A catchment area is delimited by distance, by travel time, or by a probabilistic model. Travel time is the most faithful measure: it accounts for the real network, for obstacles and for the asymmetry of a location — everything a circle drawn on a map ignores.
The split into three rings reflects how purchase frequency decreases with distance. The primary zone supplies most of the turnover, the secondary a regular supplement, the tertiary an occasional contribution.
A catchment area limited to residents systematically underestimates town-centre locations: inbound commuters, station travellers and, in tourist municipalities, overnight stays all belong in it.
Related notions
Where this notion is used
The figures, canton by canton
Population, income, benchmark commercial rent, taxation and flows — official data, refreshed at every import.
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