Semi-Automatic Technique for 3D Building Model Generation

Transcription

1 Ngw El-ASHMAWY, Egypt Key words: DTM, DSM, 3D Building Model, Phoogrmmetry, Remote Sensing. SUMMARY Three Dimensionl Building models re excessively used in vrious pplictions. Digitl Surfce Model is the ck one of the 3D uilding modeling. The utomtic DTM genertion techniques re not dequte for the uilt-up res. The mnul 3D digitiztion of the stereo imges is time consuming. Estlishing semi-utomtic technique for the 3D uilding modeling is required. The min gol of this reserch is to estlish new semi-utomtic technique for 3D uilding models sed on the utomtic DTM genertion engines nd the 2D vector representtion of the uildings. The im of this pper is to ssess the estlished semi-utomtic uilding model genertion technique. Compring the new technique output to the mnul ones re performed for ccurcy ssurnce. This technique is pplied to eril photogrphs for low uildings in villge re, s well s stereo Ikonos imges for high uildings in city re in Egypt. This study shows tht the Automtic DTM Extrction modules do not represent neither the terrin nor the Buildings in n pproprite mnner. However, the Semi-utomtic DSM genertion technique tht introduced in this pper is more ccurte in the uilding representtion, s well s, it hs ccurcy within 1.0m in height.. 1/13

2 Ngw El-ASHMAWY, Egypt 1. ABSTRACT Three Dimensionl Building models re excessively used in vrious pplictions. Digitl Surfce Model is the ck one of the 3D uilding modelling. The utomtic DTM genertion techniques re not dequte for the uilt-up res. The mnul 3D digitiztion of the stereo imges is time consuming. Estlishing semi-utomtic technique for the 3D uilding modeling is required. This pper introduces new semi-utomtic technique for 3D uilding modelling depending on the utomtic Digitl Elevtion models nd 2D vector representtion of the uildings. The im of this pper is to ssess the estlished semi-utomtic uilding model genertion technique. Compring the new technique output to the mnul ones re performed for ccurcy ssurnce. This technique is pplied to eril photogrphs s well s high resolution stereo stellite imges (IKONOS Imges) for different res in Egypt. 2. INTRODUCTION The 3D uilding models hve een used in numerous pplictions tht required, eside the plnimetry position, the elevtion informtion of the ojects on the ground, such s; communiction, militry, trnsporttion, utilities, etc.. Communiction nd urn plnning re the most common pplictions tht require the 3D uilding dt. The 3D uilding model is digitl representtion of the rel uilt-up res with three dimensionl coordintes (x, y, nd z). When the digitl elevtion dt descrie the surfce of the terrin with or without vegettion, it is clled digitl terrin model (DTM). However, when these elevtion dt represents not only the terrin surfce ut lso the non-ground ojects tht exist on the terrin surfce (usully mn-mde ojects) then it is clled digitl surfce model (DSM). DSM is 2.5 dimensionl dt, where ech point (represented y x nd y vlues) hs only one elevtion vlue (z). When the edge of high oject lies inside pixel, the pixel hs n verge elevtion vlue etween the ground nd the oject s top elevtions. Thus, the ojects heights re not represented well s they re in the relity. Consequently, the edges of the uildings re not represented correctly, s the pixels tht represent the uilding orders hve trnsition elevtion vlues etween the roof nd the ground surfce surrounding the uildings. Therefore, when DSM is used for representing the 3D uilding model, the elevtion vlues of the roofs of the uildings hs to e used insted of the terrin elevtion vlues. In other words, reconstruction of the non-ground ojects (with their correct shpe nd elevtion) on the 2/13

