CLOSE RANGE PHOTOGRAMMETR WITH CCD CAMERAS AND MATCHING METHODS - APPLIED TO THE FRACTURE SURFACE OF AN IRON BOLT Tim Suthau, John Moé, Albet Wieemann an Jens Fanzen Technical Univesit of Belin, Depatment fo Photogammet an Catogaph This epot escibes an appoach fo a estitution of the suface of a boken bolt use on Tukish ailwa biges. This is mae possible b a joint-ventue between the Istanbul Technical Univesit an the Technical Univesit of Belin. A igital 3D valuing sstem shoul give a touchless measuement of the factue of the bolt. The images wee taken with CCD cameas which wee connecte to a PC via a vieo boa. Pogam moules to the photogammetic estitution, e.g. fo calibation, bunle ajustment an fo image matching, wee available. The goal was to fin the best image acquisition conitions an the best combination of the sstem moules. As a esult one get the igital suface moel of the conjugate beaks of the bolt. Aftewas the esults allow statements about the mateial chaacteistic of the use bolts. 1. INTRODUCTION Fo man eas photogammetic poceues have significantl contibute to the touchless measuement of objects of all kin. Especiall the intouction of igital image pocessing techniques has opene new fiels of applications fo photogammet. Digital image pocessing is an efficient tool to eive geometical infomation fom igital image in a fast an economic wa. In ecent eas vesifie appoaches fo object ecognition as well as was fo etaction of geometical infomation wee evelope fo ve iffeent applications [Suthau et al., 2]. This pape escibes an appoach fo neal automatic estitution of the suface of a boken bolt use on Tukish ailwa biges Figue 1. This is mae possible b the integation an optimisation of vaious algoithms fo photogammetic estitution an image pocessing. A igital photogammetic 3D measuement sstem was applie fo image acquisition. The images wee taken with CCD cameas which wee connecte to a PC via a vieo boa. Pogam moules fo the photogammetic estitution, e.g. bunle ajustment, intesection in space, image matching an visualisation, wee available, have been evelope o aapte fo this poject. The goal was to fin the best acquisition conitions an the best combination of the sstem moules. As a esult one get the igital suface moel of the conjugate beaks of the bolt.
Aftewas the esults allows statements about the mateial chaacteistics of the use bolts. Fom investigations on the accuac of the poceue it is possible to evaluate the applicabilit of the appoach. One have istinguish between the accuac of the calibation of the cameas, of the absolute accuac of the oveall sstem an the accuac of the igital suface of the factue. Futhemoe the applicabilit of this poceue to simila tasks must be consiee. In geneal, matching objects with low gaient ae available. Figue 1. Two conjugate sufaces of the beak of an ion bolt 2. IMAGE DATA ACQUISITION AND THE CONFIGURATION OF THE SSTEM 2.1 Image ata acquisition In this wok, a commonl available CCD camea of the tpe SON C 77CE was emploe, which was connecte ove a fame gabbe. The technical ata of the camea ae liste in Table 1. Tpe of the camea Numbe of piels Signal tansfe Piel size Lens Table 1. Technical ata of the camea SON C 77CE 756 columns, 581 ows Analogue composite vieo-signal CCIR 11, 11, µm focal length 16 mm
A CCD camea can be unestoo as a metic camea because of its soli CCD mati. It is assume that the lens is stabl inteconnecte with the camea bo an the focal length is fie. The inteio oientation of the cameas must eithe be available o has to be calculate togethe with the eteio oientation uing the calibation of the cameas. 2.2 Configuation of the sstem The configuation of the measuement coespons to the case of aeophotogammet Figue 2. The efinition of the otation that is necessa to escibe the position of the cameas, is shown in Figue 2. Fo the etemination of an appopiate configuation an the calculation of the camea paametes the following input was use: - object size, - pincipal istance an - the size of the igital image. The istance between object an camea was fie to 3 cm, because the focus on the closest position stop-position. With this configuation the photogammetic estitution of objects with a size of appoimatel 1216 cm 2 is possible. The epth of focus amounts 15 cm. Figue 2. Definition of otation Figue 3. Calibation object
3. PHOTOGRAMMETRIC PROCESSING 3.1 Calibation Due to the configuation of the sstem Figue 2, the calibation object consists of a baseplate 11 cm 2 with 16 contol points an an attache cube 553 cm 3 with 9 contol points Figue 3. A bunle ajustment was applie fo calibation of the CCD camea. Thus we etemine the paametes of the inteio oientation of the cameas an the calibation of the whole sstem incluing the eteio oientation in one step. Fo the cental pespective conition, the following equations wee emploe: 9 8 7 3 2 1 c k H 1 9 8 7 6 5 4 c k H 2 Fo the istotion moel, the following equations wee emploe [Luhmann, 2]: 3 4 a 5 a 6 ² ² 4 4 2 1 A A a 7 with, - image cooinates,, - cooinates of object points i - otation mati c k, h, h,, - inteio oientation fo each camea,,, ϕ, ω, κ - eteio oientation fo each image - image aius A 1,A 2 - paametes fo aial lens istotion In the epeiment, the calibation object an the bolt was taken at a istance of appoimatel 3 cm an at the pincipal istance of 15,3 mm fom the CCD camea fom 23 iffeent iections an positions, in such a wa that the images coul be ovelappe 1%. In oe to test the absolute accuac of the configuation, we calculate the bunle ajustment with etemination of the calibation points as new points. The achieve accuac of the 3D measuement is,1 mm in - an -iection an,2 mm in. In oe to attain a bette accuac it woul be necessa to use a calibation object aoun with moe accuate calibation points an positions of the contol points aoun the object. An othe wa is the use of cameas with a highe esolution.
