1 3D CITY MODELLING WITH CYBERCITY-MODELER Kilian Ulm 1 and Daniela Poli 1 1. CyberCity AG, Zurich, Switzerland, (kilian.ulm, daniela.poli)@cybercity.tv ABSTRACT 3D city models using stereo aerial-/satellite imagery or laserscanning data are predominantly used in urban planning, architecture and marketing (e.g. tourism, real estate promotion). CyberCity generates 3D city models semi-automatically from stereo aerial and satellite images or laserscanner data with the specialised software CyberCity-Modeler (CC-Modeler ). Besides the easy-to-use texturing of facades with terrestrial images, an efficient technology was integrated for the automatic texturing of facades and roofs from (oblique) aerial images (CC-AutoTex). The 3D city models can be managed in a commercial database using ArcSDE (ESRI) and be used for further analysis in ArcGIS (ESRI). For real-time visualisation, the 3D data can be exported in Open Flight format (FLT) including levelof-detail (LOD) for geometry and textures. The professional visualisation tool TerrainView (View- Tec AG) supports level-of-details and includes sophisticated functions for the real-time visualisation of a huge 3D scenery including functionality for combining, moving and deleting objects in different formats. Web-streaming of landscape and city models allows TerrainView to view large data sets with high speed. DATA CAPTURING AND CC-MODELER Different kind of 3D City Models can be derived from several input data (Figure 1). Simple blockmodels can be automatically generated using LIDAR data (Light Detection and Ranging) and building footprints. Main roof structures can be semi-automatically created using dense LIDAR data and building footprints. Detailed roof structures including chimneys and dormers are generated using a semi-automatic feature extraction from stereo aerial imagery with the in-house developed software CyberCity-Modeler. Figure 1: Different types of 3D City Model Geometry and the used input data.
2 The technique for the generation of 3D city models with CC-Modeler is a semi-automatic procedure. An operator measures the specific roof points (x, y, z) in the stereo aerial images on a photogrammetric workstation. The points are classified as boundary points or internal points regarding their functionality in the roof. The software CC-Modeler automatically converts the measured point cloud into the 3D roof structure. The accuracy of the models is approximately 0.1-0.2 meter. Figure 2: Point definition in CC-Modeler. EDITING WITH CC-EDIT The tool CC-Edit is a CAD system specified for 3D city models and is used for improving the geometry of the models. Using Grid functions and additional functions for parallel lines, the walls can be made parallel and the angles can be improved to right angles (Figure 3). The functionality respects the fact, that in city centres or old towns, walls must not always be parallel and angles not always right angles by using a measurement error buffer, which means only deviations in the buffer are corrected. Figure 3: Object rectification (dashed line: before, solid line: after). Inconsistencies in topology between adjacent buildings and mutually overlapping roofs may arise because of measurement errors. Overlappings and gaps can be cleaned using specific functions in CC-Edit. In general, facades are not visible in aerial images and only the roof surfaces can be measured. Digital cadastral maps, which show the outer walls of buildings as part of the legal definition of real estate property, can be used to integrate realistic facades. Utilising this information, it is possible to
1st EARSeL Workshop of the SIG Urban Remote Sensing 3 model overhanging roofs and get a higher level of detail in building modelling (Figure 4). By applying this functionality, the 3D building models are consistent with existing 2D building footprints. Figure 4: Automated facade integration. Left: Roof polygon and facade from map, Right: Overhanging roofs as result. TEXTURING WITH CC-MAPPING Generally, different texturing procedures can be applied (Figure 5). Generic textures are used for automatic texturing using regional texture types. In this case, the texturing is automatic using texture libraries, but the textures do not match the reality. Using terrestrial photographs gives the best and reality-based texture quality but is a very time-consuming process. A good alternative for large 3D City Models is the in-house developed procedure for automatic texturing using (oblique) aerial images which results in realistic texturing of building facades and roofs for large area City Models. Figure 5. Texturing procedures. For high resolution facade textures (Figure 6), digital facade images that were taken on site can be easily applied by selecting the facade polygon in the 3D model in CC-Mapping and digitising the appropriate facade corner points in the facade image. The texture file is stored and linked to the 3D model file.
4 Figure 6: 3D city model of Reutlingen. Generated with CC-Modeler and CC-VisualStar. Texturing with CC-Mapping. AUTOMATIC TEXTURING FROM AERIAL IMAGES CC-AUTOTEX The texturing with CC-Mapping is easy to use, the preparative work like collecting the images and the cleaning of the images from unwanted parts (e.g. cars, trees, people etc.) is very time consuming. By using aerial images for the texturing of the roofs and facades, this work can be avoided. The aerial images are already available because the 3D model was measured from them (in stereo) and because of the photo angle from the sky, cars or trees are not existing as occlusions in the imagery. This procedure is a good alternative for texturing huge areas, terrestrial images of high quality can be used additionally. By using this technique, the 3D polygons of the 3D model is recalculated to the aerial images and the image part that belongs to the specific polygon is applied as a texture. The software compares the number of pixels and the percentage of the occlusion of the different image parts of different aerial images (because normally each polygon can be seen in more than one image) and selects the best one regarding this criteria. The selection can still be changed afterwards if the result is not satisfying. For improving the quality of the textures, oblique aerial images with a resolution of approximately 5cm per pixel can be used.