3 ctul ground surfce (DTM) is required. Thus, comintion etween digitl terrin model nd the oject model hs to e developed to crete this ccurte 3D uilding model. The min gol of this reserch is to estlish new semi-utomtic technique for 3D uilding models sed on the utomtic DTM genertion engines nd the 2D vector dt (otined from the orthogonl imges). 3. DSM GENERATION TECHNIQUES Severl techniques for generting the digitl surfce models hve een developed, some of these methods re simple nd fst nd others re more complex nd time consuming. The ville techniques for producing DTM cn e summrized s follows: 3.1 Digitiztion of Existing Topogrphic Mps Extrcting the elevtion informtion from existing topogrphic mps cn e performed y digitizing the contour lines nd/or spot heights. Then using one of vrious interpoltion techniques to get the elevtion dt to points etween the contour lines, the DTM cn e generted. This method cn e considered s one of the fstest, s well s, lest expensive methods. Inversely, the output ccurcy depends on the qulity, scle, nd ccurcy of the digitized mps s well s the professionlism of the digitiztion opertor nd his/her ccurcy. Nether the less, the generted model will descrie the terrin dt only, since the elevtion dt out the non-ground ojects re not included in the Topogrphic mps. 3.2 Ground Survey Techniques Using the tchometer instruments; such s Totl Sttions, elevtion informtion cn e otined. This is trditionl method which is clled ground survey. By this method, the X, Y, nd Z coordintes of ny point, which lies on the line of sight of the se point, cn e mesured. Usully, this method is used for cquiring dt out the ground elevtion through grid lines with equl distnce in etween. Totl Sttions cn e used for collecting position nd height informtion out ny other point. GPS s modern ground survey technique cn e used s well for collecting three dimensionl coordintes of points covering the re of interest. Then, using n interpoltion technique, X, Y, nd Z coordintes of grid points with constnt distnces cn e clculted, nd DTM cn e generted. The ground survey methods produce high-resolution nd ccurte DTM ut these methods re time consuming specilly for lrge res, nd even smll res with lot of detils. 3.3 Active Remote Sensing Techniques The most populr ctive sensors tht re used in remote sensing re rdr nd lser scnner ltimeter. These two sensors cn e used for 3D coordintes cquisition of points on the ground, nd hence, DSM genertion. 3/13

4 In Rdr imges, sed on two different psses or two different ntenns mounted on the sme pltform, the phse informtion cn e clculted depending on the otined informtion from the slight offset of the two emitted microwves. By mesuring the rnge difference of the two ckscttered wves received t the ntenn, the elevtion of the ground or the ojects on the ground cn e determined reltively to the position of the sensor. However, since the position of the sensor, t the moment of sending nd receiving the signls, is ccurtely known, the solute 3D coordintes of the whole re of interest cn e determined. Digitl surfce model cn e generted from the otined dt. Since the Rdr imges re low resolution, the generted DSM is, consequently, will e with low resolution. Lser Scnning technique is rnging system s well. The system, mounted on ord of n irplne, sends pulse signls to the ground nd receives the reflected signls. Since the light (Lser Pulses) velocity is known, y mesuring the difference in time etween sending nd receiving signls, the reltive distnce etween the sensor nd the ground point cn e determined. Knowing ccurtely the position nd ltitude of the lser system t the moment of sending pulses, the position of the points on the ground cn e clculted precisely. This technique is lind system, which required uxiliry dt for etter interprettion, such s digitl or video cmer. 3.4 Photogrmmetric Techniques Photogrmmetry techniques re, nd will still, the most common techniques for producing topogrphic mps nd generting digitl terrin models. Bsed on rigorous mthemticl models, such s Collinerity eqution, nd using stereo pir of imges (eril photos or stellite imges), the 3D coordintes of the surfce points cn e determined. Presently, digitl Photogrmmetry is common in severl countries nd there re severl softwre progrms tht speed up the eril tringultion process. Photogrmmetry cn e considered s the most ccurte technique for DSM genertion, specilly if there re qulified technicins who re le to digitize ojects in three dimensions using the stereoscope, or the equivlent instruments. However, this mnul technique is time consuming specilly in the uilt-up res. There re vrious softwre pckges hve the cpility for utomtic terrin extrction using mtching techniques, which, significntly, reduces the time nd effort for generting DTM of lrge locks. However these techniques generte poor DSM, which is not ccurte in uilt-up res. 4. AUTOMATIC TECHNIQUE FOR DSM GENERATION Rther thn mnully collecting individul 3D point positions with GPS or using direct 3D mesurements on stereo imgery, utomted techniques extrct 3D grid points s representtions of the Erth s surfce in the overlp res of two imges. This is referred to s utomted DTM extrction. Usully, the generted surfce model is in rster formt with constnt pixel size, resolution or grid points with equl intervls. 4/13

