GIS Tools - Catchment Analysis

What Is Catchment Area Analysis?

Catchment area analysis — also called service area analysis or trade area analysis — is the GIS technique of delineating the geographic area from which a facility, service or business draws its users. The catchment of a hospital is the population area from which it pulls patients; the catchment of a coffee shop is the neighborhoods within walking range; the catchment of a bus stop is the houses within a five-minute walk; the catchment of a fire station is the streets within a three-minute drive. Defining and analysing catchments answers fundamental planning questions: Who is served? Where are the gaps? Where should the next facility go?

The Catchment Analysis tool on gis.tools loads a set of facility points and produces catchment polygons using either Euclidean buffers, geodesic buffers or network-based isochrones. People search for "catchment area analysis", "service area gis", "trade area analysis", "drive time catchment", "5 minute walk catchment" and "facility coverage analysis" because catchment analysis sits at the centre of retail site selection, healthcare planning, education planning, emergency response and public transit design.

A catchment is fundamentally a polygon that answers "from where can I reach this facility within a given cost?". The cost can be distance (km, miles), time (minutes, hours), modal (walking, cycling, driving) or composite (with terrain or congestion factors).

How Catchment Analysis Works

Three families of algorithms produce catchment polygons.

Euclidean Buffers

The simplest catchment is a circle around the facility — a Euclidean buffer at a fixed radius. Fast, easy and ignores barriers, roads and topography. Appropriate for early-stage screening, dense urban environments and when you have no network data.

Geodesic Buffers

A more accurate version of the Euclidean buffer that accounts for Earth's curvature. The catchment is a great-circle disk rather than a planar circle — important for catchments that span large latitudes or large distances. See the GIS Buffer Tool for the buffer engine.

Network Isochrones

An isochrone is a polygon connecting all locations reachable within a given travel time along a routable network. Isochrones respect roads, one-ways, turn restrictions, speed limits and topography in a way that buffers cannot. Generated by Dijkstra or A* shortest-path searches from the facility outward, then traced into a polygon. The Isochrone Generator implements this for user-provided networks.

Drive-Time, Walk-Time, Bike-Time

The same isochrone algorithm runs with different cost models: walk speed (about 5 km/h), bike speed (about 15-20 km/h), drive speed (variable by road class). Each produces a different catchment polygon for the same facility.

Multi-Facility Catchments

When several facilities are analysed together, individual catchments overlap. The tool can union them to produce a single coverage polygon, or compute Voronoi-style nearest-facility polygons that assign every location to its closest facility.

Key Parameters and Options

Catchment Method

Euclidean buffer, geodesic buffer or network isochrone. Choose based on data availability and required accuracy.

Cost Type and Threshold

Distance (km, miles) or time (minutes). The threshold defines the catchment edge: a 10-minute drive catchment, a 1-mile walk catchment, etc.

Travel Mode

Walking, cycling or driving — each affects speeds and which network edges are usable.

Network Source

For isochrones, the tool uses a routable network you provide as a GeoJSON LineString layer. See Route Along Network for the routing engine.

Aggregation Strategy

Union (merge all facility catchments), intersect (find common coverage), nearest-facility (Voronoi-style assignment).

Population Overlay

Optionally overlay a population raster or polygon layer to compute population covered by the catchment.

Practical Applications

Retail Site Selection

A coffee shop chain analyses 5-minute walk catchments around candidate locations to estimate the resident population and daytime workforce within easy reach. Sites with high population density inside the walk catchment make the shortlist.

Hospital and Clinic Planning

A health authority computes 30-minute drive-time catchments around every hospital in a region to identify "medical deserts" — populated areas with no nearby acute-care facility. The analysis informs the location of new clinics.

School District Boundaries

A school district draws 1-mile walking catchments around each elementary school and 3-mile driving catchments around each high school to verify that every household can reach an appropriate school within policy thresholds.

Bus Stop and Transit Catchments

Transit planners compute 400 m walk catchments around every bus stop to identify which residential blocks are within a quarter-mile walk of transit. Aggregated to the city scale, this becomes the "transit-shed" coverage metric.

Fire Station Response

Fire departments compute 4-minute and 8-minute drive-time catchments around every station to verify compliance with NFPA 1710 response standards and to plan station relocations.

EV Charging Network

A charging-station operator analyses 5-mile catchments around every station to map coverage gaps along major highways and prioritise new station deployments.

Emergency Shelter Planning

Disaster managers compute walking catchments around every emergency shelter to ensure that vulnerable populations can reach safety on foot during a flood, fire or chemical incident.

Postal and Parcel Delivery Zones

Logistics planners compute drive-time catchments around distribution hubs to define delivery zones and balance courier workloads.

