GIS and Remote Sensing for 3D Urban Modelling by means of VRML Technology

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1 Published in: Remote Sensing for Environmental Monitoring, GIS Applications, and Geologiy VI Conference Proceedings of SPIE Vol Editors: M. Ehlers, U. Michel ISBN GIS and Remote Sensing for 3D Urban Modelling by means of VRML Technology Thorsten Bockmühl University of the Bundeswehr Munich Institut für Geoinformatik und Landmanagement Universität der Bundeswehr München 8577 Neubiberg, Germany Ulrich Michel University of Osnabrueck, Institute for Geoinformatics and Remote Sensing (IGF), Germany Kolpingstr. 7, Osnabrueck, Germany Abstract A 3D representation of a city embedded in an Internet environment is not a new technology. It is already used in many projects which present tourist information. But 3D modelling assumes a lot of manual operations and these are still not fully operational to handle, even if there exist special software (e.g. CyberCity) (1) nowadays, that support the generation of 3D scenes. The emphasis of this paper is to present a method for improving the automation of generating 3D VRML scenes by using simple and adapted procedures which are implemented in a GIS environment. Our goal was to create a backdrop which is easily modifiable and replaceable. Especially within a city we find areas (e.g. shop windows), whose appearance changes continuously. Their representations can be replaced by simple work steps in the 3D workflow. A 3D environment is based on a pool of different data e.g. cadastral maps, floor plans, and aerial images, which should have a minimum resolution of 50 cm, to guarantee the recognition of all important items. The cadastral maps should be controlled by using current aerial images to eliminate errors. More complex buildings are assembled from several simplified building sections. Subsequently, an automated modelling takes place after adding a few parameters (building height, roof form, etc.). For a realistic natural representation, the texturizing was assisted by using digital photographs of the building facade. The whole VRML scene is modular. Each wall was generated as an individual VRML file, turned and shifted into the suitable position. Every single wall can be addressed individually and this facilitates the adjustment of the textures enormously. In this way small changes of the buildings could be easily maintained without generating the whole VRML scene again. Finally, the modular city file was loaded into a main file with optional additives such as trees, viewpoints etc. This additional information could also be generated likewise automatically. For orientation purposes a general map was integrated, in which the position and line of sight of the user is indicated by an arrow. In conclusion, it can be stated that the proposed method and its implementation have been proved to be very valuable and reliable for automated 3D urban modelling. Introduction A steady rise in speed, bandwidth and availability of Internet is going on. Taking advantage of these factors, the Internet can be used to present more information in a 3D context. A 3D representation of a city embedded in an Internet environment is not a new technology. It has already been used in many projects e.g. in tourists information systems. But generation of 3D scenes requires a lot of manual

2 operations and these are still not fully operational to handle, even if there exist special software (e.g. CyberCity) for these purposes. By using 3D representations a user can navigate through an animated environment before even visiting it. In this way information can be presented in a realistic manner, and for example shops can be advertised in a more spectacular way than a common 2D homepage. The emphasis of this paper is to present a method for improving the automation of generating 3D VRML scene by using simple and adapted procedures which are implemented in a GIS environment. Our main objective has been to create a backdrop which is easily modifiable and replaceable. Especially within a city there are areas (e.g. shop windows), whose appearances change continuously. Their representations can be replaced by simple work steps in a 3D workflow. Issues and Procedures There have been several main goals to be achieved in the project: The result should illustrate the reality in an adequate way, so that a user can easily recognise the environment. To give texture to the 3D objects, images extracted from the digital photographs of the building façade have to be used, if possible and provided there is not any overlap of the facades with trees or other objects. Furthermore, the cadastral map has to be used for the correct positioning of the buildings. The result should be presented in the Internet. For the modeling, a standard language like Virtual Reality Modeling Language (VRML) has to be used (2). 3D objects should be created as automatically as possible, so that the work generates minimum time and costs. Avenue scripts had to be developed for ArcView (Esri software). In order to keep the workflow quite simple the cadastral data have to be simplified and completed only with a few parameters (e.g. building height, roof form and roof direction) of each building. The support of the 3D representation should be easy and clearly arranged, so that it could be modified in a simple way. The whole VRML scene is modular. Each wall has been generated as an individual VRML file, turned and shifted into the suitable position. Every single wall has been addressed individually, which facilitated the adjustment of the textures enormously. In this way small changes of the buildings could be easily maintained without generating the whole VRML scene again. Although, a steady rise in speed, bandwidth and availability of Internet is going on, this work aims to present the 3D representation to many users as possible. Therefore, the hunger for bandwidth should be kept small. This issue has to be achieved by simplifying the cadastral data and the use of same images for multi-texturing. Preparation Before the automatic generation of 3D objects, a few preparations have to be done. The ground layout of each building from the cadastral map has to be simplified (Fig. 1). Thus, every building should consist of only four vertices. This work step is necessary in order to fit the texture to the 3D object more easily. More complex buildings are assembled from several simplified building sections.

