ON IMPROVEMENT OF ACCURACY OF AIRBORNE LASER SCANNER DATA USING DIGITAL IMAGES

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1 ON IMROVEMENT OF ACCURACY OF AIRBORNE LASER SCANNER DATA USING DIGITAL IMAGES Kzuy NAKANO *, Hrofum CHIKATSU b* System Development Group, AERO ASAHI CORORATION, 3-1-1, Mnmd, Kwgoe, Stm, , Jpn - kzuy-nkno@erosh.co.jp b Dept. of Cvl nd Envronmentl Engneerng, Tokyo Denk Unversty, Ishzk, Htoym, Stm, , Jpn - chktsu@g.dend.c.jp Commsson I, WG I/2 KEYWORDS: LIDAR, Integrton, Aerl hotogrmmetry, GS/INS, Bundle Adjustment, Georeferencng ABSTRACT: Arborne lser scnner s useful system for cqurng pont clouds 3D dt wth rel-tme, nd t cme to be generlly used n the lte 199s. Arborne lser scnner hve effectveness n extrctng mcro topogrphy or ground surfce under the trees whch re not detect by photogrmmetry, nd mny pplctons such s cty modellng, DTM generton, montorng electrcl power lnes nd detecton of forest res were proposed. The most remrkble pont of ths system, however, s ts blty to obtn 3D coordntes of huge object ponts wth rel-tme. Arborne Lser Scnner s bsclly bsed on GS, nertl mesurement unt (IMU nd lser scnnng devce (Lser Rngefnder. GS nd IMU provde poston nd tttude nformton regrdng trjectory of pltform, nd lser scnnng devce gve dstnce between pltform nd the object ponts. Therefore, drect-georeferencng system usng GS/IMU comes to be utlzed wdely n the feld of erl photogrmmetry, nevertheless mtchng mong stereo models nd courses re not djusted snce the drect-georeferencng system re nfluenced by cycle-slp nd stellte plcement. Ths pper nvestgtes fundmentl studes to mprove of ccurcy of rborne lser scnner dt usng dgtl mges. 1. INTRODUCTION Arborne lser scnner cme to be generlly used for terrn mesurement from the vew pont of promnent chrcterstc (Bltsvs, Arborne lser conssts of 2 key technologes s spn-off of mltry technology; one s GS/IMU (.e. drect-georeferencng, other s lser scnnng devce. Current prdgm shft of pltform orentton ws cused by drect-georeferencng usng GS/IMU nevertheless system clbrton n GS/IMU system s the most crtcl fctor (Crmer nd Stllmnn, 22. In order to control qulty of erl photogrmmetry usng GS/IMU system, Geogrphcl Survey Insttute (GSI prescrbe estblshments of severl ground control ponts nd prctce of erl trngulton n genertng lrge scle mps such s 1/1,. GSI lso prescrbe mechncl ccurcy such s ±3 cm ccurcy for GS by contnuous knemtc, nd rollng nd ptchng ccurcy re.13 degrees, hedng s.35 degrees for IMU respectvely. In the mentme, GSI hs estblshed 1,224 GS sttons ll over the country for montorng of the crustl movement. Then, t become possble to utlze these GS sttons for qulty control of rborne lser scnner dt, nd GSI prescrbe mechncl ccurcy s sme s the erl photogrmmetry so tht ±25 cm ccurcy re cheved n vertclly by rborne lser scnner. On the contrry, tke nto ccount tht the drectgeoreferencng system re nfluenced by cycle-slp nd stellte plcement, some djustment for the drect-georeferencng system n rborne lser scnner should be consdered s well s estblshment of severl ground control ponts nd prctce of erl trngulton n the erl photogrmmetry usng GS/IMU. In order to mprove the ssues such s cycle-slp nd stellte plcement n the drect-georeferencng system, smultneous djustment combned wth erl photogrmmetry