ASSESSMENT OF TWO CHEAP CLOSE-RANGE FEATURE EXTRACTION SYSTEMS

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1 ASSESSMENT OF TWO CHEAP CLOSE-RANGE FEATURE EXTRACTION SYSTEMS Ahmed Elaksher a, Mohammed Elghazali b, Ashraf Sayed b, and Yasser Elmanadilli b a Shool of Civil Engineering, Purdue University, West Lafayette, IN 796-8, USA elaksher@en.purdue.edu b Faulty of Engineering, Cairo University, Giza, Egypt ABSTRACT The use of non-metri ameras in photogrammetri appliations is onsidered under very strit onstraints due to their instability and lak of fiduial oordinate system. Arhitetural building doumentation, monuments registration, and monitoring struture deformations are very essential lose-range photogrammetri appliations that require high auray and quik data aquisition. Using metri ameras, in suh situations, is quite uneonomi and non-metri ameras are in favor. In order to aelerate the proessing time of analyzing non-metri ameras digital tehniques are preferred. The aim of this researh is to investigate the use of two inexpensive tehniques for objet reonstrution using digital images produed by non-metri ameras. The first tehnique employs an inexpensive mm amera and a heap sanner, while a low-ost digital amera is used in the seond tehnique. Both tehniques are thoroughly evaluated and the RMS errors are investigated. Results show that the 6-paramter transformation model is the best model to handle geometri errors introdued by sanners. The objet reonstrution proess results show that sub millimeter auray, in objet oordinates, an be ahieved if systemati errors are onsidered.. INTRODUCTION The basi task of many photogrammetri systems is to derive objet spae oordinates from D images. Analog, semianalytial, and analytial tehniques have been employed for a long period of time in photogrammetry to extrat ground oordinates of objets from hardopy images. In reent years, digital tehniques are implemented in photogrammetri appliations. The advantages of using digital tehniques are: the ease and speed of data aquisition, the inherent on-line and real-time apabilities, and the high degree of automation. Aquiring digital images is done either by sanning hardopy images or by apturing the photographs diretly in digital format using digital sensors. The aim of this paper is to investigate the proess of apturing ground features digitally through one of the following shemes: sanning hardopy images produed by non-metri mm ameras or aquiring digital images diretly using non-metri digital ameras. Eah tehnique is evaluated and analyzed using a number of mathematial models that relate image spae oordinates with ground spae oordinates. The implemented mathematial models were adapted to handle the systemati errors produed by non-metri ameras.. PREVIOUS WORK In (Boron, 996 the auray of the UMAX sanner is investigated; the orretion method he proposed redues the sanning errors from ± pixels to ±. pixels. The orretion is exeuted in two stages. First order orretions are found for eah point in the sanner plate first then the seond order orretions for eah run are determined. In (Bolte et. al., 996 both the geometri and radiometri properties of the sanners were studied. The RM - sanner was used and it was found that its auray is equivalent to the analytial plotter. In (Karras and Mavrommati, the effets of the radial distortions in the mm ameras is studied. A number of approahes, ranging from the utilization of linear features to the retifiation of regular grids, were used. It was shown that ignoring the radial lens distortion inreases the RMS errors dramatially. In (Cruz et. al., the inner orientation of non-metri ameras was investigated. The mm amera images were sanned at 6 dpi and dpi. A omparison between the 6-parmeters and -parameters oordinate transformation models showed that the former transformation model is better than the later. In (Seedahmed and Shenk, 998 a bundle adjustment with self-alibration sheme is presented for alibrating a high auray CCD digital amera. An extended version of the ollinearity equations was implemented with orretions for the symmetri distortion, the deentering