5 The DTM from stereo imges cn e extrcted utomticlly in three-step process. These steps re: Digitl imge mtching, ground point coordintes determintion, nd DTM construction. 4.1 Digitl Imge Mtching Mtching is n importnt step in mny pplictions such s; oject recognition, stereo vision, 3D reconstruction, nd trcking. Digitl imge mtching techniques re used to utomticlly identify nd mesure the positions of common ground points ppering in the overlp re of the two djcent imges. Imge mtching depends on digitl imge correltion to find res of similrity etween the imges in the overlp re. The correltion coefficient is used to represent the mesure of similrity etween the mtched point pirs. The lrger the correltion coefficient, the more similrity etween mtched point pirs exists. The geometric nd rdiometric chrcteristics of the stereo pir imges re used s the mjor constrint of the imge mtching process to produce highly ccurte nd relile mtching imge point pirs. The epipolr geometry for the stereo pir is used to constrin the serch re used to find the mtching point to ny specified one, s shown in figure 1. Therefore, imges in the stereo pir re re-projected to the epipolr projection. The epipolr imges re creted from the rw stereo imges fter elimintion of errors in Y direction during imge-to-imge correltion (removing the Y Prllx). Using epipolr imges in the extrction of DEM improves the speed nd reliility of the mtching procedure. Adopted from Imgine OrthoBse User s Guide Figure 1. The Eppipolr Geometry 4.2 Ground Points Coordintes Determintion Once the utomted DTM extrction process hs een completed, series of distriuted 3D points is locted within the overlp re. The 3D points re susequently interpolted to crete rster DTM. 5/13

6 4.3 DTM Construction The utomticlly extrcted nd clculted 3D points re used s sis for constructing DTM. Rster DTM output method cretes continuous surfce. Conversely, ASCII files output method cretes discrete DTM surfce. The ERDAS OrthoBse pckge is used s the utomtic DTM extrction engine in this pper. These techniques generte utomtic DTM firly ccurte for the re terrin, however, for uilt-up res, mtching techniques provide uncceptle results. 5. SEMI-AUTOMATIC TECHNIQUE FOR DSM GENERATION To produce digitl surfce model, there re different utomtic nd semi-utomtic lgorithms. The dopted comprehensive method is through digitizing the uildings with their true elevtions, nd then genertes the DTM while oscuring the uildings res. Next, the oscured res re filled with elevtion dt y interpolted vlues ccording to the ground elevtions surrounding the oscured res. However, this method goes s series steps, which is time consuming. Therefore, semi-utomtic DSM genertion technique is performed. This technique is descried s illustrted in Figure 2; it depends on the utomticlly generted DTM nd the mnully digitized 2D uildings. Then comintion of the two outputs cn e used in producing the DSM. This technique sves time nd effort, s it does not require specilists in stereo visuliztion. 6/13

7 Stereo pir imges Digitizing Automtic DTM Extrction 2D Building mp Buffering Boundries of Built-up Are Automtic Digitl Surfce Model (DSM) Intersection Intersection Elevtions of Built-up Are Boundries Buildings elevtion Interpoltion DTM for Builtup Are Overlpping 3D DSM Figure 2. Description of the DSM genertion Technique A computer progrm hs een written to semi-utomte the process of converting the utomtic extrcted digitl elevtion model to relistic digitl surfce model, s shown in the flow chrt of DSM genertion process. The steps of tht procedure re: 1. Digitize uildings inside the study re mnully from the less tilted imge. 2. Collect tie points on the roof of the uildings nd generting the utomtic DTM. 3. Overlp the digitized 2D uildings nd the utomtic generted DTM, the output uildings roof elevtion my not e constnt within the uildings depending on the vlues of the DTM. 4. Use the neighourhood function ech uilding get one constnt vlue equls to the mximum vlue within the uilding. 7/13

8 5. Buffer the uilt-up res towrds outside, nd exclude these res from the utomtic DTM, to eliminte the non correct elevtion vlues, nd determine the elevtion vlues of the oundries of the uffered zones. 6. Fill the uffered zones y the verge elevtion vlues of the oundries to crete new DTM, hence, the inccurte elevtion vlues will e eliminted. 7. Comine the uilding elevtion file with the DTM only file to form the ctul DSM. (Reconstructing uildings nd non-ground ojects on the new DTM) 8. Use the elevtion vlues of the tie points (on the roof of the uildings) to check the constructed DSM. 6. EXPERIMENTAL WORK The performed semi-utomtic technique for DSM genertion hs een pplied on two different dtsets; one stereo model of eril photogrph for prt of villge (with low uildings), nd stereo IKONOS stellite imges for prt of city (with high uildings). Figure 3 shows the res of interest in the two dtsets; ) Villge eril photogrph, nd ) city stellite imge. Figure 3. Study re The tringultion of the stereo imges is out of the scope of this pper, therefore the rectified imges hve een used. for pplying the procedure descried erlier in this pper, the ERDAS Imgine Orthose is used for utomtic DTM genertion. ArcMp is used for onscreen digitizing of the uildings in the study re, nd uffering the uildings to eliminte the incorrect DTM dt, nd ILWIS is used for performing the rest of the processes for generting the DSM. 8/13