3 Figure 1: Cadastral map layer over aerial image before and after simplification The simplification can only be done semi-automatically, because the original ground layout is too detailed for further procedures. In a fully automatic procedure, there is a danger that vertices which are important for positioning building sections of a complex building will be deleted. Thus, a controlled generalisation is necessary. Therefore the best result is achieved by a manual simplification. For verification purpose an aerial image is added. This should have a minimum resolution of 50 cm, to guarantee the recognition of all important items. This minimum resolution is calculated by the formula (3) for visibility of objects in an aerial image. Object width / 8 = Pixel resolution The resolution of 50 cm has been calculated by assuming a minimum width of a building of 4 meters. In a further work-step the parameters of the building have to be added. If the parameters are not available, simple methods can be used to obtain them. The form and the direction of the roof can be extracted from the aerial image or from user s observation of the building. The two heights (from ground-to-roof and from ground-to-top) can be extracted from a stereo aerial image, too. Another method is to calculate the height from digital photographs of the building facades. Therefore, a reference height like a leveling board has to be used. Furthermore, the image has to be taken in parallel to the ground so that the image shows low distortion. The reference height within the image allows for the calculation of the height to the roof as well as the estimation of the height to the top. The accuracy of results is less than one meter, which is sufficient for the generating a building. Roof modelling The generation of the walls is quite easy, because of their orthogonal position to the ground. Their exact coordinates are given by the cadastral map. However, generating of the roof is more complicated. Each part of the roof is created according to each wall. Thus, the coordinates of the wall can be used, but two other points must be available in order to model the area of each part of the roof. These points are at the top of the roof and have to be calculated. It is necessary to mark the two walls

4 between the roof-line, this is done by intersecting the middle points. The assumption that a roof is always between the two smallest walls of a four-walled building is a possibility to create the roof-line automatically. This method is error-prone if the building has a quadratic form, which is particularly possible in the case of the simplified building parts for more complex buildings. Thus, you have to mark the walls manually. The algorithm for creating the roof is quite simple and is based on roof-line, which nearly all roof types have in common. The two other needed points of a part of the roof are on this roof-line. The exact position depends on the type chosen. In this work the following types can be created: hipped roof pavilion roof flat roof pyramidal broach roof An extension of the algorithm to create another type of roofs is absolutely possible. The work aims to create a walk-through and not an over-flight of the VRML scene. Thus, the focus has not been to create the roofs more realistic like it is explained in an article about the CyberCity-Modeller (4) but to simplify the generation of roofs. Texture Examination For this work two types of texture are differentiated: 1. Background texture Each wall gets at least one background texture, which covers a great area. The advantage is the use of small images, which are tiled in the whole wall area. These should be quadratic and the pixel count should be to the power of two. The most browsers are rendering images with these specifications in the fastest way (5). The tiling of small images can be used only with textures, which have a homogeneous appearance. For example a tiling of natural stone in a small texture creates an unreal apperance. 2. Object texture Each object (windows, doors ) should be displayed with a separate texture. The use of object textures has two advantages. On the one hand an image can be used for more object areas and so the load time would be decreased. On the other hand the images can be changed in a simple way, which is important for areas in a city (e.g. shop windows), whose appearance changes continuously. The application of two texture types is an advantage in another way. In the majority of cases, the photography of the whole wall is not possible without any overlay, like trees, cars or traffic sign. So it cannot be possible to use only one image for texturing a wall. The images of each object texture can be taken in different positions, which minimises the overlay effect in most of the cases. The position of a texture at a wall can be detected with the digital photograph of the building façade by using a modified method of the closed-range photogrammetry. Therefore the digital photographs have to be measured and corrected for perspective distortion, if it is necessary. The digital photographs should be made parallel to the object, so that no rectification is needed. This is not always possible especially in small streets. In these situations, the result is a tilted photo with perspective distortion (Fig. 2).

5 Figure 2: Perspective distortion (modified and translated) (6) To adjust the perspective distortion, linear rectification has to be used. This method transforms the perspective distorted photo in an image parallel to the object. In an exact measurement of the object, a better method has to be used to rectify the photo but by using the linear rectification the proportional distance is conserved, which is absolutely adequate for modelling the appearance of the buildings. For the measurement of the positions, two factors height and width are needed, which convert the measured pixel-value to centimetre-value for creating the VRML object. These factors have to be calculated by dividing the distance of the building through the distance of the pixel measurement. The two factors are necessary in order to get the proportional distance between the objects. Because of the inexact measurement of the building heights, some small differences can appear. Each object is rectangular and so two pairs of coordinates are needed and these have to be saved as attributes of the wall-line. With this information, the generation of the VRML building can be done automatically. Texture Adjustment The result after the automatic generation is 3D modelling without any additional textures except a default background texture (Fig.3). The arrangement of the textures has to be done manually. The following have been developed in order to simplify this work. At first the object areas have the same measurement like the real objects, so it is possible to put the textures on it without any distortions. Each wall has to be generated as an individual VRML file so the controlling of the texture adjustment is easy, because the wall is displayed by the first opening and no turning of the building is necessary. In this way small changes of the buildings could be maintained easily, without generating the whole VRML scene again.

