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The advantages of three dimensional (3D) city models can be seen in various applications including photogrammetry, urban andregional planning, computer games, etc.. They expand the visualization and analysis capabilities of Geographic Information Systemson cities, and they can be developed using web standards. However, these 3D city models consume much more storage compared totwo dimensional (2D) spatial data. They involve extra geometrical and topological information together with semantic data. Without aproper spatial data clustering method and its corresponding spatial data access method, retrieving portions of and especially searchingthese 3D city models, will not be done optimally. Even though current developments are based on an open data model allotted by theOpen Geospatial Consortium (OGC) called CityGML, its XML-based structure makes it challenging to cluster the 3D urban objects.In this research, we propose an opponent data constellation technique of space-filling curves (3D Hilbert curves) for 3D city model datarepresentation. Unlike previous methods, that try to project 3D or n-dimensional data down to 2D or 3D using Principal ComponentAnalysis (PCA) or Hilbert mappings, in this research, we extend the Hilbert space-filling curve to one higher dimension for 3D citymodel data implementations. The query performance was tested using a CityGML dataset of 1,000 building blocks and the resultsare presented in this paper. The advantages of implementing space-filling curves in 3D city modeling will improve data retrievaltime by means of optimized 3D adjacency, nearest neighbor information and 3D indexing. The Hilbert mapping, which maps a subintervalof the [0, 1] interval to the corresponding portion of the d-dimensional Hilbert’s curve, preserves the Lebesgue measure andis Lipschitz continuous. Depending on the applications, several alternatives are possible in order to cluster spatial data together in thethird dimension compared to its clustering in 2D

IMPROVING 3D SPATIAL QUERIES SEARCH: NEWFANGLED TECHNIQUE OF SPACE FILLING CURVES IN 3D CITY MODELING