Open Access Error Analysis and Accuracy Assessment of Mobile Laser Scanning System

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1 Sed Orders for Reprits to The Ope Automatio ad Cotrol Systems Joural, 2015, 7, Ope Access Error Aalysis ad Accuracy Assessmet of Mobile Laser Scaig System Shouzhi Xu 1,*, Pegfei Cheg 2, Yu Zhag 3 ad Peghui Dig 4 1 School of Geodesy ad Geomatics, Wuha Uiversity, Wuha, , P.R. Chia 2 Chia Natioal Quality Ispectio ad Testig Ceter for Surveyig ad Mappig Products, Beijig , P.R. Chia 3 Chiese Academy of Surveyig ad Mappig, Beijig , P.R. Chia 4 Natioal Calibratio Ceter for Surveyig Istrumet, Beijig , Chia. P.R. Chia Abstract: Mobile Laser Scaig System (MLS) is itegrated with GNSS, IMU, Laser Scaer ad Digital Cameras, ad other sesors takig vehicle as a carrier. It ca quickly obtai spatial locatio ad attribute iformatio o both sides of roads. With the rapid developmet of mobile mappig techology, there is a great variety of MLSs. Therefore, how to evaluate the performace of these systems is gettig more ad more attetio. I this paper, the error factors affectig the accuracy of MLS were aalyzed. Based o these aalyses, the accuracy assessmet methods were proposed for three sesors ad the system. The kow baselie value was used to assess static positioig accuracy of GNSS, GT580 was used to assess attitude accuracy of IMU, total statio was adopted to assess distace accuracy of Laser Scaer, ad multi-tooth dividig table was used to assess agle measuremet of Laser Scaer. Testig field was used to assess the plae ad elevatio accuracy of the system. The experimets show that the methods proposed i this paper are feasible eve for cmlevel high precisio of MLS. Therefore the research has practical sigificace ad referece value. Keywords: Accuracy assessmet, Error aalysis, GNSS, IMU, Laser Scaer, MLS. 1. INTRODUCTION With the advet of iformatio surveyig, the way people gather iformatio gradually developed from static collectio with a sigle sesor to dyamic acquisitio with multi-sesors. With the cocepts beig put forward such as "Smart City" ad "Digital City", the demads for threedimesioal spatial iformatio are gettig larger ad larger i agriculture, trasportatio, commuicatios, idustrial, disaster moitorig ad other fields, ad the requiremets of updatig speeds for spatial iformatio are gettig higher ad higher. Therefore, the traditioal mappig methods are far from eough to meet the growig demads for spatial iformatio due to the acquisitio cost ad quick updatig speeds. Uder this backgroud, the Mobile Laser Scaig system came ito beig. MLS is itegrated with Global Navigatio Satellite System uit (GNSS), Iertial Measuremet Uit (IMU), Laser Scaer ad Digital Cameras ad other advaced sesors i vehicle. The digital cameras are selectable. I the procedure of the vehicle, this system quickly collects a variety of iformatio about the target area, such as spatial data, attribute data, real images ad other iformatio. *Address correspodece to this author at the School of Geodesy ad Geomatics, Wuha Uiversity, Wuha, Hubei, , P.R. Chia; Tel: ; xsz2011@whu.edu.c I recet years, the research o MLS has bee maily cetralized o three aspects: systems itegratio techology researches [1-6], system parameter calibratio [7-9], ad system applicatios [10-16]. With the developmet of MLS, more ad more attetio has bee paid to its precisio ad the evaluatio of its performace has also bee icreasigly gettig importat. However, literatures are rarely ivolved this field [17-19]. I this paper, we gave a comparatively overall accuracy assessmet of MLS, icludig accuracy assessmet of each sesor ad the overall accuracy assessmet of the system, ad the steps of the correspodig assessmet methods were put forward i detail. Ad the the feasibility ad validity were demostrated by a experimet. 2. ERROR ANALYSIS OF MLS MLS is multi-disciplie combiatio, multi-sesor itegratio ad multi-data fusio system. At preset, the maistream MLSs at home ad abroad are show i Fig. (1). The workig priciple is that laser scaer is doig twodimesioal scaig perpedicular to the travel directio, takig a vehicle travelig directio as the motiodimesioal, ad costitutig three-dimesioal scaig system. At the same time, GNSS provides a vehicle s accurate locatio iformatio, while IMU provides a vehicle s spatial attitude iformatio. A three-dimesioal poit cloud data of / Betham Ope

