Advances in 3D GIS. Siyka Zlatanova Section GISt, Delft University of Technology, The Netherlands

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1 DDD - Advances in 3DGIS Siyka Zlatanova 24 DDD - Rivista trimestrale di Disegno Digitale e Design edita da Poli.Design Anno 1 n. 4 ott/dic 2002 Registrazione n. 718 del trib. di Milano Advances in 3D GIS Siyka Zlatanova Section GISt, Delft University of Technology, The Netherlands s.zlatanova@citg.tudelft.nl Abstract Many activities wait for 3D solutions in data storage, processing and analysis. Some of the most appealing ones are urban planning, environmental monitoring, telecommunications (mobile services), public rescue operations, real-estate market, utility management. The role of geo-information in all kinds of business processes is increasing as well. Most business transactions rely on information systems, as the geo-information is critical for many of them. Clearly, once the developments in the 3D GIS provide a compatible functionality and performance, spatial information services will evolve into the third dimension. Here, we attempt to summarise the recent advances in 3D GIS development. Introduction Geo-information has already proven its importance for many applications and daily use. A large number of human activities utilise 2D geo-data in some form (paper or digital maps) to complete different tasks. However, the world we are living in is three-dimensional and in many cases the two dimensions are not sufficient. The 3D objects presented as 2D projections may loose some of their properties and relations to other objects and may create difficulties to understand, analyse and evaluate the surrounding world in a critical for a certain activity moment. An increasing number of applications already seek for tools to model, store, analyse and visualise 3D data in an efficient and effective way. Urban (see [11]) and landscape planning, telecommunications, real estate market, 3D cadastre (see [9]), road, railway and building construction, utility management, shopping and tourism are among the most demanding ones. Moreover, maintenance, processing and visualisation of large data sets has been improving progressively in the last decade to approach the current stage when the user can immerse with a 3D model in different levels of mixture between reality and virtuality. Considering the recognised need for 3D data and the high level of technology developments, the logical expectation is a variety of software dealing with the third dimension. Unfortunately, the current status of the 3D software market differs. Many vendors develop intensively extensions to their software for more effectively handling 3D data. However, the killing software product, capable of dealing with all kinds of 3D data and providing the functionality needed by different application is still missing. The difficulties in devising such a product arise at each stage of the 3D modelling. The paper concentrates on a limited number of those aspects, i.e. structuring and analysis of 3D data. This paper is organised in three parts. The first part concentrates on the last developments in spatial data handling achieved under the OpenGIS specifications. Several experiments demonstrate the current functionality in 3D data maintenance. The second part briefly presents some of the last developments of traditional GIS vendors. The last part discusses research directions. A final discussion summarises the current status and trends and concludes on the future of 3D GIS. OpenGIS specifications and 3D The efficient geo-information management (especially when the third dimension is focussed) is rather complex task requiring high competence in different areas. Several years ago, leading vendors have decided to stream the efforts in GIS development by founding the OpenGIS consortium (currently consisting of more than 220 companies, government agencies and universities). The basic idea is achieving an agreement on the representation, access and dissemination of spatial information, i.e. the OpenGIS specifications (see [5]). The

2 DDD - Advances in 3DGIS Siyka Zlatanova 25 specifications describing the representation of spatial objects (i.e. the spatial) model are available in two variants abstract and implementation specifications. The abstract specifications discuss the conceptual models (already completed) and the implementation specifications (still at developing stage) provide mechanism for implementation for different technologies (e.g. SQL, CORBA). According to the OpenGIS specifications, the spatial object (named geographic feature) is represented by two models, i.e. geometric (i.e. simple feature specifications, SFS) and topological (i.e. complex feature specifications). While the geometric model provides direct access to the coordinates of individual objects, the topological model refers to the composing smaller elements (primitives) and encapsulates some of their spatial relationships. Suppose the two models are implemented, an application can benefit from the two representations, e.g. area, volume, distance can be completed on the geometric model, while analysis based on neighbourhood operations can be performed on the topological structure. Currently, the implementation focus is toward the geometric model. 