A Digital Image of the City: 3-D isovists in Lynch’s Urban Analysis

New techniques to measure 2-D and 3-D visibility over urban spaces are presented in this paper. The concept of ‘isovist’, i.e. the visible space from a vantage point, could help in providing a quantifiable basis for Kevin Lynch's urban analysis, as outlined in his book The Image of the City. We also expand on the concept of 3-D isovists and develop a new technique to calculate them on Digital Elevation Models (DEMs), whereby the same input information can easily be processed in pixel coordinates and in voxel-space, i.e. a three-dimensional array of urban visibility measures. The investigation of what we have called ‘iso-visi-matrix’ 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.