2 486 The Ope Automatio ad Cotrol Systems Joural, 2015, Volume 7 Xu et al. Fig. (1). The maistream MLSs at home ad abroad. scaed poits are obtaied through data fusio. Its positioig equatio is show as equatio (1).! " x p y p z p! + " $ % = R W LH (t)r IMU x W LH (t) y W LH (t) z E LH (t) $ %!! LH (t) k IMU IMU L R L " " x L y L z L $! + % " x L IMU y L IMU z L IMU $ $ % % (1) Where, (x p, y p,z p ) is the sca poit coordiates i the WGS-84 coordiate system, R W LH (t) is the trasformatio matrix from local coordiate system to the WGS-84 coordiate system, R LH IMU (t) is the trasformatio matrix from IMU coordiate system to local coordiate system, k L IMU is the scale factor betwee laser scaer coordiate ad IMU coordiate system, R L IMU is the trasformatio matrix from laser scaer coordiate system to IMU coordiate system, (x L, y L,z L ) is the sca poit coordiates i laser scaer coordiate system, (x L IMU, y L IMU,z L IMU ) is the scaig ceter coordiates of laser scaer i IMU coordiate system, (x W LH, y W LH,z W LH ) is traslatio values betwee local coordiate system ad WGS-84 coordiate system. The MLS errors iclude the errors related with the sesors ad system itegratio Errors Related with the Sesors 1) GNSS errors Accordig to the classificatio with error characteristics, there are stochastic errors ad system errors. Stochastic errors iclude multi-path effects ad observatio errors, while systematic errors iclude orbit errors, satellite clock errors, receiver clock errors ad atmospheric refractio errors. The correspodig error processig techology is relatively mature. The impacts of errors ca be decreased or elimiated usig the three techiques: model method, differetial method ad adjustmet method. The model method ca establish error correctio model based o aalysis of error characteristics, mechaisms ad causes, or establish empirical fittig formula accordig to a large umber of observed data. The differetial method ca elimiate or decrease its impact by differecig usig the physical correlatio of the errors betwee the observatios. The adjustmet method models error, itroduces model parameters, ad the put them together with other ukow parameters i the adjustmet. 2) IMU errors IMU errors maily iclude compoet errors, istallatio errors, iitial coditio errors, priciple errors, ad outside iterferece errors. Compoet errors maily refer to the gyro s drift ad acceleratio s zero bias ad compoet s calibratio error. Istallatio error refers to the error caused by accelerometer ad gyroscope istalled iaccurately. Iitial coditio error refers to the error formed by the iaccurate

