PICTOMETRY'S PROPRIETARY AIRBORNE DIGITAL IMAGING SYSTEM AND ITS APPLICATION IN 3D CITY MODELLING

Transcription

1 PICTOMETRY'S PROPRIETARY AIRBORNE DIGITAL IMAGING SYSTEM AND ITS APPLICATION IN 3D CITY MODELLING Yandong Wang, Steve Schultz, Frank Guffrda Pctometry Internatonal Corp. 100 Town Centre Dr. Sute A, Rochester, NY 24623, USA-(yandong.wang, steve.schultz, Commsson I, ThS-2 KEY WORDS: Photogrammetry, Dgtal, Image, Three Dmensonal, Buldng, Modelng. ABSTRACT: One of the most sgnfcant achevements n dgtal photogrammetry n the last decade s the development of large format dgtal mappng cameras. In addton to large format dgtal mappng cameras, a number of medum format dgtal mappng cameras systems have also been developed. At Pctometry, a medum format dgtal magng system has been developed and been wdely used for acquston of both vertcal and oblque mages around the world. Snce oblque mages exhbts rch 3D lke nformaton of objects on ground, they are wdely used n not only 3D measurement and vsualzaton, but also creaton of 3D cty models. In ths paper, Pctometry's propretary medum format dgtal magng system wll be brefly ntroduced. Some ssues n generaton of 3D cty models usng Pctometry dgtal oblque mages wll also be dscussed. 1. INTRODUCTION Wth the rapd development of CCD and computer technology, a number of large and medum format dgtal mappng cameras have been developed n photogrammetrc feld (Sandau et al, 2000, Hnz et al, 2001; Leberl et al, 2003; DIMAC 2008; Mostafa, 2003). In contrast to analog mappng cameras, dgtal mappng cameras exhbt a number of advantages. One of the most mportant features of dgtal mappng cameras s that they produce dgtal mages drectly wthout need of photographc processng and scannng of negatves and dapostves, and thus photogrammetrc producton cycle can be reduced. At the same tme, mage qualty s mproved snce dgtal mages are free from dust marks, spots and scratches. Most dgtal mappng cameras produce 12-bt dgtal mages whch have much larger dynamc range compared to mages produced by flms. Thus, hgh accuracy of pont measurement can be acheved wth dgtal mages, especally n dffcult stuatons such as n shadow areas. At Pctometry, a medum format dgtal magng system was developed for acqurng both vertcal and oblque dgtal mages. Up to now, more than 50 Pctometry magng systems have been deployed n the USA and tens of systems are beng used around the world. 3D cty modelng has been an actve research area n dgtal photogrammetry for a decade and a number of methods and systems have been developed for creatng 3D cty models from dgtal mages and other auxlary data automatcally or semautomatcally. Two major steps nvolved n generatng 3D cty models are creaton of buldng models and addng textures to the buldng models. Varous methods have been developed for creatng buldng models from dgtal mages automatcally or sem-automatcally (Haala and Hahn, 1995; Gülch, 1997; Henrcsson, 1996; Vosselman, 1999), or from LDAR data (Haala and Brenner, 1997; Rotternstener and Brese, 2003; Schwalbe et al, 2005). Snce both dgtal aeral mages and LDAR data supplement to each other, accurate and relable buldng extracton can be acheved by fusng dgtal mages and LDAR data (Rotternstener and Jansa, 2002; Vosselman, 2002; Hu et al, 2006). Addng texture to the created buldng models s mportant snce t makes 3D models more realstc. Dfferent approaches for creatng buldng textures have been developed to create buldng textures automatcally from aeral vertcal and oblque mages (Brenner et al, 2001; Frueh et al, 2004; Zhang et al, 2004). In ths paper, Pctometry dgtal magng system wll be brefly ntroduced. Some ssues n creaton of 3D cty models usng Pctometry dgtal oblque mages wll also be dscussed. 2. PICTOMETRY DIGITAL IMAGING SYSTEM 2.1. Components of Pctometry Dgtal Imagng System Pctometry dgtal magng system has some unque features, compared wth other dgtal mappng cameras/systems. The magng system conssts of fve dgtal cameras, an ntegrated Global Postonng System (GPS) and Inertal Measurement Unt (IMU) and a flght management system. Each camera has an array of CCD wth about 4.9k x 3.2k pxels. The fve dgtal cameras are arranged n such a way that four of them look forward, backward, left and rght drectons at a certan vew angle respectvely and one looks straght down. The camera n the vertcal drecton captures hgh-resoluton vertcal mages and the other four acqure oblque mages at dfferent vew drectons at the same tme. The onboard GPS and IMU provde an accurate poston and atttude of each sensor at exposure tme, thus the mages produced by Pctometry magng system are drectly geo-referenced mages. Lke tradtonal aeral mages, vertcal mages provde a vertcal vew of the terran surface, whle oblque mages show the sde lookng of objects on the ground such as buldngs. Vertcal mages can be used for creaton of accurate large scale orthophotos (Wang et al, 2008) and oblque mages can be utlzed for vsualzaton, measurement and 3D modelng. Pctometry oblque mages have been wdely used n varous applcatons such as publc safety, tax assessment, urban plannng, 3D cty modelng, etc. The flght management system s flght plannng software whch determnes flght lnes, control mage overlaps, etc. before and 1065