9 Figure 4 illustrtes the collecting tie points step on the roof of the uildings. Figure 4. Collecting tie points The utomtic DTM generted y orthobase pckge is shown in Figure 5. Figure 5. Automtic Generted DTM For oth dtsets, using one of tringulted imges (the less tilted one) to digitize, mnully, the uildings inside the study s 2D vector dt, Figure 6. 9/13

10 Figure 6. 2D digitized uildings Determine true uilding elevtions y intersecting the digitized uildings file with the utomtic DTM file, nd then give ech uilding the mximum elevtion vlue within the uilding itself, Figure 7. Figure 7. Buildings with true roof elevtions 10/13

11 Buffer the uilt-up res towrds outside, to eliminte the non correct elevtion vlues dt. Fill the uffered zones y their verge elevtion vlues to crete new DTM, hence, the inccurte elevtion vlues will e eliminted, Figure 8. Figure 8. Extrcted DTM Reconstructing uildings on the new DTM, Figure 9. Figure 9. Finl DSM for the Study Are 11/13

12 Finlly the DSM is generted in semi-utomtic technique, nd the vlues of the uilding heights re used to check the produced DSM, Figure 10. Figure 10. DSM Evlution step By compring the check points with the produced DSM it is found tht the difference etween the check points nd the produced DSM vlues is within 50 cm in the low uildings nd eril photogrphs cse, nd within 1.0 m in the cse of stellite imges nd high uildings. 7. CONCLUSIONS According to this study, the most comprehensive conclusions cn e summrized s follows: 1. Automtic DTM Extrction modules do not represent urn res correctly. 2. Semi-utomtic DEM genertion technique cn e used for DSM development for etter representtion of 3D surfce models (specilly for uildings nd shrp-edge ojects representtions). REFERENCES Amhr, F., Jns, J., nd Ries, C., 1998, The Genertion of True Orthophotos Using 3D Building Model in Conjunction with Conventionl DTM, Interntionl Archives of Photogrmmetry nd Remote Sensing, Vol. 32,Prt 4. Ackermn, F., 1996, "Airorne Lser Scnning for Elevtion Models", Geomtics Info Mgzine, Vol. 10, No. 10, pp Ackermn, F., 1999, "Airorne Lser Scnning Present Sttus nd Future Expecttions", ISPRS Journl of Photogrmmetry nd Remote Sensing, Vol. 54, No. 2-3, pp Bltsvis, E.P., 1999, "Airorne Lser Scnning: Existing Systems nd Firms nd Other Resources", ISPRS Journl of Photogrmmetry nd Remote Sensing, Vol. 54, No. 2-3, pp /13

13 Bottu, E., 1998, "Lser Scnning Applied to Vegettion Height nd Roughness Length Determintion", Ecole Superieure des Geometres Topogrphes (ESGT), Conservtoire Ntionl des Arts et Mtiers, Appendix C. El-Ashmwy, N., 2005, Implementtion of High Resolution Stereo Stellite Imges nd Decision Support System for Flood Extent Prediction, Unpulished Ph.D. Thesis, Ciro University. ITC, 2001, "Principls of Remote Sensing", ITC, Enschede, The Netherlnds. Leic Geosystems, 2002, Imgine OrthoBse User s Guide, Chpter 4, Leic Geosystems, Atlnt, Georgi, USA. Meek, M., Gold, Hwng, Y., Axelrd P., Born, G., Engelhrdt, D., 2002, "Orit Determintion For The Quickird Spcecrft", Proceeding of Core Technologies for Spce Systems Conference. Suveg, I., nd Vosselmn, G., 2002, Mutul Informtion Bsed Evlution of 3D Building Models, IEEE. Suveg, I., nd Vosselmn, G., 2000, 3D Reconstruction of Building Models, Interntionl Archives of Photogrmmetry nd Remote Sensing, Vol. 33, Amesterdm. Tempfli, K., 2001, "Topogrphy nd Ortophotogrphy", Hndout GFM2/3, ITC, Enschede, The Netherlnds. Vosselmn, G., nd Dijkmn, S., 2001, 3D Building Model Reconstruction From Point Clouds nd Ground Plns, Interntionl Archives of Photogrmmetry nd Remote Sensing, Vol. 34. Wehr, A., 1999, "Airorne Lser Scnning An Introduction nd Overview", ISPRS Journl of Photogrmmetry nd Remote Sensing, Vol. 54, No. 2-3, pp CONTACTS Dr. Regionl Center for Trining nd Wter Studies Ministry of Wter Resources nd Irrigtion Street No. 1, Fourth Industril Zone 6 Octoer City Giz EGYPT Tel Cell Phone: Fx Emil: nelshmwy@rctws.com 13/13