3 Error Aalysis ad Accuracy Assessmet The Ope Automatio ad Cotrol Systems Joural, 2015, Volume Table 1. The results of GNSS baselie calculated by post-processig software. Times X(m) Y(m) Z(m) Baselie values m iitial positio ad velocity iput ito IMU. Priciple errors are caused by approximatio error with mathematical models, the earth shape differece ad gravity aomaly. Outside iterferece errors maily refer to disturbig errors caused by vibratio while the vehicle is movig. 3) Laser Scaer errors Laser Scaer errors iclude ragig errors ad agle measuremet errors. Ragig errors are maily classified ito istrumetal errors ad evirometal errors. Istrumetal errors maily refer to the time delay error problem i electro-optical circuit, which also icludes a prism rotatio error, vibratio error, ad the correspodig delay circuit errors. Evirometal error is the ragig error caused by the differet reflectivity due to the reflectig surface of the material, color, texture formatio. The agle measuremet errors with optical agular ecoder maily come from mechaical processig ad istallatio. The mechaical processig ad istallatio icludes istallatio eccetric, istallatio tilt, rotatio axis shakig ad so o. The error caused by istallatio tilt ad rotatio axis shakig is very small ad ca be igored Errors Related with System Itegratio System itegratio errors iclude laser scaer s istallatio errors, time sychroizatio errors, ad coordiate trasformatio errors ad so o. (1) Istallatio errors. Each sesor has its ow coordiate system, but sesors ca ot be guarateed i accordace with the desig attitude ad positio. Thus there are three boresight agles ad lever arms. Istallatio errors cause systemic deviatios i the results, so the three boresight agles ad lever arms eed to be determied firstly before the MLS measurig. (2) Time sychroizatio errors. The time referece of each differet sesor has a certai differece, the respective time system eeds to be uified ito the UTC system. Moreover, the data iterpolatio will result i errors as the samplig frequecies are differet i each sesor. (3) Coordiate trasformatio errors. The errors exist whe coordiate trasformig occurs due to the limitatios of coordiate trasformatio model. 3. ACCURACY ASSESSMENT OF MLS 3.1. Accuracy Assessmet of Sesors 1) Static accuracy assessmet of GNSS. GNSS provides locatio iformatio for MLS. The followig method was used to assess the locatio accuracy of GNSS. The GNSS receiver was placed o kow cotrol poit, ad observed simultaeously with BJFS CORS statio four times, ad for a hour each time. The baselie value ca be obtaied usig GNSS data post-processig software. The results are show below i Table 1. As the Table 1 showed, the average baselie value was m. Therefore the ier ad exteral accuracy of GNSS ca be calculated with the stadard deviatio ad the root mea squared error (RMSE) respectively. The result showed that the ier accuracy was 3.97 mm ad the exteral accuracy was 4.79 mm. 2) Accuracy Assessmet of IMU. IMU provides attitude iformatio for MLS. The IMU accuracy evaluatio maily icludes the accuracy of roll agle, pitch agle, headig agle, ad zero drift. The high accuracy referece is provided by GT580 dual-axis digital display type maual turtable. The turtable is composed of two parts, body ad digital display table. The turtable s body is composed of ier rig shaft axis ad the outer axis. The ier is a axis bearig of 360, ad its accuracy is ±5", the accuracy of outer shaft is about ±7" with a resolutio of 1". The turtable s performace is show i Table 2, ad its appearace is show i Fig. (2). The accuracy evaluatio methods of roll agle iclude three major steps i our study. The first step is levelig the turtable precisely usig level tube before istallig IMU o the levelig turtable, ad the bootig iitializatio for 15 miutes. The secod step is adjustig the turtable ad IMU o the same axis to esure the chage of pitchig agles is very small whe the roll agle is tured. The last step is settig digital display table zero, ad the recordig the IMU observed value (α i ) ad the results (φ i ) show i data display istrumet whe rotatig turtable every 10. The observed values of IMU roll agle are show i Table 3. Ad the accuracy of roll agle (σ φ ) is calculated ad show as! " = v i 2 i=1 $1 = The accuracy assessmet methods for pitch agle ad headig agle are i the same way as the accuracy evaluatio with the roll agle. The accuracy of pitch agle (σ θ ) was equal to 0.073, ad the accuracy of headig agle (σ Ψ ) was equal to

4 488 The Ope Automatio ad Cotrol Systems Joural, 2015, Volume 7 Xu et al. Fig. (2). GT580 dual-axis digital display turtable. Table 2. The performace of GT580 biaxial digital maual turtable. Performace Idicators Outer Axis (pitch) Ier Shaft (Orietatio) Rotatio rage Agled rotatio error ±3 ±2 Locatig accuracy of agular positios ±5 / ±7 ±3 / ±5 Digital display measurig resolutio 1 1 Zero drift refers to the dispersio degree aroud the mea of the output of the fiber optic gyroscope i a statioary state. It express as agle rate which is equivalet to the stadard deviatio of the output of IMU. The formula is followig as equatio (2). B s = 1 1 % k $!1 " i=1 ( F i! F ) 2 ( ' 1/2 (2) Where: B s is zero drift F i F i is output of IMU F is the mea of output is the umber of samples. As the key performace idicators of IMU, zero drift eeds to be assessed for its accuracy. I this paper, attitude agle data of IMU was oe hour static collectio, its output frequecy was 1Hz; the output data is show i Fig. (3). As see from Fig. (3), the bias of roll ad pitch is oly 0.05 degree, but the bias of headig is 3 degree. It shows that the headig stability is very bad. Because the headig is related to the real orth, it eeds to iitialize for low accuracy IMU. Ad the Zero drift accuracy assessmet is computed by equatio (2). Zero drift of roll is Zero drift of pitch is ad Zero drift of headig is ) Accuracy assessmet of Laser Scaer The laser scaers of MLS are usually two-dimesioal laser scaers, ad its workig priciple is that the laser pulse is emitted by the diode laser pulse. It scas the target poit by rotatig prism, ad the receives ad records the reflected laser pulse through the detector thereby acquirig the three-dimesioal coordiates of the target poit. The measuremet priciple is to measure the distace by the propagatio time or phase chage, ad to obtai the agle value of the laser beam withi the istrumet by a precisio clock cotrol ecoder. The origi poit of the laser scaer coordiate system is a laser emittig a referece, x-axis directio poit is the vehicle directio, z-axis directio perpedicular to the directio of vehicle directio, ad y-axis is to meet the right had rule. Therefore, the laser spot coordiate at the foot of the laser scaer P (x L, y L,z L ) is show as equatio (3). Where, θ is a scaer agle, S is a scaer distace.! x L $! 0 $ y L = S si' (3) " z L % " S cos' % Accuracy assessmet of laser scaer icludes measurig agle ad rage accuracy. a) Rage accuracy of Laser Scaer The stadard values were obtaied by total statio Sokkia NET05; its precisio is (1+1ppm*D) mm. The rage accuracy was computed usig the observed value of the laser scaer to mius the stadard values by root-mea-square error. The rage data observed is show i Table 4. b) Agle measurig accuracy of Laser Scaer Laser scaer accuracy was assessed usig multi-tooth dividig table. It refereced the comparative method that the stadard deviatio test method roud horizotally with a total

