TERRESTRIAL LASER SCANNING CIVIL ENGINEERING APPLICATIONS

Transcription

1 Internatonal Archves of Photogrammetry, Remote Sensng an Spatal Informaton Scences, Vol. XXXVIII, Part 5 Commsson V Symposum, Newcastle upon Tyne, UK. 1 TERRESTRIAL LASER SCANNING CIVIL ENGINEERING APPLICATIONS A. Bereny, T. Lovas, A. Bars Department of Photogrammetry an Geonformatcs, Buapest Unversty of Technology an Economcs abereny@mal.bme.hu Commsson V, WG V/3 KEY WORDS: Terrestral laser scannng, accuracy analyss, laboratory tests, loa test measurement, engneerng survey ABSTRACT: Three mensonal terrestral laser scannng has alreay prove ts potental n several applcaton fels, such as topography, archaeology, mnng an cultural hertage protecton. Bese these areas a new eman emerge: accurate pont clou acquston an post-processe results n engneerng survey. Ths technology coul broaen the fel of engneerng survey, although the lmte accuracy has to be carefully consere. In orer to confrm the accuracy clame by the manufacturer, the Department of Photogrammetry an Geonformatcs carre out two complex laboratory measurements, where not only the man accuracy values were examne but the effect of fferent materals, colours an the ncent angle. 1. LABORATORY TESTS 1.1 Accuracy of stance measurement The frst laboratory test focuse on the nvestgaton of stance measurement accuracy. In theory, ths can be escrbe by smple mathematcal equatons, but partcular crcumstances an obects coul have varous nfluences on the result. The examnaton took place n the laboratory of the Department of Structural Engneerng n 6. A steel plate was bene to smulate fferent stances (wth as small ncrement steps as 1 mm from the nstrument. Reference measurements were also one wth a hgh precson gtal callper ensurng submlmeter accuracy (Fgure Basc accuracy analyss: stance fferences Durng the secon laboratory nvestgaton 9 ponts were measure wth a Regl LMS Z4 an wth a Leca TRCM 13 total staton n the laboratory of Department of Structural Engneerng. The stances were calculate n every pont combnaton (the result s a 9 by 9 matrx base on the measurement results.e. the coornates from both nstruments. Table 1 shows the fferences between the calculate stance values n mm. Note that ponts labelle wth SOCS (Scanner's Own Coornate System are measure wth the scanner an the ponts labelle wth numbers are the same ponts measure wth the total staton. SOCS , 3, 3, 4,1 3,7 4,1 5,6 7, 6, 9,,8,8,6,3, 1,1 3 3, 9, 3,7 5,4 5,7 6,1 8,1 9,4 4 3,,8 3,7 1,1,, 3, 4,8 5 4,1,8 5,4 1,1,,7 1,9 3,7 6 3,7,6 5,7,,,1,1 3, 7 4,1,3 6,1,,7,1 1,8 3, 8 5,6, 8, 3, 1,9,1 1,8 1,1 9 7, 1,1 9,4 4,8 3,7 3, 3, 1,1 Table 1. Dstance fferences [mm] Fgure 1: Reference measurement wth hgh precson callper The plate was scanne wth a Regl LMS Z4 nstrument n each poston an reference ponts were measure as well. The results confrme the factory gven accuracy.e. the RMSE remane uner ±5 mm. Although etale accuracy assessment nees error propagaton calculatons, these results may prove a goo startng pont. The authors lke to emphasse that the evaluaton manly focuses on the proceure nstea of the performance of the partcular nstrument. One of the man goals was to evelop a metho that s applcable for laser scanners regarless to manufacturer, bran or stance measurement metho of the nvestgate scanner. 8