nd rborne lser scnner become possble under consderton of current stuton tht non-metrc dgtl cmer s equpped wth rborne lser scnner system. However, prevous nteror orentton procedure newly comes up s the nother ssue snce nteror orentton procedure for non-metrc dgtl cmer s generlly performed beforehnd usng test sheet or test trget, nd the prevous nteror orentton procedure should be removed for del drect-georeferencng system. Wth ths motve, smultneous djustment such s bundle djustment wth self-clbrton s proposed n ths pper so tht exteror orentton prmeters whch re obtned from GS/IMU system, 3D objects coordnte whch re cqured from lser scnner nd the nteror orentton prmeters re djusted smultneously. Though, the combned orenttons n block djustment hd been proposed n lte 19 s (Ackermnn, & et l., 1972, EL-Hkm & Fg, 1981, Chktsu, & et l., 1988, t s enormously expected tht the proposed djustment wll mke possble to utlze rborne lser scnner n genertng lrge scle mp, nd effcent erl photogrmmetry except the geodetc dt such s ground control ponts nd the erl trngulton wll be ccomplshed. 2. WORKFLOW In order to cheve the smultneous djustment, bref procedures whch re nvestgted n ths pper re s follows. 373

2 The Interntonl Archves of the hotogrmmetry, Remote Sensng nd Sptl Informton Scences. Vol. XXXVII. rt B1. Bejng 28 Acquston of Approxmte Vlues + Approxmte vlues for trjectory of pltform re clculted by post processng usng GS nd IMU. + Approxmte vlues for exteror orentton prmeters re gven from drect-georeferencng system. + Nomnl vlues for nteror orentton prmeters such s focl length, prncpl ponts nd so on re dopted s ntl vlues. + Corse pont clouds 3D dt re generted usng the trjectory vlues nd exteror orentton prmeters. Cmer Clbrton + Bundle djustment wth self-clbrton s performed usng corse pont clouds 3D dt. + Trjectory of pltform s reclculted by GS/IMU nd clbrton results. Genertng DSM + All pont clouds dt re corrected usng reclculted trjectory of pltform. + DSM s generted. + Interestng ponts such s edge of rod pnt re extrcted s feture pont on the mge, nd the mge coordntes re cqured. + Intl ground coordntes of the nterestng ponts re clculted usng the mge coordntes, ntl nteror prmeters nd exteror orentton prmeters whch re obtned from drect-georeferencng. + ont clouds dt (X, Y, Z round the nterestng pont re trnsformed to mge coordntes (u, v by collnerty equton (1. + ostonl reltonshp between the pseudo GC nd the pont clouds on the mge re computed usng ech mge coordnte. + Heghts of the nterestng ponts (the pseudo GCs were computed from weghted nterpolton equton (2. + Two-dmensonl coordntes of the nterestng ponts (the pseudo GCs were computed from equton (3. 3. CAMERA CALIBRATION The uthors hve been concentrtng on developng close rnge mesurement system for consumer grde dgtl cmers usng trplet mges (Chktsu & et l., 26. The mesurement system ws dopted nto dgtl erl photogrmmetry n ths pper snce trplet mges hve followng chrcterstcs. + Trplet mges hve dvntges n genertng stereo prs. + Trplet mges hve flexblty for multple mges. + Trplet mges hve blty to ncrese geometrc restrcton. On the other hnd, lens dstorton s the most mportnt prmeter mong nteror orentton prmeters, nd mny dstorton models were proposed (Fryer nd Brown, In order to pply consumer grde dgtl cmers to dgtl photogrmmetry, t ws concluded tht lens dstorton ws corrected suffcently usng rdl polynoml 5th degree model (Chktsu, 27. Therefore, rdl polynoml 5th degree ws dopted n ths pper, nd ccurcy performnces for the smultneous djustment were nvestgted regrdng followng clbrtons models. 