5 Error Aalysis ad Accuracy Assessmet The Ope Automatio ad Cotrol Systems Joural, 2015, Volume (a). Roll output (b). Pitch output Fig. (3). The output of IMU attitude i a hour. (c). Headig output

6 490 The Ope Automatio ad Cotrol Systems Joural, 2015, Volume 7 Xu et al. Table 3. The observed value of IMU roll agle i degrees. Id Observed Value Turtable Value Agle of Observed Value Agle of Turtable Residual Error(vi) Table 4. Laser scaer distace observatio i meters. Distace Laser Scaer Total Statio Error

7 Error Aalysis ad Accuracy Assessmet The Ope Automatio ad Cotrol Systems Joural, 2015, Volume Table 4. cotd Distace 50 Laser Scaer Total Statio Error The rage accuracy is: 0.005m@10m, 0.016m@20m, 0.027m@50m. Fig. (4). Multi-tooth dividig table. statio ad electroic theodolite, that is, the multi-tooth dividig table was take as a agle of the stadard device, ad compared with the observed values obtaied by laser scaer. Multi-tooth dividig table is show i Fig. (4) that is a kid of circular dividig stadard istrumet with high accuracy. Geerally, it cosists of two toothed plates with the same tooth ad modulus. Whe such table is used, a tooth plate is fixed ad is off with aother. It meshes whe rotatig to the desired agle, so as to obtai positioig ad high degree. Multi-tooth dividig table i 552 was used, ad the maximum error is about 0.3!!. Agular accuracy assessmet methods cotai two phases. The first phase was to level precisely the multi-tooth dividig table, fix the laser scaer to the platform ad the level it together with foudatio (Fig. 4). The secod phase was to set the multi-tooth dividig table zero, ad rotate clockwise with a certai agle each time. Ad the the scaer also followed the same rotatig agle. The scaer scaed the target whe the rotatio fiishes, ad the correspodig agle value was extracted from the target. The laser scaer agle value ad its accuracy are show i Table Evaluatio of the System Accuracy The fial data processed from MLS are threedimesioal coordiates of poit clouds. Therefore, the coordiate precisio of poit clouds is a importat idicator of the system performace evaluatio. Coordiate precisio of poit clouds icludes plae precisio ad elevatio accuracy. The accuracy assessmet method is as follows: the feature poits o both sides of the road, whose threedimesioal coordiates ca be obtaied by the traditioal way with high precisio, ca be regarded as the kow poits ad their three-dimesioal coordiates as the stadard values. Ad the the system accuracy ca be evaluated by comparig the results obtaied from MLS with the stadard values of the kow feature poits. System accuracy assessmet was carried out i the testig field (Fig. 5), which was 1000m log from east to west ad 550m wide from orth to south. The testig field was composed of the 45 kow cotrol poits measured by RTK (with a expected accuracy of 1cm + 2ppm i horizotal plae ad 2cm + 2ppm i height) ad 200 measured poits by total statios (Sokkia NET05).