2 Internatonal Archves of Photogrammetry, Remote Sensng an Spatal Informaton Scences, Vol. XXXVIII, Part 5 Commsson V Symposum, Newcastle upon Tyne, UK. 1 Note that the results hghly epen on the use technology an other envronment/geometry relate factors. 1.3 Detale accuracy analyss: 3D error propagaton Beses the alreay nvestgate stance measurement an stance fference analyss, the accuracy assessment of the 3D coornates of every sngle pont s also very mportant. Instea of analysng the coornates prove by the nstrument rectly, the "raw" measurement results were processe, such as horzontal (H an vertcal (V angle, an the prevously qualfe stance (D, n orer to avo rounng errors an to make the proceure nstrument-nepenent: The stance between two ponts:, p ( X ( X X, Y, Z, X ( Y Y, Y, Z ( Z Z The last step s the calculaton of the error propagaton functon; wth the squares of the stanar evaton of the coornates (assumng nepenent measurements:. (6 ( X, Y, Z ftls( H, V, D (1 Note that the vertcal angles were transforme from zenth stance to heght angles n orer to smplfy the calculaton of the error propagaton. μ, p X p X μ X μ X p Y p Y μ Y μ Y p Z p Z μ Z μ. Z (7 X f Y f Z f TLSY TLSZ ( H, V, D D cos( H cos( V, ( H, V, D D sn( H cos( V, ( H, V, D D sn( V. ( For example: p ( X X X, (8 The squares of the stanar evaton values of the coornates (ue to paper sze lmtatons only the x value s shown for every ponts (assumng that the measure values are nepenent: μ X g X f V ( H,V,D,μ,μ μ V V H,μ D f D a sample partal ervatve: f H f H D sn( H cos( V. μ D, μ Due to sze lmtatons the further ervatves are not publshe. Durng the calculatons the manufacturer's accuracy value (±5 mm was use as an ntal value that was ve to three components (proectons parallel to the axes. The resultant of these proecte vectors was calculate n orer to verfy the metho: H (3 (4 Table shows the results of the calculaton. SOCS ,,9 11,4 1,9 1,7 1, 1,3 11,3 13, 1,3 8, 8, 8,8 9,7 9,8 1,7 3,9 1,3 1,3 11,5 11,1 9,7 1, 11,5 4 11,3 8, 1,4 5,4 9, 1, 9,3 1,1 5 1,9 8, 11,5 5,4 8, 9,7 8,7 9,3 6 1,7 8,8 11,1 9, 8, 1, 8, 8,7 7 1, 9,7 9,7 1, 9,7 1, 4,1 6,5 8 1,3 9,8 1, 9, 8,7 8, 4,1 6, 9 11,3 1,6 11,5 1,1 9,3 8,7 6,5 6, Table. Devaton of the stances [mm] For the better unerstanng, the man statstcal values are gathere as follows: mn( μ avg( μ max( μ,9 mm, 8,3 mm, 13, mm. (9 μ μ μ μ 5 mm. (5 X Y Z The error propagaton showe that the RMSE of the scanner's coornate etermnaton precson s the same that can be foun n the techncal specfcaton; moreover, n most cases t s better. 81

3 Internatonal Archves of Photogrammetry, Remote Sensng an Spatal Informaton Scences, Vol. XXXVIII, Part 5 Commsson V Symposum, Newcastle upon Tyne, UK Effect of materals an colours After provng the accuracy clame by the manufacturer, the emphass was put on atonal contons that coul affect the measurements an thus the qualty of pont clous. The secon laboratory measurements were one n the laboratory of the Department of Structural Engneerng n 9. In the frst test fferent materals often use at constructon stes were nvestgate: natural woo bark, natural woo, sawn woo, varnshe woo, steel, concrete, an brck. Each materal was scanne from the same poston. The post-processng of the separate pont clous was one by self evelope software to mnmze the effect of potental human error. The prmary results were mnmum, maxmum an mean reflectvty values an average pont ensty for each examne materal. The results verfe the expectatons; the steel plate's reflectvty values were the smallest, because, as all shny surfaces, t "ffuses" the laser beam. From the reflectvty pont of vew the brck s the best materal, because ts reflectvty s 1.7 tmes hgher than that of the steel's (Table 3. Materal N o of Reflectvty ponts Mn. Max. Mean brck 3597,7,81,51 concrete 3598,17,7,198 concrete (pante 366,141,7,186 natural woo 3597,168,5,4 natural woo 359,184,54,16 bark raw woo 3597,3,31,3 sawn woo 3596,199,54,6 steel 361,133,195,147 varnshe woo 365,3,7,46 Table 3: Reflectvty values of fferent materals The effect of fferent colours on the reflecte laser beam coul be very mportant on constructon stes, where the measurements have to be one pror to the fnal pantng or surface treatment. A test obect was pante n matte black, grey, an whte colours (Fgure, an the pont clou was analyse. The results showe that the lowest