A Digital Image of the City: 3D Isovists in Lynch's Urban Analysis

New techniques to measure 2D and 3D visibility over urban spaces are presented in this paper. The concept of the isovist, that is, the visible space from a vantage point, could help in providing a quantifiable basis for Lynch's urban analysis, as outlined in his book Lynch, 1960 The Image of the City (MIT Press, Cambridge, MA). We also expand on the concept of 3D isovists and develop a new technique to calculate them on digital elevation models, whereby the same input information can be processed easily in pixel coordinates and in voxel space, that is, a 3D array of urban-visibility measures. The investigation of what we have called the ‘isovisimatrix’ seems to allow a very useful interpretation of visibility from a visual-perception viewpoint. The approach presented here suggests helpful applications for urban design, in particular for predicting the visual impact of new buildings on the physical context and understanding how open spaces may be used by people depending on visibility features.