8 492 The Ope Automatio ad Cotrol Systems Joural, 2015, Volume 7 Xu et al. Fig. (5). The testig field. Accuracy assessmet procedure maily icluded four steps. The first step was to combie GNSS data ad IMU data by itegrated avigatio software i order to obtai MLS vehicle track. The secod step was to preprocess laser scaer data usig laser pretreatmet software. The third step was to fuse track data ad laser data for obtaiig WGS-84 coordiates of poit clouds by usig poit cloud processig software. The last step was to extract coordiates of feature poits (such as: buildig corer, corer widows, poles, traffic sigs, etc.) i the testig field, ad compare them with the kow poits, ad the assess their accuracy with equatio (4). $ (x p " x r ) 2 i=1! x = ± ( y p " y r ) 2 i=1 %! y = ± (h p " h r ) 2 i=1! h = ±! p =! 2 2 x +! ' y (4) Where, (x p, y p,z p ) is a poit coordiate which is computed by MLS, (x r, y r,z r ) is the coordiate of the referece poit, σ p is the plae accuracy,! h is elevatio accuracy. Plae error ad elevatio error of MLS are show i Fig. (6) ad Fig. (7). We selected early more tha 90 obvious feature poits i the testig field to assess accuracy, ad the plae accuracy was 0.187m ad elevatio accuracy was 0.251m accordig to equatio (4). 4. DISCUSSION The accuracy of MLS is related to the accuracy of each sesor besides the level arm ad boresight agle. The level arm refers to the distace from the ceter of the GNSS coordiate system to the ceter of the IMU coordiate system, ad the distace from the ceter of the Laser Scaer coordiate system to the ceter of the IMU coordiate system. The boresight agel refers to the agel betwee laser scaer coordiate system to IMU coordiate system. Therefore, it requires precise calibratio of level arm ad boresight agle before accuracy assessmet. GNSS uit of MLS is a dyamic surveyig process. But we assessed the static surveyig accuracy of GNSS, because it is hard to assess the dyamic surveyig accuracy, it

9 Error Aalysis ad Accuracy Assessmet The Ope Automatio ad Cotrol Systems Joural, 2015, Volume Fig. (6). The differetial value of coordiates i x directio ad y directio. Fig. (7). The differetial values of height directio. Table 5. Laser scaer agle measuremet i degrees. ID Laser Scaer Multi-tooth Dividig Table Error

10 494 The Ope Automatio ad Cotrol Systems Joural, 2015, Volume 7 Xu et al. Table 5. cotd ID Laser Scaer Multi-tooth Dividig Table Error Agular accuracy was obtaied by root-mea-square error method is is difficult to obtai the vehicle positio i real time for accuracy assessmet. This problem may be solved i the future. I this paper, the preset methods were verified with the decimeter level of MLS, the methods also ca assess the cetimeter level of MLS because the testig field has high precisio which the coordiates reach to 1~2cm. It has a greater error whe extract the feature poits coordiate for assessig the overall accuracy of MLS. The error is related to the agular resolutio ad scaig distace of the laser scaer, thus how to reduce extractio errors will be researched i the future. CONCLUSION Curretly, sice there are may varieties ad differet properties of MLSs, how to evaluate the performace of this system is particularly importat. I this paper, a set of key performace evaluatio methods are proposed to assess the accuracy of the sesors ad the system. The methods are proved to be effective ad reliable for MLS itegrated accuracy assessmet. The developmet tred of MLS i future is to improve the system by usig more accurate sesors ad more sophisticated techology, further expadig the scope of its applicatio areas ad applicatios. With the developmet of Chiese Smart City, the system is boud to have a great value ad market demad. CONFLICT OF INTEREST The authors cofirm that this article cotet has o coflict of iterest. ACKNOWLEDGEMENTS This research is fiacially supported by the Surveyig ad Mappig Geographic Iformatio Public Idustry Scietific Research Projects uder grat No Special thaks to Xiujua Wu from Chiese Academy of Surveyig ad Mappig (CASM) for her help i improvemet of the Eglish laguage. The authors would also like to thak Sheghua Xu for his costructive suggestios ad valuable commets. REFERENCES [1] K. Novak, ad J. D. Bossler, Developmet ad applicatio of the highway mappig system of Ohio state uiversity, Photogrammetric Record, vol. 15, pp , [2] K. P. Schwarz, ad N. El-Sheimy, Kiematic multi-sesor systems for close rage digital imagig, Iteratioal Archives of Photogrammetry ad Remote Sesig, vol. XXXI, pp , [3] N. El-Sheimy, A mobile multi-sesor system for GIS applicatio i urba ceters, Iteratioal Archives of Photogrammetry ad Remote Sesig, vol. XXXI, pp , [4] G. P. He, Desig of a mobile mappig system for GIS data collectio, Iteratioal Archives of Photogrammetry ad Remote Sesig, vol. XXXI, pp , July [5] G. Huter, C. Cox, ad J. Kremer, Developmet of a commercial laser scaig mobile mappig system Street Mapper, I: 2 d Iteratioal Workshop the Future of Remote Sesig, Atwerp, October 17-18, [6] X. S. Lu, B. Shi, ad D. Wag, 3DSUR-a example of the developmet of mobile mappig systems i Chia, Survey Review, vol. 43, pp , [7] P. Rieger, N. Studicka, ad M. Pfeigbauer, Boresight aligmet method for mobile laser scaig systems, Joural of Applied Geodesy, vol. 4, pp.13-21, [8] R. L. Scouarec, T. Touze, J. B. Lacambre, ad N. Seube, A ew reliable boresight calibratio method for mobile laser scaig applicatios, The Iteratioal Archives of the Photogrammetry, Remote Sesig ad Spatial Iformatio Scieces, vol. XL-3/W1, pp , [9] C. J. Che, H. Liu, Y. Liu, ad X. Zhuo, High accuracy calibratio for vehicle-based mobile laser scaig ad urba paoramic imagig ad surveyig system, Multispectral Image Acquisitio, Processig ad Aalysis, vol. 8917, pp Y Y-8, [10] D. R. Li, Q. W. Hu, S. Guo, ad Z. Che, Lad-based mobile mappig system with its applicatio for the Olympic games, Chiese Sciece Bulleti, vol. 54, pp , 2009.

