INTRODUCTIONGeo-DBMSs make it possible to manage large spatial datasets in databases that can be accessed by multiple users at the same time. These spatial datasets usually contain 2D data, while more and more applications depend on 3D data. Some examples are 3D cadastres [1], telecommunications [2] and town planning [3]. These applications mainly come from the ever-growing tendency of using living space multifunctional by building in the vertical direction, e.g. apartments, buildings over spanning a road, tunnels and bridges [1]. The present Geo-DBMSs do not support 3D primitives, but 3D spatial objects can be modelled by using 2D primitives such as polygons. This is possible by using 3D coordinates, which are supported by the Geo-DBMSs. In this way, several 2D polygons bound a 3D object. These 2D polygons can be stored in one record (multi-polygon) or multiple records.The absence of a real 3D primitive in the Geo-DBMSs however, results into two problems:• The Geo-DBMSs do not recognize 3D spatial objects, because they do not have a 3D primitive to model the 3D object. This results into DBMS functions not working properly (e.g. there is no validation for the 3D object as a whole and functions only work with the projection of these objects, because the third dimension is ignored [4]).• In the case 2D objects, that bound a 3D object, are stored in multiple records, a 1:n relationship exists between the object and the number of records; a better administration of these large datasets requires a 1:1 relationship between objects in reality and objects in the database.Geo-DBMSs were developed to store spatial data, because they could guarantee the safety of the data (in 2D). But with the arrival of applications depending upon correct 3D data, new techniques need to be developed to support 3D data as well. The solution for this problem is to implement a real 3D primitive, including validation functions and functions that e.g. return the volume or the distance in 3D between objects. This improves the maintainability of 3D geo-datasets [5] and opens the door to more realistic applications [2], [3]. This paper will show how 3D spatial objects can be modelled, i.e. stored, validated and queried, in a Geo-DBMS using a 3D primitive and how these objects can be visualised. The innovation of this research is that the developed concepts have been translated into prototype implementations of a true 3D primitive in a DBMS environment (Oracle Spatial 9i Spatial [11]). As far as we know, this is the first time ever that a Geo-DBMS directly supports
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