distortion, the image plane unflatness, and the in-plane image distortion. The results showed the neessity to orret systemati errors. In (Zolfaghari and Malian, non-metri ameras are used to reord arhitetural and historial buildings. The work shows the effetiveness of using non-metri ameras for apturing this type of features. Setion presents the alibration proess of flatbed sanners. Setion summarize the mathematial models used to transfer image oordinates to ground oordinates. The objet reonstrution proess is presented in setion. Conlusions are disussed in setion.. GEOMETRIC CALIBRATION OF FLATBED SCANNERS During the sanning proess, the positions of the sanned features are orrupted ausing the distanes between them to hange. For heap sanners, the distortions inrease due to the bad funtioning of the mehanial, optial, and eletroni

2 parts of the sanner. In this setion different mathematial models are used to handle the geometri errors introdued by the sanners, the performane of eah model is evaluated and the best model is remarked. CSE Howled Pakard sanner. Two sanning resolutions were used (dpi, and 6dpi.. Two-Dimensions Coordinate Transformations In order to study and ompensate the introdued geometrial distortions in the sanning proess we employed four D oordinate transformation models. In the first model, Equation (, three parameters are used to represent two shifts and a rotation angle between the hardopy oordinate system and the sanner oordinate system. In the seond model one more parameter is added to onsider the uniform sale between the two systems, Equation (. The third model represents the -paramter transformation model. Six parameters are used to represent two translations, the rotation angle between the two systems, two different saling fators, and a skew fator, Equation (. The fourth model is the 8-paramter transformation model, Equation (. A omplete study on the D transformation models is disussed in (Mikhail et. al.,. = Y sinθ = S * Y sinθ = Y sinθ + Y Yo + Y α ax + by + aox + boy + = Y ax + by + a X + + o boy Where Sx Yo X, Y = hardopy oordinates, X,Y = sanner oordinates, X o, Y o = shift parameters, θ = rotation angle, S = uniform sale fator, S x, S y = non uniform sale fators ( ( + S y Y Yo ( ( α = skew fator, and a, a, a, b, b, b,, = the 8-paramter o o transformation model parameters.. Experiment Setup Some experiments are performed to evaluate the sanning proess as well as to hoose the most appropriate model that minimizes the produed geometri distortions. The proess is based on sanning a preise hardopy image of known and aurate point oordinates, Figure. The measured oordinates are used as the referene oordinates for the hardopy points. The experiment is performed with a Figure. Configuration of Referene Image The alibration proess is summarized in three steps. First the sanned oordinates of all points are measured digitally. The points are divided to ontrol points and hek points. The ontrol point oordinates are used to estimate the D transformation parameters for eah D transformation model. Then the estimated parameters are used to onvert the sanned oordinates of the hek points to the hardopy oordinate system. The RMS errors are alulated for eah model and are shown in Figures. The Y diretion represents the sanning diretion. The results show that the 6-parameter transformation model is the best model to reompense the geometri errors introdued by the sanners Model( Model( Model( Model( RMS (X, dpi RMS (X, 6dpi RMS (Y, dpi RMS (Y, 6dpi Figure. RMS Errors for the Sanning Proess (Pixels. MATHEMATICAL MODELS FOR CLOSE RANGE PHOTOGRAMMETRY Due to the wider use of non-metri ameras for different photogrammetri purposes suh as in registering arheologial buildings, doumenting historial writings, and reording historial buildings, there has been an inreasing demand to alibrate non-metri ameras. Using non-metri ameras inorporate many problems, inluding: -Non-metri ameras lak a stable inner orientation tehnique and irregular methods of alibration. -Most non-metri ameras have neither a reseau plate nor fiduial oordinate system to at as a referene for photographi measurements.