Step-by-Step Workflow in gis.tools

Open the Catchment Analysis page on gis.tools.
Drag a point layer (GeoJSON, Shapefile, CSV) of facility locations into the page.
Choose the catchment method: Euclidean buffer, geodesic buffer or network isochrone.
For buffers, set the distance threshold; for isochrones, set the time threshold and travel mode.
For network mode, drag in a routable network as a LineString layer.
Choose aggregation: union, intersect or nearest-facility (Voronoi).
Click Compute. The tool generates one polygon per facility (and optionally a unioned coverage layer).
Preview on the map. Style by facility ID or by aggregate coverage area.
Optionally overlay a population layer and use Zonal Statistics to compute population reached.
Export the catchment polygons as GeoJSON, KML or Shapefile.

Worked Example: 5-Minute Walking Catchment Around 12 Bus Stops

A small transit agency wants to know how many people live within a 5-minute walk of its 12 high-frequency bus stops. You drop the stop locations into Catchment Analysis and choose Euclidean buffer mode (no network data available). At a typical 5 km/h walking speed, 5 minutes is about 400 m, so you set a 400 m radius. The tool generates 12 circular catchments. You union them and compute the total area covered. Then you load a 100 m WorldPop population raster and run Zonal Statistics to sum population inside the unioned catchment polygon. The result: 18000 people live within a 5-minute walk of the high-frequency network — about 22% of the city. Repeating the analysis for the proposed expanded network shows coverage increases to 31%, justifying the expansion to council.

Common Pitfalls and Gotchas

Euclidean buffers ignore barriers, rivers, freeways and topography — they overstate true accessibility in obstructed terrain.
Walking speeds vary by population (children, elderly, disabled). Use slower speeds for inclusive analysis.
Network isochrones depend on the quality of the network data; missing roads produce missing catchments.
Drive-time catchments computed without congestion factors overstate off-peak access.
Catchment polygons computed in a geographic CRS at fixed degree radii are distorted at high latitudes — buffer in a metric CRS first.
Overlapping catchments double-count population if you sum populations across all polygons; union first.
Catchment thresholds are policy choices, not facts — a 30-minute drive catchment is not the same as a 60-minute one, and the choice changes the conclusion.
Snapping facilities to the wrong network point produces nonsense isochrones; always snap to the nearest road segment first.

Tips for Best Results

Use network isochrones whenever you have a routable network — they are dramatically more accurate than buffers in any non-trivial terrain.
For walking catchments, use slower speeds (3.5-4 km/h) for accessibility-sensitive analyses.
Snap facilities to the network with Snap Points to Line before running isochrones.
Combine catchment polygons with a population raster and Zonal Statistics for accessibility metrics.
Compute multiple thresholds (5, 10, 15 minutes) to produce nested catchment rings.
Use nearest-facility (Voronoi) assignment when allocating users to facilities for capacity planning.
Document the cost model assumptions (mode, speed, threshold) explicitly in the output.
Validate against known travel times where possible.

Comparison with Other GIS Approaches

ArcGIS Network Analyst's Service Area solver is the gold standard for network catchments. QGIS provides a similar service area function via the QGIS Network Analyst plugin or the GRASS v.net.iso module. Specialised SaaS tools like TravelTime API and Mapbox Isochrone API offer high-quality isochrones via REST. PostGIS plus pgRouting handles catchment computation in databases. The Catchment Analysis tool on gis.tools delivers buffer-based and network-isochrone catchments in a browser without an account or install. The trade-off is that the tool depends on the network data you provide; it is not pre-loaded with a global routable network.

Performance Considerations

Buffer-based catchments scale O(n) in facility count and are essentially instant. Network isochrone computation is O(n * m) where m is the number of network nodes within the threshold from each facility — slower, especially for large networks. For dozens of facilities on city-scale networks the computation runs in seconds. Very large networks (national-scale) should be precomputed offline.

Data Privacy and Browser-Based Processing

Facility locations and network data stay in the browser. Catchment polygons are computed locally and exported as download blobs. No upload, no telemetry, no logging. This is essential for sensitive facility locations (women's shelters, addiction services, classified installations) and any compliance-bound workflow.

Related GIS Concepts

Service Area: ArcGIS terminology for the same concept.

Trade Area: Retail-industry term for catchment areas defined by customer source.

Isochrone: A polygon of equal travel time. The most accurate catchment representation when network data is available.

Voronoi Assignment: A nearest-facility partition where every location is assigned to its closest facility. See Voronoi Polygons Generator.

Two-Step Floating Catchment Area (2SFCA): An accessibility metric that accounts for both the supply at facilities and the demand within their catchments.

Frequently Asked Questions

What's the difference between a catchment and a service area?

They are synonyms. Service area is the ArcGIS term; catchment area is more common in retail and healthcare planning.

Should I use buffers or isochrones?

Isochrones are more accurate when you have a network. Buffers are faster and acceptable for early screening or dense uniform urban areas.

Can I compute walking, cycling and driving catchments?

Yes — choose the travel mode and the tool uses appropriate speeds for each.

How do I overlay population on a catchment?

Use Zonal Statistics with the catchment polygon and a population raster.

Can I produce nested catchment rings?

Yes — run the analysis at multiple thresholds (5, 10, 15 minutes) and stack the polygons.

Does the tool include a global road network?

No — provide your own network as a LineString GeoJSON. OpenStreetMap exports work well.

About Catchment Analysis