3.2 Photogammetic pocessing moel The photogammetic calibation elives the oientation paametes of the cameas an the whole sstem. With the application of these paametes an the measuements in the images of the epesente object it is possible to eive 3D cooinates of this object. The measuement of image cooinates took place with methos of igital image pocessing. A sstem oveview is illustate in Figue 4. Object Camea 1 n Image 1 n Image pocessing / Matching Intesection in space 3D - Moel Figue 4. Sstem oveview 3.3 Evaluation of the object The task was to suve the suface of the beaks of the bolt. So it is not possible to make a estitution of single lines an points, which chaacteize the object. Theefoe we use the matching methos an intesection in space to etemine the 3D Moel of ou suface. 3.3.1 Image matching The use matching metho was a combination between aea base matching an least squae matching. As esult we get the image cooinates of all conjugate points on the suface of the factue of the bolt. Fo ou epeiences we achieve a matching ate at 9%
Figue 5. The goo matching ate must be bone b a favouable base-to-height atio. That means the angle between the two images was ve small. Theefoe the peequisite fo intesection in space is not ve sufficient. Figue 5a. Oiginal image Figue 5b. Image with matching points 3.3.2 Detemination of object cooinates an visualisation Afte the camea was calibate successfull an the conjugate point wee matche, the object cooinates of the matche points coul be calculate. The use metho was intesection in space. The esult is the igital suface moel of the conjugate beaks of the bolt. The 3D Moel was visualize with the help of the pogam SURFER. A pespective view is illustate in figue 6, a contou map of the same ata is illustate in figue 7. Figue 6. Pespective view Figue 7. Contou map
4. Evaluation of esults an accuac The achieve accuac of the calibation is ±,5 mm in plane an ±,15 mm in height in the pat of the image with the contol points, calculate with all 23 images. The esult of the intesection in space with onl 2 images an a ba base to height atio is not so goo. The achieve accuac of the whole sstem is estimate ±,3 mm in plane an ±2mm in height. But fo the suface, the accuac between points on the factue ae impotant. The accuac of the spatial istance between an points aoun the object is.1 mm. 5. Conclusions In the pesente pape, an appoach fo neal automatic etemination of a igital suface moel of beaks of the bolt has been intouce. An impovement of the calibation e.g. a calibation object aoun the object woul lea to an inceasing pecision. Also the using of multi-image matching an a bette hight to base atio woul be the basis fo bette esults an accuac. A suveing of othe objects is possible, if the pe-conition suface of the object with a low gaient is fulfille. The size of the object plas no ole, but must be consiee egaing the esolution an the accuac howeve. With the pesente pape it coul be shown that the utilisation of CCD cameas an suitable softwae enables an effective suveing of object in the fiel of close-ange photogammet. REFERENCES Luhmann, T., Nahbeeichsphotogammetie - Gunlagen, Methoen un Anwenungen., Wichmann Velag, Heielbeg 2 Suthau, T., Hemmleb, M., uan, D. an P.G. Jost-Binkmann, Photogammetic measuement of linea objects with CCD cameas supe-elastic wies in othoontics as an eample, IAPRS, Vol. III, Amsteam 2