3D topological implementation specifications are not available yet. Many DBMS (Oracle, Ingres, Informix) offer already maintenance of spatial objects as well as many front-end engines (Microstation, AutoCAD, ArcGIS) access the spatial data stored in DBMS. Here, we will briefly describe the implementations by Oracle Spatial and MicroStation GeoGraphics ispatial. Geo-DBMS (Oracle Spatial): A spatial objects in Oracle Spatial is defined by the geometric type. Currently, the supported geometric types are 2D (point, line, polygon) but 3D coordinates are accepted, i.e. it is possible to describe 3D points, 3D lines and 3D polygons. Lines and polygons are represented as an ordered set of coordinates (2D or 3D). Selfintersecting lines are allowed but self-intersecting polygons are not supported. Polygons with holes are maintained as well. Actually, the geometric type is an object-relational model (accessible as the standard data types such as integer, date) and contains information about type, dimension, coordinate system, holes of objects, and provide the list with the coordinates. Using appropriate coding, it is possible to represent also real 3D spatial objects, e.g. as a set of polygons or a collection of polygons. CAD (GeoGraphics ispatial): In contract to Oracle, the definition of a spatial object in GeoGraphics cannot be done without specifying the semantic meaning. Three levels of semantic hierarchy are maintained. Feature represents one or more objects from real world (e.g. the bank building, the school building). Category groups features with a similar theme (e.g. buildings, rivers). Finally, project refers to as the root and represents the data for the entire study area. One project can have many categories but a category may belong to only one project. To be able to distinguish between different spatial objects stored in Oracle Spatial, each object has to be assigned to a feature (i.e. its semantics has to be clarified). Furthermore, edited and newly created objects cannot be posted in the database without attributing predefined features to them. Spatial layers hold the geometry of the objects, which actually correspond to geometric types of Oracle Spatial. Figure 1: The Aula: a building of the TUDelft campus area and the 3D model (left) in GeoGraphics (right)

3 DDD - Advances in 3DGIS Siyka Zlatanova 26 To investigate the functionality of the two software products in representing, maintaining and visualising 3D spatial objects, we completed several case studies following two different approaches. In the fist approach, we had the 3D data organised in Oracle Spatial in user-defined relational tables and the task was to access, query and edit them from GeoGraphis ispatial. In the second approach, the 3D data were available in a DGN file and had to be imported in geometric structure of Oracle Spatial. The first approach appeared to be more complex, requiring good skills in both software products. The detailed description of all the required steps can be found in [12]. The second approach is relatively simple and straightforward. In both approaches, it was possible to populate, query, visualise and edit 3D objects. The query can be performed on the basis of the semantic characteristics of the objects as they are defined in GeoGraphics ispatial. For example, query on feature buildings will result in visualising all the buildings. If only one feature (e.g. the Aula ) is attached to only one spatial object, then this object will be only extracted from the database (see Figure 1). Figure 2: Query of spatial objects: Show buildings in a the Vield of View (left) and Show building (right) Our experiments have showed that although both packages follow closely the OpenGIS concepts, still some differences exist. For example, the geometric representation of Oracle Spatial slightly differs the SFS. Semantic characteristics of a feature are organised in GeoGraphics ispatial, as one significant part of the information (semantic hierarchy, links to geometry types) is maintained at a database level. However, the notations (table names, columns, object definitions) have very specific application-oriented meaning. If the user decides to keep the database and change the CAD package, he/she needs to create the featuregeometry link from scratch. Real 3D geometric types are missing but the description of 3D data is possible. The Z value is maintained together with the X,Y values, i.e. it is not an attribute. Although topological primitives are not implemented yet, Oracle Spatial offers a number of spatial operations. They operate with X,Y coordinates but some of them accept the Z coordinate as the computations are still in 2D. We have implemented an operator using the Oracle Spatial function with_in_distace to compute the field_of_view for a given angle and direction of view. Figure 2, left shows a snapshot with the result of the query. Despite the 3D view the quesry is completed on the basis of the 2D coordinates. Apparently the greatest benefits of the DBMS-CAD integration are in the area of visualisation and editing of data. As frequently commented, the amount of data to be visualised in 3D increases tremendously and requires supplementary techniques (LOD, on-fly simplification, etc.) for fast rendering. Having 3D data stored in a database, the user has the possibility to extract only a limited set of data (e.g. one neighbourhood instead of one town) and thus critically reduce the time for loading. Locating, editing and examining a particular object become also quick, simple and convenient. Indeed, the elements that can be edited correspond to the geometry representation in Oracle Spatial. Figure 2, right shows a building