3.1 Clbrton usng seudo GCs seudo GC s nterestng ponts on the mge nd seudo GCs hve followng dvntge. - It doesn t requre for seudo GCs s ntl vlue to hve ccurte coordnte. - seudo GCs cn be extrcted s feture pont on the mge. - 3D coordntes for seudo GCs re computed from pont clouds dt. Therefore, cmer clbrton usng pseudo GCs ws consdered s prmry stge for the smultneous djustment. Fgure 1 shows concept of genertng pseudo GC, nd X,Y,Z coordnte for the seudo GCs re computed by followng procedures. u = f v = f Fgure 1. Concept of pseudo GC + ( Y Y ( Y Y ( Y Y ( Y Y where u, v: mge coordntes f : focl length j : rotton mtrx X, Y, Z: ground coordntes of the pont clouds X, Y, Z : exposure stton (1 374

3 The Interntonl Archves of the hotogrmmetry, Remote Sensng nd Sptl Informton Scences. Vol. XXXVII. rt B1. Bejng 28 Z w = = w Z w ( u u v v (2 the trplet mges were generted 3 stereo prs, nterestng pont (X,Y,Z hs dfferent 3 ground coordntes for ech stereo prs whch re shown n Fgure 3. Therefore, trplet restrcton ws ppled so tht the ech resdul Δ for the X,Y,Z become mnmum n clbrton procedures. Here, ech resdul Δ re computed from equton (4. Where Z :heght of the pont clouds u, v :mge coordntes of the pont clouds Z : hght of the nterestng pont u, v: mge coordntes of the nterestng pont u = f v = f ( Y Y ( Y Y ( Y Y ( Y Y where u, v: mge coordntes f : focl length j : rotton mtrx X, Y, Z: ground coordntes of nterestng pont X, Y, Z : exposure stton of pltform (3 Δ Δ Δ ve. AB BC AC = = ve. = ve. = ve. W W AB BC AC where, j :ground coordntes of the nterestng pont Δ j : dfferences of ech stereo pr W :weght of ech pont A B C (4 Dstnces EO IO Imges AB Imge Coord. BC ont clouds Intl Ground AC Collecton ont clouds Fgure 3. Concept of trplet restrcton Fgure 2. Flowchrt of pseudo GCs generton 3.2 Smultneous djustment Convert to mge Coord. Determne weght Compute Ground In order to mprove the clbrton ccurcy, trplet restrcton ws dopted nto the clbrton usng seudo GCs. Due to 3.3 Dt cquston 4. EXERIMENT In order to confrm the vldty of the proposed smultneous djustment, dt cquston ws performed n our test feld. Test feld hs 117 GCs, nd 26 GCs cn be utlzed n trplet re. GCs were obtned by sttc observtons usng GS tht were set on the edge of rod pnts. Tble 1 shows dt component whch used n ths nvestgton, nd Fgure 4 shows the centre mge usng n ths nvestgton. It ws tken wth bout 127 m, nd photo scle s bout 1/21,. Therefore, GSD (Ground smplng dstnce shows bout 2cm. Fgure 5, 6 shows GC mge, nd t cn be relzed tht mge qulty s not clerly. ontng were performed n ech clbrton wth 1/1 pxel ccurcy, but t s speculted tht pontng ccurcy hven t 1/1 pxel ccurcy. 375

