanners achieved asystematic shift of up to +6 mm in the deriveddistances. The sphere spacing error was betterthan 3mm for most of the scanners, exceptionswere Trimble GX, Riegl LMS420 and FaroLS880.


JohanSSon, m., 2003: Explorations into the behav-iour of three different high-resolution ground-based laser scanners in the built environment. –CIPA WG 6 International Workshop on Scan-ning for Cultural Heritage Recording, Corfu,Greece: 33–38.

kern, F., 2008: Prüfen und Kalibrieren von terres-trischen Laserscannern. – Photogrammetrie –Laserscanning – Optische 3D-Messtechnik,Wichmann, Heidelberg: 306–316.

kern, F. & huxhagen, u., 2008: Ansätze zur sys-tematischen Kalibrierung und Prüfung von ter-restrischen Laserscannern (TLS). – Terrestri-sches Laserscanning (TLS2008), DVW-Schrift-enreihe 54: 111–124.

kerSten, th., SternBerg, h., mechelke, k. &acevedo Pardo, c., 2004: Terrestrischer Laser-scanner Mensi GS100/GS200 – Untersuchungenund Projekte an der HAW Hamburg. – Photo-grammetrie – Laserscanning – Optische 3D-Messtechnik, Wichmann, Heidelberg: 98–107.

kerSten, th., SternBerg, h. & mechelke, k.,2005: Investigations into the Accuracy Behav-iour of the Terrestrial Laser Scanning SystemTrimble GS100. – Optical 3-D MeasurementTechniques VII I: 122–131.

lichti, d. d. & Franke, J., 2005: Self-calibration ofthe iQsun 880 laser scanner. –Optical 3-D Meas-urement Techniques VII I: 112–121.

mechelke, k., kerSten, th., & lindStaedt, m.,2007: Comparative Investigations into the Accu-racy Behaviour of the New Generation of Ter-restrial Laser Scanning Systems. – Optical 3-DMeasurement Techniques VIII I: 319–327.

mechelke, k., kerSten, th., & lindStaedt, m.,2008: Geometrische Genauigkeitsuntersuchun-gen neuester terrestrischer Laserscanningsys-teme – Leica ScanStation 2 und Z+F IMAGER5006. – Photogrammetrie – Laserscanning –Optische 3D-Messtechnik, Wichmann, Heidel-berg: 317–328.

neitzel, F., 2006: Untersuchung des Achssystemsund des Taumelfehlers terrestrischer Laserscan-ner mit tachymetrischem Messprinzip. – Terres-trisches Laser-Scanning (TLS2006), DVW-Schriftenreihe 51: 15–34.

reShetyuk, y., 2006. Investigation and calibrationof pulsed time-of-flight terrestrial laser scan-ners. – Licentiate thesis in Geodesy, Royal Insti-tute of Technology (KTH), Stockholm, Sweden.

rietdorF, a., 2005: Automatisierte Auswertungund Kalibrierung von scannenden Messsyste-men mit tachymetrischem Messprinzip. – DGKReihe C, Nr. 582, München.

Schulz, t., 2007: Calibration of Terrestrial LaserScanner for Engineering Geodesy. – Disserta-tion No. 17036, ETH Zürich.

ters for the investigation of terrestrial laserscanning systems is already summarised byheiSter (2006) and practically tested by kern

& huxhagen (2008). Future investigations inTLS should refer to these definitions.

References

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Address of the Authors:

thomaS kerSten, klauS mechelke, maren lindStaedt, harald SternBerg, HafenCity UniversitätHamburg, Department Geomatik, D-22297 Ham-burg, Tel.: +49-40-42827-5343, Fax: +49-40-42827-5399, e-mail: {Thomas.Kersten, Klaus.Mechelke,Maren.Lindstaedt, Harald.Sternberg}@hcu-hamburg.de

Manuskript eingereicht: Februar 2009Angenommen: Mai 2009

Schulz, t. & ingenSand, h., 2004: Influencing Var-iables, Precision and Accuracy of TerrestrialLaser Scanners. – ‘Ingeo 2004’, Bratislava,www.geometh-data.ethz.ch/downloads/SchulzT_TS2_Bratislava_2004.pdf.

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vdi/vde 2634, 2002: Optische 3D-Messsysteme– Systeme mit flächenhafter Antastung. – VDI/

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