2 durng the flght for both vertcal and oblque mages. In order to better use and vsualze both oblque and ortho mages, Electronc Feld Study (EFS) has been developed at Pctometry. Both vertcal and oblque mages can be easly vewed n EFS, and spatal measurement such as dstance and heght of objects on the ground can be easly performed on both oblque and vertcal mages. The results can be exported nto ArcGIS drectly to update the exstng geo-spatal nformaton n the database Camera Calbraton Camera calbraton s an mportant process n photogrammetrc mappng to ensure extracton of accurate and relable 3D nformaton from magery. In camera calbraton, the prncpal length of the camera, the coordnates of the prncpal pont n mage coordnate system and the coeffcents of lens dstorton ncludng radal dstorton, tangental dstorton and affnty and shearng are computed. Varous calbraton methods have been developed for calbraton of dgtal mappng cameras and the performance of some medum format dgtal cameras can be found n Cramer (2004). At Pctometry, a calbraton system has been developed for calbraton of ts dgtal cameras and the system was provded to EROS center of USGS at Soux Falls for establshng a calbraton system for calbraton of varous dgtal mappng cameras n mappng communty (Pctometry, 2002). Bascally, the Pctometry's calbraton system ncludes an ndoor calbraton cage wth evenly dstrbuted targets as shown n Fgure 1 and software Australs whch s the well-known calbraton software (Fraser and Edmundson, 2000). Radal Lens Dostorton (um) (b) Calbraton target Fgure 1. Camera calbraton cage and target Radal Dstance (mm) Fgure 2. Radal lens dstorton (a) Calbraton cage In calbraton, the camera to be calbrated captures a number of mages aganst the calbraton cage from dfferent locatons to form a network, and Australs s then used to measure the targets n the mages automatcally and accurately and to perform a free network bundle adjustment to compute the nteror orentaton parameters and dstorton coeffcents of the camera. Fgure 2 shows the calbraton result of Pctometry dgtal camera. It can be seen that the radal lens dstorton of the camera s very small n most area of the mage and hgh accuracy of pont measurement can be acheved after correcton of lens dstorton. The advantage of Pctometry's calbraton system les on ts effcency and relablty. It s very easy to run calbraton of dgtal cameras wth Pctometry's calbraton system and very economc, compared wth other approaches such as n-stu calbraton method. Fgure 3 shows the radal lens dstorton of a Pctometry dgtal camera obtaned at calbratons repeated n a short tme perod. It can be seen that the results are consstent and the dfference between two calbratons s very small. It s very mportant that each dgtal mappng camera s calbrated regularly so that the changes of camera's nteror orentaton parameters and lens dstorton parameters between two consecutve calbratons are wthn the defned tolerance and do not affect the mappng accuracy. Radal Lens Dostorton (um) Radal Dstance (mm) Fgure 3. Radal lens dstorton at repeated calbratons 1066

