Trade-area methodology

@@ -11,7 +11,7 @@ status: active
audience: customer-woodfine
bcsc_class: current-fact
language_protocol: PROSE-TOPIC
last_edited: 2026-08-25
last_edited: 2026-08-26
editor: pointsav-engineering
short_description: "Honest labelling of demand geography — why straight-line distance bands are never called catchments, and the planned move to isochrones and observed origins."
paired_with: site-selection/trade-area-methodology.es.md
@@ -24,7 +24,7 @@ The trade-area methodology governs how the Woodfine location intelligence map (g

The live map renders a distance band around each cluster centroid. **Until the observed-trade-area pipeline is integrated, that band is a straight-line radius, not a measured catchment.** The labelling rule is unambiguous: any geometry derived from a straight-line formula reads as "distance band (straight-line)" on the map face and in the detail panel — never "catchment" and never "trade area."

The Method modal carries a plain statement: *"Distance bands are straight-line radii around the cluster centroid; they approximate, but do not measure, where customers come from."*
The map states plainly: *"Distance bands are straight-line radii around the cluster centroid; they approximate, but do not measure, where customers come from."*

This is a deliberate honesty edit. A clearly-labelled approximation is defensible in front of a reviewer; a circle mislabelled as a measured catchment is not.

@@ -38,7 +38,7 @@ A trade area is the set of places customers actually travel from. Two methods 

An isochrone replaces the straight-line radius with the area reachable within a stated drive time (for example 10, 20, or 30 minutes) along the road network. Isochrones respect barriers that circles ignore — rivers, motorway access, mountain passes, one-way coastal routes — so two clusters with identical straight-line radii can have substantially different reachable areas. Drive-time isochrones are the retail-geography standard that site-selection reviewers expect.

The intended implementation uses a self-hosted routing engine over the OSM extracts already ingested in the archive. A dependency on a metered third-party isochrone API is not the intended path for a product built on sovereign, self-contained data infrastructure. The specific routing engine is an open question in the engineering backlog.
The intended implementation uses a self-hosted routing engine over the OSM extracts already ingested in the archive. A dependency on a metered third-party isochrone API is not the intended path for a product built on sovereign, self-contained data infrastructure.

### Observed origin-destination polygons

@@ -46,13 +46,13 @@ Rather than modelling where customers could come from, an O-D polygon draws wher

- **United States — US LEHD LODES.** Worker origin-destination flows aggregated to H3 cells, 49 states, approximately 684,000 H3 cells. Supports a workplace-origin trade area for US clusters.
- **Spain — MITMA.** The Spanish Ministry of Transport mobility matrices are ingested for 58 ES clusters, providing an observed origin distribution rather than a modelled ring.
- **`layer6-mobility-work.pmtiles`.** The combined US LODES and MITMA work-origin surface is built and served as a map tile layer. The data needed to render observed origin polygons for US and Spain clusters is present; the integration that turns a cluster click into an observed-origin polygon, rather than a ring, is planned but not yet wired.
- **Combined mobility surface.** The combined US LODES and MITMA work-origin data is built and served as a map layer today. The data needed to render observed origin polygons for US and Spain clusters is present; turning a cluster click into an observed-origin polygon, rather than a ring, is planned but not yet available.

The intended O-D trade area for a cluster is the set of origin cells contributing the top share — for example 70–80% — of observed trips or workers to that cluster's destination cells. Coverage is uneven (US and Spain today; UK, France, and Germany researched as viable next sources), so the planned rollout is country-by-country. Clusters without O-D coverage keep the clearly-labelled distance band as an explicit interim.

### Why both methods are complementary

Drive-time isochrones answer the question "who can reach this site." Observed O-D polygons answer "who actually shops or works here." The intended map exposes the best available representation per cluster, with the Method modal stating which model produced the polygon on screen and over what vintage. A measured polygon and a drawn circle are never combined under a single label.
Drive-time isochrones answer the question "who can reach this site." Observed O-D polygons answer "who actually shops or works here." The intended map exposes the best available representation per cluster, stating on screen which model produced the polygon and over what vintage. A measured polygon and a drawn circle are never combined under a single label.

