Gamma map is defined as the graph representing the interior layout of a building as well as its external space in terms of permeability. Every interior space or subdivision of a space can be conceptualised as a point and represented as a circle, with its relations of permeability represented by lines linking it to others; the space outside is considered as a point, and represented as a circle with a cross.
Hillier, B. & Hanson, J. (1984), The Social Logic of Space, Cambridge University Press: Cambridge. pp.147-9