reflectvty colour s black an the hghest s whte (Table 4. Note the sgnfcant fference between the two mean values! N o of Reflectvty Colour ponts (per m Mn. Max. Mean whte 364,7,58,3 grey 364,156,11,181 black 3619,35,19,85 Table 4: Reflectvty values of fferent materals 1.5 Incent angle Incent angle nvestgaton coul be very useful f measurements have to be accomplshe e.g. n tny rooms or narrow hallways, snce the gathere ata coul contan errors (so calle ghost ponts because of the too small ncent angle of the reflecte laser beam. To avo ths type of measurement error, a threshol value of ncent angle was erve. In orer to analyze the crtcal laser beam ncent angle s effects on reflectvty, a steel obect wth two reflectors on ts plane se was capture n fferent postons (Fgure 3. The plate was scanne n 9 fferent postons, then the pont ensty (per m, ntensty, the coornates of the marke ponts, an the angle of the plate n every sngle poston were analyze Table 5. Scan N o Angle N o of ponts (per m 1 9, , , , , , , , ,14 17 Table 5: Incent angle an pont ensty values The results showe that the 8 th scan of the steel plate prove sgnfcantly less ponts than the prevous ones therefore ths scan has been omtte from the evaluaton process. Fgure : Test obect wth black, grey an whte colour an ts ntensty representaton (the coloure areas were nvestgate Fgure 3: Test obect for ncent angle analyss an pont clous capture n fferent rotaton angles 8

4 Internatonal Archves of Photogrammetry, Remote Sensng an Spatal Informaton Scences, Vol. XXXVIII, Part 5 Commsson V Symposum, Newcastle upon Tyne, UK. 1 The parameters erve from the 7 th scan showe that ths scan contans outlers (gross errors; thus ths scan has nether been taken nto conseraton. The evaluaton proves relable results above ~1 (17. The authors woul lke to notce that ths value strongly epens on the materal of the scanne obect, so ths partcular nvestgaton prove reasonable results only for shny steel surfaces.. APPLICATION EXAMPLES.1 Loa test measurements To confrm that terrestral laser scannng s a reasonable ata acquston technque not only n the usual applcaton fels but even n cvl engneerng applcatons. In the followng a specfc fel of engneerng survey, the loa test of brges wll be scusse. Ths type of survey requres a lot of manpower, several nstruments an a rather complex post-processng phase. Last, but not least, engneerng survey nees to prove very accurate results (~1 mm. Although terrestral laser scannng's accuracy s lmte (±5 mm, the applcaton potental of ths technque was prove urng the measurement campagns. Loa tests are usually followng a very smlar scenaro, one partcular test s escrbe as follows. In the th loa case the brge s unloae; reference measurements can be one. In the frst loa case (an any other cases except the ntermeate unloae cases the brge s loae wth 43 ton trucks accorng to the preefne loa plan (fferent number of trucks locate n fferent postons on the brge. In each case the stresses an the splacements (vertcal, horzontal, 3D are measure wth fferent types of nstruments: stran gauges, hgh-precson levels, total statons. These measurement technques have one feature n common: they coul only prove ata about screte ponts. After all phases were measure, post-processng begns, an the expecte (smulate an moelle an calculate values are to be compare n orer to valate the brge s loa capacty. Megyer Brge By ths partcular loa test two scanners were eploye to maxmze the pont ensty an to make sure that postprocessng wll prove the best results. Applyng two scanners smultaneously was also necessary because the scannng postons were relatvely close to the structure, an the ata loss cause by too small ncent angles s to be avoe. The scannng resoluton was set accorng to the tme requre by the hgh-precson levellng (,3 for each scanner. One scan of