11 Error Aalysis ad Accuracy Assessmet The Ope Automatio ad Cotrol Systems Joural, 2015, Volume [11] A. Kukko, H. Kaartie, J. Hyyppa, ad Y. Che, Multiplatform mobile laser scaig: usability ad performace, Sesors, vol. 12, pp , [12] M. Bitec, R. Lidebergh, K. Khoshelham, ad A. P. Va Waarde, Evaluatio of a LiDAR lad-based mobile mappig system for moitorig sady coasts, Remote Sesig, vol. 3, pp , [13] M. Rutziger, Tree Modellig from mobile laser scaig datasets, The Photogrammetric Record, vol. 26, pp , [14] H. Y. Gua, J. Li, Y. T. Yu, C. Wag, M. Chapma, ad B. Yag, Usig mobile laser scaig data for automated extractio of road markigs, ISPRS Joural of Photogrammetry ad Remote Sesig, vol. 87, pp , [15] L. N. Fag, ad B. S. Yag, Automated extractig structural roads from mobile laser scaig poit clouds, Acta Geodaetica et Cartographica Siica, vol. 42, pp , [16] L. L. Zhu, J. H. Hyyppa, ad A. Kukko, Photorealistic buildig recostructio from mobile laser scaig data, Remote Sesig, vol. 3, pp , [17] M. Vaaja, A. Kukko, H. Kaartie, M. Kurkela, E. Kasvi, C. Fleer, H. Hyyppä, J. Hyyppä, J. Järvelä, ad P. Alho, Data processig ad quality evaluatio of a boat-based mobile laser scaig system, Sesors, vol. 13, pp , [18] X. S. Lu, Z. Y. Zheg, D. Wag, ad B. Shi, Evaluatio o theoretical accuracy of 3dsurs system scaer poit, Acta Geodaetica et Cartographica Siica, vol. 39, pp , [19] H. Kaartie, J. Hyyppa, A. Kukko, A. Jaakkola, ad H. Hyyppä, Bechmarkig the performace of mobile laser scaig systems usig a permaet test field, Sesors, vol. 12, pp , Received: September 16, 2014 Revised: December 23, 2014 Accepted: December 31, 2014 Xu et al.; Licesee Betham Ope. This is a ope access article licesed uder the terms of the Creative Commos Attributio No-Commercial Licese ( which permits urestricted, o-commercial use, distributio ad reproductio i ay medium, provided the work is properly cited.