Access Catchment Map¶
Artifact — instantiates Position-Based Leverage Design
Maps who or what can be reached from each candidate location within meaningful cost, time, permission, or distance bands.
An Access Catchment Map answers one question for every place you might stand: how much of what you care about lies within reach of it? Around each candidate location it draws the set of people, demand, assets, or opportunities that can actually be served from there inside a meaningful band — a fifteen-minute drive, a two-hop referral, a permitted jurisdiction, a shipping-cost ceiling. Its defining move is to grade the whole field by outward reach from a position, converting a vague sense that a spot "looks central" into a countable catchment. It is a coverage artifact, not a ranking or a routing plan: it says what each location can touch, and deliberately leaves the question of which touch is worth the most, and how to hold it, to other mechanisms.
Example¶
A county health department has one-time capital funding for three new community clinics and a shortlist of twelve possible sites. Rather than argue site-by-site, the planners build an Access Catchment Map. For each candidate they draw a 20-minute drive-time isochrone, then a 45-minute transit isochrone for households without a car, and overlay uninsured-population density, existing-clinic coverage, and language needs. The map immediately reorders the debate. Two of the "obvious" sites near the downtown hospital turn out to have almost fully overlapping catchments — the second one adds very little net new population it can reach. A less glamorous site on the county's eastern edge, by contrast, covers a band of several thousand residents who currently sit outside any clinic's 20-minute reach at all. The department ends up choosing the two sites that maximize net new reachable population under transit constraints rather than the three that looked most prominent on a wall map, and it can show the county board exactly whom each site does and does not serve.
How it works¶
- Fix the band, not just the point. Reach is meaningless without a threshold: cost ceiling, travel time, hop count, or permission scope. The band is chosen to match how the served party actually decides to engage.
- Compute reach from each candidate. For every position in the set, enumerate what falls inside its band — usually with a distance-decay weighting so a target at the edge of the band counts for less than one at the center.
- Lay catchments over each other. The payoff of the artifact is in the overlaps and gaps: where two positions serve the same catchment (redundancy), and where no position reaches (a coverage hole).
- Distinguish inbound from outbound reach. "Whom can I serve" and "who can reach me" are different maps when travel is asymmetric (one-way roads, referral-only access); the artifact keeps them separate when it matters.
Tuning parameters¶
- Band definition — the reach threshold (minutes, dollars, hops, permission tier). Widening it makes every position look better and washes out the differences that matter; too narrow and the map is all gaps.
- Distance-decay function — how sharply value falls with distance inside the band. A steep decay rewards proximity; a flat one treats the whole band as equally reachable and can overstate edge coverage.
- Weighting of what is reached — raw counts versus need-weighted or value-weighted targets. Weighting surfaces underserved demand but embeds a contestable judgment about whose access counts more.
- Spatial resolution — the grain of the target grid. Fine grids find real gaps; coarse ones invite the boundary artifacts below.
- Directionality — one-way (you→them) or two-way reach; picking the wrong one flatters positions that are easy to leave but hard to arrive at.
When it helps, and when it misleads¶
Its strength is that it makes redundancy and blind spots visible: it separates positions that look central from positions that actually enlarge coverage, and it names exactly who lives in the gap. That is precisely the input a siting or outreach decision needs and rarely computes.
Its central failure mode is mistaking reach for value — a position with an enormous catchment of the wrong targets is worse than a small catchment of the right ones, and the map's tidy polygons invite that confusion. It is also sensitive to how the bands are drawn: shifting the travel-time cutoff or the zone boundaries can move which site "wins," a version of the modifiable areal unit problem well known in the two-step floating catchment area literature.[n1] And it is a snapshot — a road closure, a new competitor, or a demand shift can silently invalidate it. The guarding discipline is to run the map at two or three band settings and keep it only if the ranking is stable, to weight targets by need rather than count, and to date the artifact and refresh it rather than trust last year's isochrones.
How it implements the components¶
Access Catchment Map fills the reach-and-coverage side of the archetype's machinery — the components that establish what a position can touch, not what it is worth holding:
value_graded_field_map— the catchment overlay is the value-graded field, grading each part of the terrain by how reachable it is and from where.position_candidate_set— the map is built over an explicit list of candidate locations, giving the archetype the set of positions to compare.adjacency_and_reach_model— its core computation: what lies within cost/time/permission/distance of each position, with decay and directionality.
It does not chart where you sit among complements, substitutes, and governance gates (platform_or_ecosystem_position_map) — that ecosystem view is Platform Positioning Map, its nearest twin, which answers "what is my relational standing" rather than "how much can I reach." Nor does it score which reachable positions are defensible or worth the holding cost (defensibility_and_holding_cost_assessment, advantage_vector_profile); those belong to Terrain or Topology Position Review and Market Entry Positioning Matrix.
Related¶
- Instantiates: Position-Based Leverage Design — supplies the reach/coverage input the rest of the appraisal builds on.
- Sibling mechanisms: Platform Positioning Map · Chokepoint or Gateway Analysis · Interior-Lines Route Model · Market Entry Positioning Matrix · Network Centrality Analysis · Overton-Window Position Scan · Prepositioning and Staging Plan · Ranking or Shelf-Placement Audit · Terrain or Topology Position Review
Editorial Notes¶
Form Classification¶
Form family: Representation, Specification & Plan
Rationale: The mechanism maps who or what can be reached from each candidate location within meaningful cost, time, permission, or distance bands, so its operative form is a static or prospective information artifact.
Independent corroboration: The frozen evidence defines Access Catchment Map as 'Maps who or what can be reached from each candidate location within meaningful cost, time, permission, or distance bands', so its operative form is Representation, Specification & Plan.
Review outcome: Independent reviewer agreement; high confidence.
Origin Attribution¶
Primary origin: Architecture & Urban Planning
Origin pattern: Convergent development
Present-day reach: Multi-domain
Rationale: Isochrones, service catchments, site comparison, overlap, and gap analysis are established spatial-planning and facility-location practices.
Related originating lineages:
- Medicine & Healthcare — Health-services geography developed influential floating-catchment methods for relating population demand to reachable clinical supply.
- Operations Research — Facility-location and coverage models formalize the selection of sites that maximize net reachable demand under constraints.
- Public Administration & Policy — Public-service siting and equity-weighted reach translate the spatial model into policy allocation decisions.
Review resolution: Spatial and urban planning provide catchments and isochrones; facility-location analysis, health-access geography, and public-service planning independently formalized consequential variants.
Attribution caveat: The named 2SFCA method sits between health geography and planning; the taxonomy has no separate geography domain, making urban planning the closest primary home.
Review outcome: Reconciled after independent review; high confidence.
Notes¶
[n1] The two-step floating catchment area (2SFCA) method, developed in health-services geography by Radke & Mu and popularized by Luo & Wang, computes accessibility by pairing supply and demand within a travel-time threshold. It is the canonical formalization of a catchment map — and the literature around it is where the boundary-and-cutoff sensitivity ("modifiable areal unit problem") is most carefully documented. ↩