a sngle loa case resulte about 6 mllon (! ponts, an laste about mnutes. The result ata sets conssts of sngle 3D ponts from whch geometrc elements have to be obtane urng the postprocessng phase; splacement of any pont of the structure can be measure, no preefne ponts are neee. Note that entfcaton of the pont pars coul be ffcult, thus geometrc elements ftte on the evaluate part of the structure were use nstea. Durng the post-processng t was clearly seen that the brge also move urng the more than mnute long loa cases, so the laser scanner not capture a snapshot, but the pont clou also contans the (mnor splacements occurre urng the loa case. The result of ths effect can be clearly seen on Fgure 5, the splacement values are more correlate on the se where the scannng has begun. Note that shae areas coul cause faulty post-processng results, thus these parts of the pont clou have to be remove pror to the post-processng. The vertcal splacement of the eck were calculate an compare to the results from hgh precson levellng as reference measurement. The results prove that terrestral laser scannng coul be use n such engneerng survey applcatons. However, ths state-of-the-art technology s not to be consere as a replacement of the exstng technologes but as a useful atonal ata acquston metho. A number of results were obtane that were not or cannot be measure urng the loa test, such as pylon tlt an cable movements. These results assure the authors to contnue the examnaton of the 3D terrestral laser scannng n other engneerng applcatons. The loa test of the cable-staye Megyer Brge was one n August 8. Ths brge has a specal structure that conssts of nne brge structures; atonally, ths s the longest rver brge n Hungary (1 861 m. The scanne part s 591 m long an connects the left rver bank (Pest se wth the Szentenre Islan (Fgure 4. Fgure 5: Aggregate results (hgh precson levellng an laser scannng of the eck's vertcal movement Szebény Motorway Brge Fgure 4: The Megyer Brge Scannng the loa test measurement of the motorway brge at Szebény le to specfc experments; beses provng the usual elverables (.e. vertcal splacements compare to an valate by hgh precson levellng, new scannng metho was teste. The apple Leca C1 laser scanner allowe the measurement even at extremely low temperature (-4-5 C egrees. Snce ths brge spans over a valley (not over a rver, eployng the 83

5 Internatonal Archves of Photogrammetry, Remote Sensng an Spatal Informaton Scences, Vol. XXXVIII, Part 5 Commsson V Symposum, Newcastle upon Tyne, UK. 1 scanner rght uner the grer was feasble, therefore splacements occurre n (or close to the laser beam s recton were capture (Fgure 6. Ths kn of surveyng from unerneath the structure enables the analyss of the cross-sectons,.e. ervng the egree of asymmetry n splacements by capturng both ses of the structure. Ths scannng staton allowe clear vsblty to the pllar heas, therefore enable the analyss of ther horzontal movements (.e. tlt stances an angles. The evaluaton process nclue the vertcal splacement nvestgaton of the man cable an the eck. A 3D curve was ftte on the man cable n the th measurement's pont clou, an use as reference. Then the curve fttng was repeate n the secon pont clou, an the fferences were analyze between the curves. The results showe that the maxmal fference (.e. the vertcal splacement of the man cable s 1.8 cm, an can be observe n the mle of the brge (Fgure 8. Fgure 8: Erzsébet Brge evaluate results Fgure 6: Scannng staton unerneath the Szebény Brge. Effect of ambent temperature for steel brges - Erzsébet Brge Base on the postve experences of several loa tests, the technology s capablty n other engneerng applcatons was evaluate as well. One of these experments s the nvestgaton of the effect of ambent temperature on steel brges. Erzsébet brge's span s 9 m, t