3 Due to these two disadvantages, the use of non-metri ameras in photogrammetri appliations is onsidered under very strit onstraints. However if non-metri ameras are alibrated properly they ould be used in more appliations. In setion. a brief summary of the systemati errors introdued by the non-metri ameras is presented. Setion. desribes the mathematial models employed to transform image spae oordinates to the objet spae oordinates taking into aount the effet of the systemati errors.. Systemati Errors Introdued By Non-Metri Cameras Systemati errors in non-metri ameras are the deviation of the physial imaging event from the projetion mathematial model, Karara (989. There are two types of systemati errors; the external systemati errors, and the internal systemati errors. The soures of the external systemati errors are: the atmospheri refration and the omparator errors. However, aording to (Marzan and Karara, 976 the atmospheri refration errors are negleted for objet distanes less than meters. The auses of the internal systemati errors are: the lens distortion, the film deformation, and film unflatness. Lens distortion is onsidered as the major soure of errors for the non-metri ameras. Lens distortion is introdued by two omponents: symmetri lens distortion and asymmetri lens distortion. The adopted mathematial model for symmetri lens distortion aording to (Brown, 97 is given by Equation (. 7 + r = k r + k r + k r... ( Where k,k,k = onstants, r = symmetri lens orretion in the radial diretion The asymmetrial lens distortion is due to the lenses deentering. We employed the orretion model presented by (Faig and Moniwa, 97, Equations (6. x = b( r + x + b xy y = b( r + y + bxy Where b, b = onstants, r = radial distane from the point of symmetry, x, y = point oordinates in image spae, and x, y = symmetri lens orretions The film deformation inside the amera is aused by bad flatness of the film. The film deformation outside the amera is due to lak of ontrol over temperature, and the humidity during proessing and storage of non-metri films. In (Robson, 99 the 6-paramer transformation model is used to model the total film deformation. (6 ollinearity representation with no systemati error orretion parameters. Nine transformation parameters are inluded in this model. The seond model is a modified version of the ollinearity representation, 6 more parameters are added to handle the systemati errors, Equation (7. The third model is the DLT representation, (Abdel-Aziz and Karara, 97. The fourth model is an adapted form of the DLT representation, Equation (8. The DLT is a linear treatment of a non-linear problem so its results are approximate. a + a x + a y + k xr = o m(x X + m(y Y + m(z Z f m(x X + m(y Y + m(z Z ( 7 b + b x + b y + k yr = o m(x X + m -f m (X X + m (Y Y + m(z Z (Y Y + m (Z Z Where f = amera interior parameters, X, Y, Z = exposure station oordinates, x, y = image spae oordinates, X, Y, Z = objet spae oordinates, m through m = rotation matrix elements, k, k, b, b, b, a, a, a = systemati error o o orretion parameters x + lx + ly + lz + l + l9xx + lxy ++ lxz + + kxr = y + lx + l6y + l7z + l8 + l9xx + lxy ++ lxz + + kyr = (8 Where x, y = image spae oordinates, X, Y, Z = objet spae oordinates, l through l = DLT parameters, k, k = systemati error orretion parameters. DIGITAL APPROACH FOR OBJECT SPACE RECONS TRUCTION Two objet spae reonstrution systems are presented in this paper. The first system employs a heap mm amera, Zenit E, and an inexpensive flatbed sanner, while the seond system uses only a low-resolution digital amera, Casio QV- A. First target points are fixed as shown in Figure. The points are fixed on wood plates with different thikness varying from mm to m. A theodolite and a total station are employed to find the referene ground oordinates of the target points.. Transformation Between Image Coordinates and Ground Coordinates We employed four models to transform the image oordinates and the objet oordinates. The first model is the

