How IRIS draws a catchment

Before you can forecast what a site will earn, you have to decide whose money is on the table. That is the catchment: the pool of people who could realistically reach the door. Most tools draw it as a circle on a map, a fixed radius around the pin. It is quick, and it is wrong, because no one travels in circles.

A radius ignores how people actually move

A one-kilometre radius treats a house across a canal the same as a house on the same street. In reality, one is a two-minute walk and the other is a ten-minute detour to the nearest bridge. A postcode is no better: it is an administrative boundary drawn for the post office, not a description of how a neighbourhood feeds a storefront. Both answer a question nobody asked.

We build the catchment from travel time instead. Starting at the door, IRIS traces isochrones, the lines you can reach within a given number of minutes, separately on foot, by bike, and by car, because the mix that matters depends on the location. A car isochrone follows the real road network, respects one-way streets, and bends around the things that slow a journey. The result is a shape, not a circle, and the shape usually tells you something the circle hid.

Nearer counts for more

Not everyone inside the catchment counts equally. Someone whose office is a two-minute walk away will pass the door often; someone ten minutes out will not. So we apply distance decay: demand is weighted down the further it sits from the site, in line with how willingly people actually travel for what you sell. The two-minute office and the ten-minute office are both "inside the catchment", but the first carries far more weight than the second. A raw headcount would flatten that difference and mislead you.

Barriers cut the catchment

Cities are full of edges that a circle sails straight through. A canal with a bridge every few hundred metres, a rail line, a motorway, a river: each one severs the walking and cycling routes on the far side, even when the straight-line distance looks trivial. IRIS honours these barriers, so the catchment stops where people's journeys actually stop. This is often where a naive radius overstates demand the most.

The Singel, worked through (an illustrative example)

Take the empty unit on the Singel in Amsterdam Centrum. Its primary catchment, weighted by travel time and cut by the surrounding canals and streets, holds 71,237 people (an illustrative sample figure), reachable within roughly a 12-minute drive. That is not everyone who lives in the postcode, and not everyone inside a tidy circle. It is the demand that can genuinely get there.

The catchment is the foundation the rest of the forecast rests on. Draw it as a circle and every number after it inherits the error. Draw it by how people move, and you have an honest pool of demand that can realistically reach the door. The next step is reading who those people are, and why they are near the site at all.