connects Pest an Bua; ts pylons are not stanng n the Danube but on the rverbank. To nvestgate the effect of the ambent temperature a scannng on 16 th of November 8 was mae, as reference ( th measurement. Two scannng postons were eploye, one on the left se of the Danube (Pest an one on the rght se (Bua. The ambent temperature was 7 C, the traffc on the brge was relatvely low because t was scanne aroun mnght. Ths s very mportant because there are several publc transportaton bus lnes cross the brge that has maor effect on brge movements. The scannng postons was eploye a bt farther from the brge, thus the small ncent angles coul not cause problems or errors n the pont clou. Control ponts (6-7 te ponts were marke on each se of the Danube, to be able to make measurements for the secon tme n the same reference system. The secon measurement was one n 6 th of August 9, n C ambent temperature (Fgure 7. The tme frame of scannng was chosen aroun mnght agan, to mnmze the ynamc effect cause by the traffc. The movement of the eck was calculate nrectly, because from the scannng postons only the lower structures of the brge were vsble, however these parts move together wth the eck. 3. CONCLUSION The state-of-the-art three mensonal terrestral laser scannng has alreay prove ts potental n several applcaton fels, such as actve qualty control, monument protecton, an mnng applcatons. The author s goal was to broaen the technology's applcaton fel n engneerng survey applcatons as well. To confrm the factory gven accuracy values laboratory tests were carre out, whch prove the manufacturer's clam of accuracy. These tests focuse not only on the 3D accuracy but on the effect of fferent materals, colours an ncent angle. Beses the laboratory nvestgaton, the technology was teste n on-fel applcatons as well. Ths new ata acquston technque s well applcable n engneerng survey applcatons such as brge loa test measurements an etecton of splacements cause by fferent ambent temperatures. As every sngle geoetc measurement metho, terrestral laser scannng also has ts shortcomngs. Frst especally n the fel of engneerng survey the lmte accuracy has to be consere. Although the nstrument's verfe accuracy s ±5 mm, the terrestral laser scannng s a capable atonal technology beses the conventonal engneerng geoetc methos. Instea of measurng preefne, ecate ponts, the entre vsble structure can be capture that enable varous nvestgatons n the post-processng (e.g. cable movements, pylon eformaton, etc. ACKNOWLEDGEMENT Fgure 7: Erzsébet brge Authors woul lke to thank to Burken Lt. for provng the scanner for the nvestgatons. Ths paper was supporte by the János Bolya Research Scholarshp of the Hungaran Acaemy of Scences. 84

6 Internatonal Archves of Photogrammetry, Remote Sensng an Spatal Informaton Scences, Vol. XXXVIII, Part 5 Commsson V Symposum, Newcastle upon Tyne, UK. 1 REFERENCES Berény, A., Lovas, T., 9. Laser Scannng n Deformaton Measurements. GIM Internatonal, 3(3, pp Berény, A., Lovas, T., Bars, Á., Duna, L., 9. Potental of Terrestral Laserscannng n Loa Test Measurements of Brges. Peroca Polytechca, 53(1, pp Lovas, T., Bars, Á., Detrekő, Á., Duna, L., Csák, Z., Polgár, A., Berény, A., Kbéy, Z., Szőcs, K., 8. Terrestral Laserscannng n Deformaton Measurements of Structures. In: Internatonal Archves of Photogrammetry an Remote Sensng, Vol. XXXVII, Part B5, pp Lovas, T., Bars, Á., Polgár, A., Kbéy, Z., Berény, A., Detrekő, Á., Duna, L., 8, Potental of Terrestral Laserscannng Deformaton Measurement of Structures, Proc. ASPRS Annual Conference, Portlan, USA, Aprl 8 May, p. 1. Kersten, T., Mechelke, K., Lnstaet, M., Sternberg, H., 8, Geometrc Accuracy Investgatons of the Latest Terrestral Laser Scannng, FIG Workng Week 8, Integratng Generatons, Stockholm, Sween, June 14-19, p heet_z4_9-1-9.pf (accesse 15 Mar. 1 85