4 The Interntonl Archves of the hotogrmmetry, Remote Sensng nd Sptl Informton Scences. Vol. XXXVII. rt B1. Bejng 28 Domn Item Contents Test feld All GCs 117 ponts ltform Helcopter AS-35BA Flght Speed 13 km/h Alttude 127 m GS Rte 1 sec IMU Rte 2 Hz Lser Scn rte 7 khz Scn ngle 4 degree (full Dgtl Cmer Focl length 6 mm Sensor sze 9 x 9 μm Imge sze 544 x 48 Tble 1. Dt component Fgure 6. Imge qulty 3.4 Accurcy Evluton Tble 2 shows root men squre error for 26 check ponts, nd permssble errors mens restrctons for check ponts whch re estblshed by GSI n genertng ech scle mp usng drectgeoreferencng system. RA (Reltve Accurcy mens the vlue whch ws computed from equton (5 RA = 2 σ x + 1 H 2 σ y + 2 σ z (5 where H:Flght Heght σ:root men squre error for x, y, z Fgure 4. Centre mge It cn t be found sgnfcnt dfferences between both clbrton models, but t cn be confrmed tht the horzontl nd vertcl ccurcy re mproved by the smultneous djustment. Furthermore, t cn be found tht the both clbrton ccurcy compre wth permssble error show less thn 1/1 nd more thn 1/5. Consequently, t s concluded tht clbrton usng pseudo GCs s prctcl method n drect-georeferencng, nd the ccurcy ws mproved by trplet restrcton. Fgure 5. GC n rkng Type σ x σ y σ z RA Usng seudo GCs /171 Smultneous djustment /1819 ermssble error /4356 1/ /2444 1/ /183 1/ /542 1/5 H =127m n RA Tble 2. RMSE for check ponts 5. CONCLUSION In order to develop new clbrton methods combned wth GS/IMU nd Lser Scnner dt, smultneous djustment ws nvestgted n ths pper. 376

5 The Interntonl Archves of the hotogrmmetry, Remote Sensng nd Sptl Informton Scences. Vol. XXXVII. rt B1. Bejng 28 It s confrmed tht clbrton usng pseudo GCs nd smultneous djustment show less thn 1/1 nd more thn 1/5 whch s restrcted by GSI n genertng ech scle mp usng drect-georeferencng system, nd the ccurcy ws mproved by trplet restrcton. Therefore, t s concluded tht smultneous djustment usng pseudo GCs nd trplet restrcton s prctcl method n drect-georeferencng snce the smultneous djustment hve blty to perform nteror nd exteror orentton wthout ny GC nor erl trngulton. However, There re ssues, however, for further work. These problems re mprovement of ccurcy nd utomtc generton of pseudo GCs REFERENCES Bltsvs E, A comprson between photogrmmetry nd lser scnnng, ISRS Journl of hotogrmmetry nd Remote Sensng, Vol. 54, pp Crmer M. nd Stllmnn D., 22, System clbrton for drect georeferencng, Interntonl Archves of hotogrmmetry, Remote Sensng nd Sptl Informton Scences 34 (22 (rt 3A, pp EL-Hkm S.F., nd Fg W, 1981, A combned Adjustment of Geodetc nd hotogrmmetrc Observtons, hotogrmmetrc Engneerng nd Remote sensng, vol.47, No.1, pp Chktsu H., Ksugy N., Mur S., 1988, An Adjustment of hotogrmmetry Combned wth the Geodetc Dt nd GS, 16th Interntonl Socety for hotogrmmetry nd Remote Sensng, Vol.26, Chktsu H., nd Odke T., 26, Ubqutous Dgtl hotogrmmetry by Consumer Grde Dgtl Cmer, Interntonl Archves of the hotogrmmetry, Remote Sensng nd Sptl Informton Scences, Vol. XXXVI, ART 5 (CD- Rom, ISSN Fryer J.G nd Brown D.C., 1986, Lens Dstorton for Close- Rnge hotogrmmetry, hotogrmmetrc Engneerng nd Remote Sensng, Vol.52, No.1, pp Chktsu H., 27, Re-evluton of nteror orentton prmeters of consumer grde dgtl cmer n dgtl photogrmmetry, 5th ARIDA SEMINAR on Innovtons n 3D Modellng nd Vsulzton of Dgtl Imgery-Bsed Sptl Informton, tect.cg Ackermnn F, Ebner H, Klen H, 1972, Combned block djustment of AR Dt nd Independent hotogrmmetrc Models, The Cndn Surveyor, Vol. 26, pp

6 The Interntonl Archves of the hotogrmmetry, Remote Sensng nd Sptl Informton Scences. Vol. XXXVII. rt B1. Bejng