4 DDD - Advances in 3DGIS Siyka Zlatanova 27 and the wall (the polygon in green) that is accessible for editing. As it can be seen, the face is a separate polygon, i.e. 3D topology of the building is not preserved. GIS vendors and 3D Nowadays, 2D GISs are widely used to handle many spatial issues in a very efficient manner. However, the same kind of systems fail to operate if more advanced 3D tasks are demanded. There are already few systems available in the market that can be categorised as systems providing 3D solutions. We analysed four of them that constitute the largest share of the GIS market, i.e. ArcView 3D Analyst, (ESRI), Imagine VirtualGIS (ERDAS), GeoMedia (Integraph Inc.) and Geomatica (PCIGeomatics). The results are reported in [12]. Here I will give a short summary. All the systems provide excellent tools for 3D visualisation, animation and navigation through 3D textured models (Figure 3). Imagive Virtual GIS offers two interesting 3D visualisation features. The system allows the user to include and visualise 3D models (e.g. 3D models of trees in a forest area) in a selected polygon or along a line in an easy and convenient way (and thus largely increase the realism). A logo layer is introduced that can accommodate 2D image into the 3D scene and stretch it over the entire view as foreground (e.g. cockpit in a fly animation, Figure 3, right). Most of them offer sufficient tools to manipulate 2.5D data such as surface generation, volume computation, draping images, terrain inter-visibility, etc. Access to data distributed on different servers is also greatly improved. All the systems revealed little provision of 3D GIS functionality in terms of 3D structuring, 3D manipulation and 3D analysis. The full 3D geometry (the z-coordinate is basically an attribute), 3D topological relationships and 3D analysis are still areas to be entered by the general-purpose GIS vendor. Figure 3: ArcScene, ESRI (left), TerraBuilder (middle) and Imagine VirtualGIS, ERDAS (right) Research in 3D GIS The research in 3D GIS is intensive and covers all aspects of the collecting, storing and analysing real world. 3D analysis and the related issues (topological models, frameworks for representing spatial relationships, 3D visualisation) are mostly in the focus of investigations. The 3D topological model is the key issue, since it is related to the representation of a large group of spatial relationships, e.g. inclusion, adjacency, equality, direction, intersection, connectivity. As it was mentioned above, OpenGIS consortium has not achieved a consensus on 3D topological implementation specifications. The problems with the third dimension are many more. For example, the well known relationship between arc and face, i.e. an arc has two neighbouring faces (i.e. left and right), is not valid in the 3D space (i.e. one arc can have several neighbouring faces) A large number of 3D models are reported in the literature, but the research concentrates around few basic ideas, as the level of explicitly described spatial relationships varies. Few examples follow. Molenaar [4] presents a 3D topological model called 3D Formal Vector Data Structure (3DFDS). The model is the first that takes into account semantic and geometric characteristics of spatial object. Nodes, arcs, edges and faces describe the four basic feature types named points, lines, surfaces and bodies. The model has been used by

5 DDD - Advances in 3DGIS Siyka Zlatanova 28 many for describing spatial objects in DBMS or objects-oriented implementations. Abdul- Rahman [1] presents an object-oriented implementation on the Molenaar s data model. Work by Pilouk [7] focuses on the use of Tetrahedron Network (TEN), maintaining triangles, tetrahedrons, arcs and nodes to describe spatial objects. Zlatanova [10] discusses some aspects of the data structuring and 3D visualisation with respect to data query over the Web. The proposed data structure lacks arcs in order to improve the performance of the system. The Urban Data Model (see [3]) represents the geometry of a body or a surface by triangles as each triangle is defined by a set of nodes. More examples of 3D topological models can be found in [13] It is rather difficult to give priorities to only one 3D topological model. Advantages of a topological model in one aspect may occur as disadvantages in another aspect. For example, the arbitrary number of nodes per face (3DFDS, SSS) can be seen as advantage and disadvantage for different applications. It is very convenient for modelling complex 3D objects (e.g. buildings) since an inappropriate partitioning (from user point of view) is not necessary. However, the same freedom in face description may lead to problems in visualisation (the rendering engines handle only triangles). Furthermore, the operators for consistency check may become very complex. Due to the omission of arcs, data structures (SSS, UDM) can benefit form the significantly faster data traverse. However, navigating through surfaces (e.g. follow shortest path ) may become time-consuming. Representing bodies as a set of faces (e.g. SSS) yields advantages for extracting the coordinates of the objects, but navigation queries might be disturbed since the co-boundary relationships (i.e. face-body) is not maintained. Bearing in mind the large variety of topological models (also the implemented in 2D systems), a possible solution could be the maintenance of topological models in a metadata table (in DBMS) and a supplementary set of rules can provide a consistent conversion between them (see [6]). Mechanisms for representing spatial relationships are yet another hot area of investigation. OpenGIS consortium has adopted two frameworks known as Egenhofer operators and Clementini operators based on the 9-intersection model (see [5]). Although the topology is considered the most appropriate mechanism to describe spatial relationships, the study on applicability of other mathematical frameworks