3 3. 3D CITY MODELLING WITH PICTOMETRY DIGITAL IMAGERY Oblque mages have advantage aganst vertcal mages n creatng buldng textures snce they provde better sde vew of buldng facades. Pctometry magng system captures oblque mages from dfferent drectons whch are deal for generatng buldng textures. The vertcal mages taken n the same area can be used to generate 3D buldng models or to refne 3D buldng models created from other data source such as LDAR data. In ths secton, some ssues n generatng 3D cty models usng Pctometry dgtal mages wll be addressed. vector from the center of the façade to the camera center of an oblque mage and the one wth hghest score s chosen. Snce Pctometry oblque mages are captured at a certan angle, a reference vector wth a certan angle to the buldng facade wthn the vertcal plane passng through the normal vector nstead of a normal vector s chosen as shown n Fgure 6 and a 3.1. Refnng 3D Buldng Models 3D buldng models can be extracted from both aeral mages and LDAR data. In automatc buldng extracton from aeral mages, mage matchng technque s usually used to extract 3D nformaton of buldngs. One major problem wth automatc approaches s that the extracton may fal when occlusons and shadows occur n the mages. Modern LDAR systems are capable of recevng multple returns wth some penetratng vegetaton, and thus, the effect of occlusons can be reduced by combnng the nformaton from dfferent returns, e.g. frst and last returns and buldngs can be extracted relably. Fgure 4 shows an example of buldng footprnts extracted from LDAR data n downtown area of Buffalo, New York. However, the problem wth extracton of buldng models from LDAR data s that the extracted models may not be very accurate because of pont spacng, scannng angle, the performance of lne extracton algorthm, etc. Therefore, buldng models derved from LDAR data need to be refned, n order to create accurate 3D cty models. To correct buldng models, they are projected back on the vertcal mage trangulated wth accurate ground control ponts. The dfference between a projected roof edge and ts correspondng edge extracted from the mage s usually just a few pxels, as shown n Fgure 5. Therefore, an affne transformaton can be used to correct the buldng models and the transformaton parameters can be estmated by usng the dstance between the projected roof edges and the extracted edges from the mage. Fgure 5. Projected buldng footprnt on aeral mage score s gven to an oblque mage based on the angle between the reference vector and the vector from the center of the façade to the camera center of the oblque mage. At the same tme, a vsblty analyss s performed to make sure that the façade s not blocked by other buldngs. Vector to camera center Reference vector Buldng facade Normal vector Fgure 6. Selecton of oblque mage 3.3. Texturng wth Oblque Image Fgure 4. Buldng footprnts extracted from LDAR data 3.2. Selecton of Oblque Image Due to the mage overlap, each buldng s maged on several oblque mages. It s very mportant to choose one from them to gve the best texture of the buldng. In Zebedn et al (2006), a score s assgned to all oblque mages based on the angle between the normal vector of the facade to be textured and the Once a rght oblque mage s chosen, the next step s to pck up the rght mage porton and add t to the buldng façade. To make sure the rght mage porton s selected, t s necessary to check whether the buldng façade projected onto the oblque mage matches the buldng edges on the mage. The projected boundares of the façade should match the correspondng buldng edges n the mage when the oblque mage has accurate exteror orentaton (EO) parameters. However, they may not exactly concde wth the actual edges of the buldng as shown n Fgure 7, when mage's EO parameters from GPS/IMU are used drectly. To create accurate 3D cty models, accurate EO parameters of mages must be used. The usual way 1067

4 to obtan accurate EO parameters of mages s by automatc trangulaton (AT). However, most commercal AT software cannot handle oblque mages because of the nature of oblque mages. A least squares method s proposed n ths paper. In the proposed method, the dfferences between edges of the buldng façade extracted from the oblque mage and the correspondng REFERENCES Brenner, C. Haala, N. and Frtsch, D., Towards fully automated 3D cty model generaton. In: E. P. Baltsavas, A. Grün and L. V, Gool. eds., Automatc Extracton of Man-Made Objects from Aeral and Space Images (III). A.A. Balkema, Ascona, Swtzerland, pp Cramer, M Performance of Medum Format Dgtal Aeral Sensor Systems. In: the Internatonal Archves of Photogrammetry and Remote Sensng, Istanbul, Turkey, Vol. 35, Part B5, pp DIMAC Fraser C. and Edmundson K., Desgn and Implcaton of a Computatonal Processng System for Off-Lne Dgtal Close- Range Photogrammetry. ISPRS Journal of Photogrammetry & Remote Sensng, 55(2) , Frueh, C., Sammon, R. and Zakhor, A., Automated texture Mappng of 3D Cty Models wth Oblque Aeral Imagery. In: Proceedngs of 2nd Internatonal Symposum on 3D Data Processng, Vsualzaton and Transmsson, Thessalonk, Greece, pp Fgure 7. Projected buldng boundary on oblque mage projected boundares are used to estmate the correctons of EO parameters. The dfferences of x and y coordnates of end ponts of edges are used as observatons gven n formula (1) and the correctons of EO parameters are treated as unknowns. Δx Δy x ' ' = x x = y y y where:, = coordnates of th end pont on oblque mage ' ' x, y = coordnates of correspondng projected end pont. The EO parameters from GPS/IMU are used as approxmate values and ther correctons are computed by the least squares adjustment. Once the accurate EO parameters of the mage are computed, the rght mage porton can be determned by projectng the buldng façade onto the oblque mage wth the corrected EO parameters and added to the 3D buldng model. 4. SUMMERY In ths paper, Pctometry's propretary dgtal magng system s ntroduced brefly and some ssues n 3D cty modelng usng Pctometry's dgtal mages are dscussed. The unque feature of Pctometry's dgtal magng system s that t produces both drect geo-referencng dgtal vertcal and oblque mages at one tme. The vertcal mages can be used for photogrammetrc mappng such as generaton of accurate large scale dgtal orthophotos whle oblque mages can be utlzed for other applcatons such as publc safety, 3D cty modelng, etc. In 3D cty modelng, vertcal mages can be used to create buldng models or refne buldng models generated from LDAR data and buldng textures can be created from oblque mages. (1) Gülch, E., Applcaton of Sem-Automatc Buldng Acquston. In: A. Grün, E.P. Baltsavas, O. Henrcsson, eds., Automatc Extracton of Man-Made Objects from Aeral and Space Images (II), Brkhäuser Verlag, Basel, Swtzerland, pp Haala, N. and Hahn M., Data Fuson for the Detecton and Reconstructon of Buldngs. In: A. Grün, O. Kübler & P. Agours, eds., Automatc Extracton of Man-Made Objects from Aeral and Space Images, Brkhäuser Verlag, Basel, Swtzerland, pp Haala, N. and Brenner, C., Generaton of 3D Cty Models from Arborne Laser Scannng Data. In: Proceedng of EARSEL Workshop on LIDAR remote sensng of land and sea, Tallnn, Estona. Henrcsson, O., Analyss of Image Structures Usng Color Attrbutes and Smlarty Relatons. PhD thess, ETH, Zurch, Swtzerland. Hnz, A., Dörstel C. and Heer H., DMC - The Dgtal Sensor Technology of Z/I -Imagng. In: the proceedng of Photogrammetrc Week 01', Eds. D. Frtsch and R. Spller, Wchmann Verlag, pp Hu, J., You, S. and Neumann, U., Integratng LDAR, Aeral Image and Ground Images for Complete Urban Buldng Modelng. In: Proceedngs of the Thrd Internatonal Symposum on 3D Data Processng, Vsualzaton, and Transmsson (3DPVT'06, Chapel Hll, USA, pp Leberl, F., Gruber M., Pontcell M., Bernoegger S. and Perko S., The Ultracam Large Format Aeral Dgtal Camera System, In: the Proceedngs of the Amercan Socety for Photogrammetry & Remote Sensng, 5-9 May, 2003, Anchorage, Alaska. Mostafa, M., Desgn and Performance of the DSS. In: the proceedng of Photogrammetrc Week 03'. Ed. D Frtsch, pp