6 Figure 3: Default background texture of the buildings Building scene The building scene is illustrated in Figure 4. city layer building layer wall layer Figure 4: Modular building scene Each individual wall (geb1_1.wrl, ) has to be turned and shifted into the suitable position in a separate file which includes the whole building (geb1.wrl). This modular building scene organizes lots of individual walls and gives the possibility to change one building without generating the whole VRML scene again. This is true in case of small changes. In the city layer, each building is shifted into the suitable position and is included in a separate file (index.wrl). Finally the modular city file has to be loaded into a main file with optional additives such as trees, viewpoints etc. Also this additional information could be generated likewise automatically. These additives can be point information like trees or viewpoints or line information (which becomes the route or path of an animation). For point data, a separate VRML file should be included at each position. For line data, a route from point-to-point of the line is created which serves as the line of sight to the next point. Simplify the reality All aspects of the reality cannot be represented because of its high complexity. Necessarily, the reality has to be simplified in such ways. The modelling of four roof types had to be converted in this project. Because of the high complexity some aspects of roofs have to be simplified as shown in Figure 5. A more realistic representation had to be done manually.

7 Figure 5: Real buildings (top); VRML buildings (bottom) The objective of this project has been to generate a walk-through and this means some areas of the environment should not be visible e.g. the backside of the buildings. For this reason, these back walls have only background texture as shown in figure 6. Including in HTML By loading the modular city file and the additives in a main file, it is possible to deactivate or change the view. This has been realised by including the main file in a HTML file. JavaScript has been used for the interaction between user and VRML scene. The HTML page displays the first layer in which all information of the result is presented (Fig. 7).

8 Figure 6: Backside VRML buildings Figure 7: Embedded VRML scene in HTML side In addition to the VRML scene there is a general map, an information field and a control panel. The general map makes the orientation possible for the user navigating in the virtual reality. The user gets information about the position and the line of sight by an arrow in the map. It is possible to display information about buildings or other objects in an information field by making them click-sensitive. Thus, special information can be integrated in a separate HTML page and displayed in the field. Figure 7 shows an example of a control panel which the user can use to manipulate the VRML scene. Conclusion The developed method makes the generation of 3D representation of a city possible in a simplified way. Beside the easy semi-automatic generation the crucial advantage is that all objects can be modified by using easy work-steps without generating the whole VRML scene again. The representation of the buildings as the main component of the 3D representation can be completed with many additives to make the result more realistically. These additives can be point information like trees or viewpoints or line information which becomes the way of an animation. It has to be decided prior to the application of the method, how detailed the result should be. It has to be shown that there are many options to complete the 3D-environment with dynamic elements like

9 animation. The number of these elements influences the time of work. Furthermore, the loading time is enhanced with increasing degree of reality. The method which has been presented in this paper has been used in some different parts of the city Osnabrück, Germany and furthermore for a New-Old-Scenario of a building scheme in a small town. Discussion Some aspects which are not realised in the work have to be discussed for a possible further development. As it shown in Simplify the reality only four roof types have to be converted for modelling. These types occurred in the test area as the most. An implementation of some other roof types is possible based on the same algorithm. The whole VRML scene has to be created by using the assumption that all objects are on the same continues ground level, which may be the case of cities in Lower-Saxton, Germany, where the test area is. In a more mountainous landscape it is necessary to implement the differences between the relief. The ground can be illustrated with a VRML node called EvaluationGrid, which creates a ground with different heights. The problem is the positioning of the object, specially the buildings. The decision to implement the result in VRML and not in the further developed standard X3D (7) is based on VRML, which is more established. An implementation in X3D is possible to use the expansions of the newer standard. References (1) (2) ISO/IEC :1997 and ISO/IEC :2004 (3) SCHIEWE, J. (2005): Status and future perspectives of the application potential of digital airborne sensor systems. In: International Journal of Applied Earth Observation and Geoinformation, 6: p (4) POLI, D. (2006): Reality-based 3D City Models from Aerial and Satellite Data. In: GEOInforamtics, Volume 9: p (5) POMASKA, G. (2002): Internet-Präsentation von Bauprojekten. Bauwerl-Verlag, Berlin. (6) WEINMANN, G. (1988): Architektur-Photogrammetrie. Wichmann Verlag, Karlsrue. (7) ISO/IEC 19775:2004 Extensible 3D (X3D)