Previously, GIS-based visibility analysis has been conducted mainly in two dimensions, based on the concept of an isovist in the built environment or the concept of a viewshed in terrain and landscape analysis. The Viewsphere, a GIS approach towards 3D visibility analysis is proposed for measuring visible urban space quantitatively in a way that is different from its predecessors, the isovist and the viewshed. A test case of Singapore's urban space was conducted by evaluating the visibility of three alternative urban design scenarios and their potential impacts on the visual quality of open space. Both directional and nondirectional approaches were applied to the mapping of visibility based on the 2D and 3D indices. The proposition that 3D visibility indices are more effective than 2D indices was verified. The findings show that the 3D indices are sensitive to the changes of z-dimension.

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