5 Pctometry 2008: Rotternstener, F., Brese, C., A New Method for Buldng Extracton n Urban Areas from Hgh-resoluton LDAR Data. In: the Internatonal Archves of Photogrammetry and Remote Sensng, Graz, Austra, Vol. 34, Part 3A, pp Rotternstener, F., Jansa, J., Automatc Extracton of Buldngs from LDAR Data and Aeral Images. In: the Internatonal Archves of Photogrammetry and Remote Sensng, Ottawa, Canada, Vol. 34, Part 4, pp Sandau, R., Braunecker B., Drescher H., Eckardt A., Hlbert S., Hutton J., Krchhofer W., Lthopoulos E., Reulke R. and Wck S., Desgn Prncples of the LH Systems ADS40 Arborne Dgtal Sensor. In: Internatonal Archves of Photogrammetry and Remote Sensng, Amsterdam, the Netherlands, Vol. 33, Part B1, pp Schwalbe, E., Maas, H.-G. and Sedel F., D Buldng Model generaton from Arborne Laser Scanner Data Usng 2D GIS Data and Orthogonal Pont Cloud Projectons. In: Internatonal Archves of Photogrammetry and Remote Sensng, Enschede, the Netherlands, Vol. 36, Part 3/W19, pp Vosselman, G., Buldng Reconstructon Usng Planar Faces n Very Hgh Densty Heght Data. In: the Internatonal Archves of Photogrammetry and Remote Sensng, Munch, Germany, Vol. 32, Part 3/2W5, pp Vosselman, G., Fuson of Laser Scannng Data, Maps, and Aeral Photographs for Buldng Reconstructon. IEEE Internatonal Geoscence and Remote Sensng Symposum and the 24th Canadan Symposum on Remote Sensng, IGARSS'02, Toronto, Canada, June 24-28, on CD-ROM, 4 pages. Wang, Y., Schultz, S. and Guffrda, F., Generaton of Orthophotos Usng Pctometry's Dgtal Images. In Proceedngs of ASPRS Annual Conference (CD ROM), Portland, Oregon. Zhang, Z., Wu, J., Zhang, Y., Zhang, Y. And Zhang, J., Mult-Vew 3D Cty Model Generaton wth Image Sequences. In: the Internatonal Archves of Photogrammetry and Remote Sensng, Istanbul, Turkey, Vol. 34, Part 5, pp Zebedn, L., Klaus, A., Gruber, B. And Karner, K., Facade Recognton from Aeral Images by Mult-vew Plane Sweepng. In: the Internatonal Archves of Photogrammetry and Remote Sensng, Bonn, Germany, Vol. 36, Part 3, pp

6 1070