Catchment (trade area)

What a catchment is

In retail geography a catchment, or trade area, is the part of the map where a site's actual or potential customers come from. Thomas Wieland, writing in The R Journal, describes a market area as the spatial equivalent of the sum of a location's customers or sales, and as the result of where consumers choose to shop. A catchment is used to estimate the residents, households and spending a site can draw on. Moreton Bay Regional Council's planning scheme policy on economic impact assessment, for example, asks applicants for a retail development to identify its trade area and report the area's existing population and households, its socio-economic profile benchmarked against the region and Queensland, ten-year projections of population and households, and the retail spending it generates.

In Australian schooling the word has a second meaning. Government schools have catchments or intake zones, which the NSW Department of Education publishes as the local enrolment area each residential address belongs to.

Ways to draw a catchment

• Rings. A circle of set radius around the site. Rings are quick to draw and easy to compare between sites, and they take no account of roads, rivers, rail lines or other barriers.
• Drive-time and walk-time areas (isochrones). The area reachable within a set number of minutes along the road or path network. A 2026 study of walking catchments around rail stations in Malaysia's Klang Valley compared 5, 10 and 15-minute isochrones with 400 m, 800 m and 1,500 m circles, and found the network catchments smaller and less regular, with the circles generally overstating the area people could reach.
• Drawn boundaries. A polygon drawn to follow barriers and the reach of competing centres. Moreton Bay's policy asks for the trade area to be set having regard to the existing network of retail centres, the road, public and active transport network, and any physical or psychological barriers that influence shoppers.
• Customer-derived areas. Areas built from where existing customers live, using loyalty data, sales records or point-of-sale surveys (known as customer spotting).
• Modelled areas. Market area models such as the Huff model treat each consumer's choice of store as a probability, and a store's market area is the sum of the expected flows of customers or spending from each surrounding area.

Wieland divides these into empirical approaches, built from observed customers, and theoretical approaches, built from models, and notes that practical work combines both by fitting models to observed data.

Census figures for a catchment

The ABS publishes Census figures for its own areas, so figures for a catchment are assembled from the small areas inside it, such as SA1s or mesh blocks. Where the boundary cuts through one of those areas, part of its count has to be allocated to the catchment, which is what area weighting does. Counts can be added across the pieces, while percentages and medians have to be calculated again from the combined counts. The ABS doesn't publish SEIFA scores for areas users define, and its guidance states that converting data to areas outside the ASGS, such as school catchments, needs a custom correspondence.

Census counts place people where they usually live. A site that draws workers, students or visitors, such as one in a CBD or beside a hospital or university, also has customers that a resident count of its catchment leaves out.

Catchments and Censible

Censible describes a catchment as a circle around a point (1, 3, 5 or 10 km in the app), a drive of 5, 10, 15, 20 or 30 minutes, a walk of 5, 10, 15 or 20 minutes, or a shape drawn on the map. Drive and walk times follow OpenStreetMap's roads and paths and assume no traffic, so an 8 am drive covers less ground than the outline shows.

Each catchment is built from 2026 SA1s weighted by where their residents live, as described under area weighting, with counts added up before percentages and medians are calculated. Sites gives every location in a set the same catchment, so their figures can be compared.

Sources: Wieland, T. (2017), Market Area Analysis for Retail and Service Locations with MCI, The R Journal 9(1); Moreton Bay Regional Council, Planning scheme policy: Economic impact assessment; Abdul Halim, Teh and Rosni (2026), GIS-Based Walking Map Isochrones in Measuring Transit Catchment, ISPRS Archives.