Automatic Building Extrusion from a TIN model Using LiDAR and Ordnance Survey Landline Data Rebecca O.C. Tse, Maciej Dakowicz, Christopher Gold and Dave Kidner University of Glamorgan, Treforest, Mid Glamorgan, CF37 1DL Telephone: +44 (0) 1443 483617 Fax: +44 (0) 1443 654022 rtse@glam.ac.uk, mdakowic@glam.ac.uk, christophergold@voronoi.com and dbkidner@glam.ac.uk 1. Introduction LiDAR (Light Detection and Ranging) data is widely used to construct 3D terrain models which provide realistic impressions of the urban environment and models of the buildings. This paper presents the possibility of constructing a 3D terrain model using LiDAR data and the 2D building outlines from Ordnance Survey (OS) Landline data. A TIN (Triangulated Irregular Network) model is constructed by filtering the LiDAR data to generate a bare-earth model. 2D building boundaries are added on the terrain surface by using a line-tracing algorithm. The building is extruded from the terrain surface by using CAD-type Euler Operators, therefore the topological connectivity is kept for further spatial analysis. The use of Euler Operators allows the modification of the 3D terrain model (surface) interactively. 2. Building Re-Construction by Using LiDAR Data The latest airborne laser scanning technology allows the capture of very dense 3D point clouds from the terrain and surface features, therefore 3D building reconstruction from LiDAR becomes feasible. This is an active research topic in GIS. However different steps are involved before actually using the laser scanning data. A filtering algorithm is used to generate a bare-earth model which removes all the buildings, trees and terrain objects. Morphologic filtering is one of the algorithms which is commonly used to solve this problem, for example slope based filtering (Vosselman, 2000) and modified slope based filtering (Roggero, 2001). Building detection or extraction algorithms are used to extract the building boundaries from the laser scanning data. Many researchers are interested in building extraction. There is still room for improvement in automatic building extraction because it is not totally reliable. In this paper, Landline data is used to avoid the problem. Constructive Solid Geometry (CSG) and VRML are the two common methods for modelling and rendering the buildings on the terrain surface respectively. Suveg and Vosselman (Brenner, 1999; Suveg and Vosselman, 2004) used CSG to generate a complex building with the Boolean operations of union, intersection and differences. Rottensteiner and Briese used VRML to display the generated buildings (Rottensteiner and Briese, 2003). However the topological relationships are not kept during the construction. All of the rendered buildings are superimposed on the terrain surface without actually connecting to it. If the topological connectivity is preserved, more kinds of spatial analyses can be performed. 3. Methodology for Adding Buildings to a TIN Several steps are used to create a 3D terrain model from the laser scanning data. They are: Create a TIN model with the filtered LiDAR data. Filter the laser scanning data by
one of the methods mentioned above. Add Landline data to the terrain surface using the line tracing algorithm described below. Calculate the average heights of the buildings from the LiDAR data. Extrude the building by using CAD-type Euler Operators while keeping the topological connectivity. 4. Line Tracing Algorithm In order to model buildings, we need to estimate the ground surface at the foot of the walls described by the OS Landline data. We also need to have triangle edges that follow these lines. The line tracing algorithm is: 1. Insert two points on the terrain surface (Points A and B in figure. 1) 2. Check if any triangle edge connects these two points. i. Stop if this is true. ii. Insert a point half way between these two points if no edge connects them. 3. For each half of the line repeats step 2 recursively until points A and B are connected by triangle edges. We add the points on the terrain surface and estimate the height of the points by using the surface interpolation method of (Dakowicz and Gold, 2002). Figure 1 A Delaunay Triangulation with points A and B
Figure 2 Inserted points (square points) in between of points A and B 5. Building Extrusion Using Euler Operators We start by creating a 2.5D TIN model and Euler Operators are used to extend the TIN (Tse and Gold, 2001). Building boundaries are added on the terrain surface by using the Landline data. Then the boundaries are extruded from the ground surface to the height of the building by using Euler Operators. The selected building boundary in figure 3 will be extruded. We extend the TIN model by extruding buildings from the terrain surface in figures 4 and 5. Figure 3 the building boundaries are added on the terrain surface
Figure 4 Buildings are extruded with different heights Figure 5 shows the extended TIN with buildings of the University of Glamorgan We may then perform further spatial analysis in our model because the topological connectivity is preserved. Interactive editing is allowed, for example, bridges and tunnels, as in (Tse and Gold, 2002). We created a TIN model by using Euler Operators. Then we used an additional Euler Operator to extend the TIN model with bridges and tunnels. This method keeps the topological connectivity. The details can be found in (Tse and Gold 2001; Tse and Gold 2002).
6. Conclusion and Future Work This paper shows the possibility of using laser scanning and Landline data to generate a 3D terrain model automatically. The line tracing algorithm allows us to insert the building boundaries on the terrain surface and the extended TIN model preserves the topological connectivity (Tse and Gold, 2002). In the presentation, we will demonstration the use of laser scanning data to filter and create a bare-earth model. OS Landline data will be used to add the outline of the buildings. We will estimate the height of the buildings by using the laser scanning data, and extrude the buildings, and edit them to create more complex shapes. 7. Acknowledgement The research discussed in this paper was supported by the Ordnance Survey, Southampton (Research & Innovation). 8. Reference: BRENNER, C. 1999. Interactive Modelling Tools for 3D In Proceedings of Photogrammetric Week 99 Building Reconstruction (Wchmann Verlag, Heidelberg), p.23-34. DAKOWICZ, M. and GOLD, G. 2002. Extracting Meaningful Slopes from Terrain Contours. In Proceedings of Computational Science - ICCS 2002, Lecture Notes in Computer Science (Amsterdam, The Netherlands), p. 144-153. ROGGERO, M. 2001. Airborne laser scanning: clustering in raw data. In Proceedings of IAPRS (Annapolis, MD), vol XXIV-3/W4, p. 227-232. ROTTENSTEINER, F. and BRIESE, C. 2003. Automatic Generation of Building Models from LIDAR Data and the Integration of Aerial Images. In Proceedings of the ISPRS working group III/3 workshop 3-D reconstruction from airborne laserscanner and InSAR data (Dresden, Germany, Institute of Photogrammetry and Remote Sensing Dresden University of Technology). SUVEG, I. and VOSSELMAN, G. 2004. Reconstruction of 3D Building Models from Aerial Images and Maps, ISPRS Journal of Photogrammetry & Remote Sensing, 58(3-4), p. 202-224. TSE, R.O.C and GOLD, C.M. 2001. Terrain, Dinosaurs and Cadastres-Options for Three-Dimension Modelling. In Proceedings of International Workshop on 3D Cadastres (Delft, the Netherlands), p243-257. TSE, R.O.C. and GOLD, C.M. 2002. TIN Meets CAD - Extending the TIN Concept in GIS. In Proceedings of Computational Science - ICCS 2002, International Conference, Proceedings of Part III. Lecture Notes in Computer Science (Amsterdam, the Netherlands), p. 135-143. VOSSELMAN, G. 2000. Slope based filtering of laser altimetry data. In Proceedings of IAPRS (Amesterdam, The Netherlands),XXXIII, Part B3, p935-942. Biography Rebecca Tse currently is studying for her Phd at the School of Computing of the University of Glamorgan. Her research interests are 3D terrain models, CAD-type Euler Operators and LiDAR data. She received an MPhil degree from the Hong Kong Polytechnic University in 2003 and the research topic was Semi-automated construction of fully 3D terrain models.