4 Case Case Case Case Figure. Test Area Configuration. Aquisition of Digital images from the mm amera and the sanner In this setion the analog amera, Zenit E, was used together with the HP sanner to produe the digital images. First the amera was used to ollet a number of image pairs with different B/H ratio, and these images are sanned with different resolutions. Sixteen well-distributed ontrol points were hosen to solve the resetion problem for eah image. The intersetion proess is used to evaluate the experiments. Table shows the different ombinations of the B/H ratios and the used resolutions. Figure -a, b, and show the RMS errors for the different experiments. Case H (m B (m B/H Sanning Resolution (dpi Table. First System Configuration Case Case Case Case Model ( Model ( Model ( Model ( Figure -a. First System RMS Errors (mm, X Diretion Model ( Model ( Model ( Model ( Figure -b. First System RMS Errors (mm, Y Diretion Case Case Case Case Model ( Model ( Model ( Model ( Figure -. First System RMS Errors (mm, Z Diretion. Aquisition of Digital Images Using The Digital Camera The digital amera was used to produe the digital images diretly. The images were taken with B/H ratios shown in Table. The resetion and intersetion proesses were applied to solve for the amera parameters and the ground oordinates of the target points. Figure -a, b, and show the RMS errors for the different experiments using the digital amera. Case H (m B (m B/H Table. Seond System Configuration

5 Case ( Case ( Case ( Model ( Model ( Model ( Model ( Figure -a. Seond System RMS Errors (mm, X Diretion Case ( Case ( Case ( Model ( Model ( Model ( Model ( Figure -b. Seond System RMS Errors (mm, Y Diretion REFERENCES Abdel-Aziz Y. and Karara H. M., 97. Diret Linear Transformation from Comparator Coordinates into Objet Spae Coordinates in Close-Range Photogrammetry. Papers from the Amerian Soiety of Photogrammetry Symposium on Close-Range Photogrammetry, Urbana, Illinois, pp. -8. Bolte et. al.,996: Bolte U., Jakobsen K., and Wehrmann H., 996. Geometri and Radiometri Analysis of a Photogrammetri Image Sanner. In: International Arhives of Photogrammetry and Remote Sensing, Vienna, Austria, Vol., Part B, pp Boron A., 996. Calibration of Digital Images Produed With The Use of UMAX SE Sanner. In: International Arhives of Photogrammetry and Remote Sensing, Vienna, Austria, Vol., Part B, pp. -. Brown D. C., 97. Close Range Camera Calibration. Photogrammetri Engineering and Remote Sensing, Vol. 7, No. 8, pp Cruz et. al., : Cruz S., Cardenal J., and Delgado J.,. A Program For Automati Inner Orientation of Digitized Non-Metri Images ( And 7 Mm. In: Proeeding of the 9 th ISPRS Congress, Amsterdam, The Netherlands, Vol. XXXIII, Part B/, pp Case ( Case ( Case ( Model ( Model ( Model ( Model ( Faig W. and Moniwa H., 97. Parameters of Interior Orientation and Their Correlations. In: Proeeding of ASP Fall Convention, Buena Vista, Florida, USA, pp Karara H., M., 989. Manual of Non Topographi Photogrammetry, nd Edition, ASPRS, Falls Churh, Virginia. Karras E. G. and Mavrommati D.,. Simple Calibration Tehniques For Non-Metri Cameras. In: International Committee for Arhitetural Photogrammetry, Potsdam, Germany. Figure -. Seond System RMS Errors (mm, Z Diretion. CONCLUSIONS - The best mathematial model for the treatment of the geometri errors introdued during the sanning proess is 6- paramters transformation model. - The (B/H ratio must be onsidered during the imaging proess in order to ahieve similar auray in the X,Y,Z ground oordinates. - The auray of the ground oordinates extrated from the digital images aptured by sanning hardopy images ould reah sub millimeter auray with high quality sanning resolution (dpi and good (B/H ratio. - The auray of the low resolution digital ameras ould reah sub millimeter with good (B/H ratio. Marzan G. T. and Karara H. M., 976. Rational Design for Close Range Photogrammetry. Photogrammetri Series, No., University of Illinois, Urbana, Illinois. Mikhail et. al., : Mikhail E.M., Bethel J., and MGlone J.,. Introdution to Modern Photogrammetry. Join Wiley & Sons. In., New York. Robson S., 99. Film Deformation in Non Metri Cameras Under Weak Geometri Conditions-An Unorreted Disaster. In: International Arhives of Photogrammetry and Remote Sensing, Washington D.C., USA, Vol. 9, Part B, pp Seedahmed G. and Shenk T., 998. Experimental Digital Camera Calibration. ISPRS Commission III Symposium on Objet Reognition and Sene Classifiation from Multispetral and Multisensor Pixels, Ohio, Columbus, USA, pp Zolfaghari M. and Malian A.,. Non Metri Cameras in Arhiteutral photogrammetry. In: Proeeding of the 9 th ISPRS Congress, Amsterdam, The Netherlands, Vol. XXXIII, Part B/, pp. -8.