## The previous radius formula

@@ -60,7 +60,7 @@ The distance band used in the initial product version was computed from the clus

This is a geometric artifact, not a demand quantity. It describes how spread out the stores are; it carries no information about how far customers actually travel. Two failure modes follow directly: a dense urban cluster gets a small ring because its stores sit close together, even though its trade area may be large; a sprawling exurban cluster gets a large ring because its stores sit far apart, not because it draws from far away.

Neither the inflation factor nor the floor has a published derivation. They are tuning constants that make the picture look reasonable. Until observed O-D or drive-time boundaries are adopted, any interim distance band discloses that it rests on this unpublished, geometry-only formula in the Method modal rather than applying it silently.
Neither the inflation factor nor the floor has a published derivation. They are tuning constants that make the picture look reasonable. Until observed O-D or drive-time boundaries are adopted, any interim distance band discloses that it rests on this unpublished, geometry-only formula rather than applying it silently.

The intended end state removes this span-based formula from the live pipeline entirely, replacing it with a boundary whose parameter is a quantity a domain expert can evaluate on its merits — a stated drive-time, a stated percentile of modelled demand, or a stated population threshold.

@@ -71,10 +71,10 @@ The map renders in Web Mercator (EPSG:3857). Web Mercator preserves shape locall
The methodology therefore computes distances and areas geodesically:

- Distance tests use the haversine (great-circle) distance between geographic coordinates. The method is consistent across the platform's full co-location footprint (24 countries as of the most recent full processing run, 2026-08-06) and both the North American and European frames.
- Polygon construction uses metric geodesic buffers — turf's metric buffer builds geometry in true ground distance and projects to Web Mercator only for display, so the rendered polygon is the correct ground shape even though it appears as a latitude-stretched oval on screen.
- Polygon construction uses metric geodesic buffers, built in true ground distance and projected to Web Mercator only for display, so the rendered polygon is the correct ground shape even though it appears as a latitude-stretched oval on screen.
- Area figures — catchment area, density per square kilometre — are computed on the geodesic polygon, not from screen pixels.

The Method modal carries the caveat: *"Distances and areas are computed on the ellipsoid; the map is drawn in Web Mercator, which stretches with latitude — a band at 60°N covers less ground than the same band drawn at 25°N."*
The map states the caveat: *"Distances and areas are computed on the ellipsoid; the map is drawn in Web Mercator, which stretches with latitude — a band at 60°N covers less ground than the same band drawn at 25°N."*

## Spatial framework

@@ -86,7 +86,7 @@ A single H3 cell may fall inside the trade areas of several clusters. This is in

The intended migration from crow-flies rings is phased:

1. **Honesty edit.** Relabel every straight-line ring "distance band (straight-line)"; surface the radius formula's reliance on an undocumented tuning factor and floor, and the Web Mercator caveat, in the Method modal.
1. **Honesty edit.** Relabel every straight-line ring "distance band (straight-line)"; surface the radius formula's reliance on an undocumented tuning factor and floor, and the Web Mercator caveat, on the map.
2. **Observed O-D where data exists.** Wire the ingested LODES and MITMA mobility data into the cluster-click so US and Spain clusters show an observed work-origin polygon; clusters without coverage retain the labelled distance band.
3. **Drive-time isochrones.** Stand up a self-hosted routing engine and offer a fixed-drive-time band as the default reachable-area view, retiring the span-based interim formula from the live pipeline.
4. **Distance-decay calibration.** Fit decay curves to the O-D flows to set published percentile-of-demand boundaries, extending defensible polygons to countries with viable O-D sources.
@@ -102,5 +102,5 @@ Trade-area membership is the basis for population aggregation (WorldPop 2026), s
- [[co-location-tiering-scoring]] — how clusters are assigned composition tiers and the planned strength score
- [[spend-population-provenance]] — the estimation chain for population and spend within the trade area
- the settlement-level rollup built over co-location clusters
- the orchestration layer that runs the spatial analysis pipeline
- the process that runs the spatial analysis
- upstream retail clustering feeding the co-location index