1st EARSeL Workshop of the SIG Urban Remote Sensing 5 Figure 7: 3D Model of Salzburg, streamed over the Web. Automatic texturing from oblique aerial images with CC-AutoTex. Project realised in cooperation between CyberCity AG, Forest Mapping Management GmbH, ViewTec AG. UPDATE AND CONTINUATION WITH CC-VISUALSTAR CC-VisualStar is a digital photogrammetric station, which was especially developed for 3D city modeling. CC-VisualStar also has standard photogrammetric features like stereo-model orientation, automatic and semi-automatic measurement of aerial triangulation, DTM and DSM, orthophoto computation including mosaicking. For updating 3D models, the existing 3D City Model is imported from a GIS database or file system and is displayed together with the aerial stereo model. This means that the old city model is overlaid with the new image information and differences and changes are visible immediately. Buildings, which do not exist anymore, can be deleted from the database. New buildings can be recognised because they are visible in the stereo model and not yet represented by the necessary vectors. Figure 8. GUI CC-VisualStar. The 3D City Model is overlaid with stereo aerial images for Quality Control.
6 MANAGEMENT IN ARCGIS (ESRI) The 3D City Model can be exported to shapefile, geodatabase or can be managed in a commercial database using ArcSDE (ESRI). Further analysis with ArcGIS can be applied and 3D City Models displayed using ArcGlobe (3D Analyst). Line-of-sight or viewshed analysis can be performed very accurate with the high-quality 3D models that are generated by photogrammetric means. Figure 9. 3D Model of Los Angeles as a Personal Geodatabase displayed in ArcGlobe (ESRI 3D Analyst). EXPORT IN OPENFLIGHT AND VISUALIZATION IN REAL-TIME The textured 3D city models are exported in FLT format and can be viewed in a real-time visualisation. Different level-of-details are applied for geometry and textures. The textures can be stored in the optimised image format DDS DXT1. TerrainView, the software solution of ViewTec (www.viewtec.ch), can manage huge 3D landscape and city model data. It is a tool for the interactive visualisation of 3D models/objects in different formats (e.g. 3DS, OBJ, VRML etc.). By using additional modules like weather simulation (clouds, rain, snow) it is possible to achieve a high degree of realism. In different distances from the viewer, building data and terrain data are displayed in different details whereby the concept of level-of-detail (LOD) is applied for the optimisation of the performance and the memory. TerrainView is used as a planning tool by combining planned objects (e.g. buildings) with the existing virtual environment for decision making during the planning process.
7 Figure 10. 3D Model of Bonn (Germany) visualised in TerrainView (ViewTec AG). TRUE-ORTHOPHOTO For the visualisation of 3D City Models, using a True-Orthophoto as terrain texture is recommended (Figure 11). In an Orthophoto the facades of the buildings can be seen on the ground, sometimes covering the street because of the perspective of the aerial images and the fact that only the Digital Height Model was used for the rectification of the aerial images. In case of a True- Orthophoto, additionally the 3D Building Model is used for the rectification which results in True- Orthophotos where no building walls can be seen from the top, which is necessary for a realistic visualisation of the ground. The gaps which happen by moving the roof textures during the processing are filled and radiometrically corrected automatically with information from other images. Figure 11. Los Angeles True-Orthophoto (left) compared with Orthophoto (right). True-Orthophoto generation by CyberCity AG 3D LANDMARKS In case of important landmarks like churches etc., detailed building models using terrestrial laserscanning or close-range photogrammetry can be generated (Figure 12). High-resolution textures are applied by using digital photographs taken on-site.
8 Figure 12. 3D Landmark ( HarmanBecker) WEB-BASED 3D-GIS The 3D city models represent a fundamental basis for Web-based 3D GIS. As example, Map2day.at is a web-based 3D-GIS service which provides user interaction between 2D city maps and 3D visualisations and query based information. As a marketing tool, it allows organisations to present their services and products via the internet and shows their physical location in both a 2D and 3D format. Queries can be run on the GIS database by selecting a category (e.g. tourism, public health etc.) and select a group (e.g. hotels, golf courses etc.) from the query window. Additional user requirements are met by having area/distance measurement capabilities, support for 12 languages, and individual customisation of the interface appearance allowing for different color schemes. The relation between the 2d map, and the free navigation 3d visualisation is updated dynamical. The queried results can be approached automatically with smooth flight in the 3D environment, which consists of high resolution imagery, Digital Terrain Models and 3D city models.
9 REFERENCES Ulm, K. 2004. Virtuelle 3D-Stadtmodelle - Technologie und Anwendung. GeoBit 8-2004, August 2004. Ulm, K. 2004. 3D-Stadtmodellierung mit dem CyberCity-Modeler. Digital Production 04/04, Juli/August 2004 Steidler, F., Beck, M. 2004. CyberCity-Modeler - Generation, Updating and Continuation of 3D-City models with on-line-editing - Visualization with TerrainView 2.0. CORP 2004. Ulm, K. 2003 Reality-based 3D city models with CyberCity-Modeler (CC-Modeler TM ) and laserscanner data. Optical 3D Measurement Techniques, 22-25 September 2003, ETH Zürich. Steidler, F., Gruen, A., Wang, X. 2002. Generation and visualization of 3D-city and facility models using CyberCity Modeler. MapAsia 2002, August 2002.