continues (see [2]). Another significant area of 3D GIS research is devoted to Web applications. The Web has already shown a great potential in improving accessibility to 2D spatial information (raster or vector maps) hosted in different computer systems over the Internet. The first attempt to disseminate and explore 3D data, i.e. VRML, appeared to be rather heavy for encoding real geo-data due to the lack of a successful compression concept. Despite the drawbacks, the language became a tool for research visualisation. Researchers could concentrate on data structuring and analysis and leave the rendering issues to VR browsers offered freely on Internet. GeoVRML (VRML extended with geo-nodes) and Geographic Modelling language (GML) are another promising opportunities for representing 3D data on the Web. Based on XML concepts, GML provides larger freedom, flexibility and operability than VRML. Discussion The study on current implementations of OpenGIS specifications displayed that DBMS&CAD&GIS developers have rapidly adopted the idea of common standards. The first step is made, i.e. DBMS support spatial objects and many front-end engines make use of it. The performance of the DBMS (in maintenance of spatial objects) is already quite good and continues to improve with each new release (see [8]). The third dimension with respect to topological issues is still in the hands of the researchers. While quite many 3D topological models are reported in the literature, 3D spatial operations and analysis still have to be

6 DDD - Advances in 3DGIS Siyka Zlatanova 29 addressed by the researchers. 3D buffering, 3D shortest route, 3D inter-visibilities are some of the most appealing for investigations. The 3D progress in traditional GIS packages is great but mainly in the area of data presentation and surface analysis. The models are still 2D and the Z coordinate is maintained as an attribute. 3D GIS requires appropriate means not only for visualising and exploring 3D textured models but also for building and storing them. Observations on the demand for 3D City models show user preferences for photo-true texturing. Currently, a limited number of packages offer means for mapping images onto geometry (e.g. facades). Trading photo-true texture requires a variety of topics to be considered, e.g. standardisation of parameters for image-geometry references and image organisation at a database level. Despite the variety of issues waiting for an appropriate solution, the recent advances in geo-processing change the understanding of GIS. Instead of a monolith, desktop, individual system, GIS is becoming an integration of strong database management (ensuring data consistency and user control) and powerful editing and visualisation environment (inheriting advanced computer graphics achievements). References [1] Abdul-Rahman, A., The design and implementation of two and three-dimensional triangular irregular network (TIN) based GIS, PhD thesis, University of Glasgow, Scotland, United Kingdom, 250 p. [2] Billen R., S. Zlatanova, P. Mathonet and F. Boniver, 2002, The Dimensional Model: a framework to distinguish spatial relationships, in: Advances in Spatial Data handling, D.Richardson, P.van Oosterom (Eds.), Springer, Ottawa, Canada, 9-12 July, pp [3] Coors, V., 2002, 3D GIS in Networking environments, CEUS (to be published), 17 p. [4] Molenaar, M., 1990, A formal data structure for 3D vector maps, in: Proceedings of EGIS 90, Vol. 2, Amsterdam, The Netherlands, pp [5] Open GIS Consortium, Inc., 1999, The OpenGIS abstract specification, topic 1: Feature geometry. Technical Report Version 4 ( doc), OGC, URL: [6] Oosterom. P.J.M. van, J.E. Stoter, S. Zlatanova, W.C. Quak, 2002, The balance between Geometry and Topology, Advances in Spatial Data handling, D.Richardson, P.van Oosterom (Eds.), Ottawa, Canada, 9-12 July [7] Pilouk, M., 1996, Integrated modelling for 3D GIS, PhD thesis, ITC, The Netherlands [8] Quak, W., P. Oosterom and T. Tijssen, 2002, Testing current DBMS products with realistic spatial data, in: Proceedings of the 5th AGILE conference on Geographic Information Science, ISBN X, April, Palma de Mallorca, Spain, pp [9] Stoter J.E., 2002, 3D Cadastres, state of the art: from 2D parcels to 3D registrations, GIM International, the world magazine for Geomatics, February, [10] Zlatanova, S., 2000, 3D GIS for Urban development, PhD thesis, ITC, The Netherlands, 222 p. [11] Zlatanova, S. and T. Bandrova, 1998, User requirements for the third dimensionality, seminar of Cartography 1998: Maps of the future, Vol. 1, Sofia, pp [12] Zlatanova, S., A.A. Rahman and M.Pilouk, 2002, 3D GIS: current status and perspectives, in Proceedings of the Joint Conference on Geo-spatial theory, Processing and Applications, 8-12 July, Ottawa, Canada, 6p. CDROM. [13] Zlatanova, S. A.A. Rahman, and W. Shi, Topology for 3D spatial objects, Journal of Geospatial Engineering (in press). Siyka Zlatanova is a researcher at the GISt section, Delft University of Technology, The Netherlands (since 2000). She has graduated as a surveyor at the University of Architecture, Civil Engineering and Geodesy (UACG), Sofia, Bulgaria in 1983 and has obtained her PhD degree on 3D GIS for urban modelling at the Graz University of Technology, Austria in She worked as a software programmer at the Central Cadastre in Sofia, Bulgaria ( ), an assistant-professor at UACG, Sofia, Bulgaria ( ) and a researcher at the Institute of International Institute for Geo-information Science and Earth Observations (ITC), The Netherlands ( ). Her interests are in 3D GIS: 3D reconstruction, 3D data structures, 3D spatial analysis and 3D visualisation.