Isochrone Generator

What Is an Isochrone?

An isochrone is a map polygon showing all the places reachable from a starting point within a given travel time (for example, everywhere within a 15-minute drive). The word comes from Greek for "equal time," and an isochrone map draws successive polygons at different time thresholds — typically 5, 10, 15, 30 minutes — to produce a travel time map that visualizes accessibility. Isochrones are the answer to questions like "how far can a fire engine travel in four minutes?", "where can employees live to be within a 30-minute commute?", or "which neighborhoods are within a 10-minute walk of a transit stop?". Isochrone generator tools are central to urban planning, emergency response, retail site selection, healthcare access analysis, and real estate valuation.

The Isochrone Generator on gis.tools produces travel time polygons from any line network you provide. Drop in a road, trail, or transit layer, set the origin point, specify one or more time thresholds, pick a travel speed profile, and the tool computes the reachable polygons directly in your browser. Because gis.tools routing uses a network you supply, you can run drive time analysis on private or custom networks that public services like TravelTime or Mapbox Isochrone API don't cover.

Isochrones are sometimes called reach polygons, drive time zones, service areas, or accessibility polygons. The complementary concept is the isodistance (equal-distance reach), which uses distance instead of time as the threshold. Both share the same underlying graph traversal — only the edge weighting differs.

How Isochrone Generation Works

1. Build the network graph: the input line layer is converted to a graph with nodes at endpoints and intersections, edges along each segment.
2. Assign edge weights: each edge's weight is its traversal time, typically length divided by segment speed.
3. Run reach analysis from origin: a modified Dijkstra's algorithm flood-fills outward from the origin, computing the minimum travel time to every reachable node within the maximum time threshold.
4. Generate the isochrone polygon: all nodes within the time threshold form a point cloud; the polygon is computed as the concave hull (or alpha shape) of these points, sometimes buffered slightly to smooth the boundary.
5. Repeat for each time threshold: multiple thresholds produce nested polygons visualizing the gradient of accessibility.

Speed Profiles

Isochrones are highly sensitive to speed assumptions. A single "driving" speed of 50 km/h produces very different results from a detailed profile with 30 km/h on residential streets, 50 km/h on collectors, and 100 km/h on freeways. The gis.tools isochrone generator lets you specify speeds per edge via an attribute column or a global default.

Straight-Line vs Network Isochrones

A naive "isochrone" using straight-line distance from the origin is just a circular buffer and completely ignores barriers, one-way streets, and real network structure. Network-based isochrones are far more accurate and are the standard for serious accessibility analysis.

Key Parameters and Options

Origin Point

Set by clicking on the map or providing coordinates. Multiple origins can be batched.

Time Thresholds

Specify one or more thresholds (e.g., 5, 10, 15 minutes). Each produces a separate polygon.

Travel Mode

Choose the profile: driving, walking, cycling, or custom. The profile determines default speeds.

Speed Attribute

Optional per-edge speed column for fine-grained speed control.

One-way Handling

Respect edge direction for one-way streets.

Output Smoothing

How tightly the isochrone polygon follows the reached network nodes. Tighter polygons are more accurate but more irregular.

Practical Applications

Emergency Service Planning

Fire departments use 4-minute drive time polygons from every station to verify coverage against NFPA 1710 response time standards. Identifying coverage gaps justifies new station placements. EMS and police agencies use similar thresholds.

Retail Site Selection

Retailers use 10, 20, and 30-minute drive time isochrones around proposed sites to compute the population, income, and competitor density within their likely catchment.

Healthcare Access Analysis

Public health agencies compute 30-minute drive time polygons around hospitals and specialty clinics to measure the percentage of a population with timely access to care.

Transit Accessibility

Walking isochrones (5 and 10 minutes) from transit stops quantify the "walkshed" each stop serves, helping planners identify underserved neighborhoods.

Real Estate and Commute Analysis

Buyers filter homes by commute time to work; isochrones from a workplace define the set of neighborhoods that meet their tolerance.

Delivery and Logistics

Courier companies use isochrones to define same-day delivery zones around distribution centers.

Tourism and Recreation

Isochrones from hotels or trailheads show what is reachable during a half-day or full-day trip.

Evacuation Planning

Time-based reach polygons from evacuation centers estimate how long it takes populations to reach safety under different mode assumptions.

Step-by-Step Workflow in gis.tools

1. Open the Isochrone Generator
2. Drag in a network line layer (roads, trails, transit)
3. Clean topology with Topology Helpers if needed
4. Click to set the origin point
5. Enter time thresholds (e.g., 5, 10, 15, 30 minutes)
6. Pick a travel mode and speed profile
7. Click "Generate Isochrones"
8. Review the nested polygons on the map
9. Export as GeoJSON, KML, or Shapefile

Worked Example

A fire chief wants to evaluate coverage from a new station site. She loads the road centerline network (with speed limits in a SPEED_KPH column), clicks the candidate site as the origin, and sets thresholds at 2, 4, 6, and 8 minutes with a driving profile that uses the SPEED_KPH attribute. The tool returns four nested polygons showing the reach at each time threshold. She overlays the polygons on residential parcels and counts how many households fall within 4 minutes. Comparing to the NFPA 1710 response time standard, she concludes the new site would improve coverage for 2,100 households not currently within 4 minutes of any existing station.

Common Pitfalls and Gotchas

• Poor network topology: disconnected edges create artificial barriers. Fix with Topology Helpers before routing.
• Speed assumptions matter enormously: doubling the speed roughly doubles the reach area. Document your assumptions.
• Missing one-way attributes produce overly optimistic isochrones on roads that should only be traversable in one direction.
• Lack of turn restrictions produces reach areas that use turns that are physically impossible in the real network.
• Edge-of-network artifacts: the polygon may end abruptly where the network ends, not because of a true reach limit.
• Concave hull parameter sensitivity: tight hulls are more accurate but more jagged; loose hulls are smoother but overstate reach.
• CRS and distance units: length-to-time calculations require meters, not degrees.
• Multiple origins can't be computed in one pass; run the tool once per origin.

Tips for Best Results

• Use a detailed per-edge speed attribute for the most accurate isochrones
• Clean network topology before running with Topology Helpers
• Document your speed profile and time thresholds in the output metadata
• Compare network isochrones to simple buffers to show how much buffers overestimate reach
• Pair with Catchment Analysis for multi-origin coverage studies
• Reproject to a metric CRS before computing

Comparison with Other GIS Approaches

Dedicated isochrone services like TravelTime, Mapbox Isochrone API, HERE, and Google Distance Matrix provide highly accurate isochrones using comprehensive road network data and traffic models. ArcGIS Network Analyst offers Service Area analysis with full turn restrictions. QGIS has the TravelTime plugin and the Network Analysis tools. PostGIS + pgRouting supports isochrone generation via driving distance queries. The gis.tools browser version uses your provided network and doesn't require an API key, data limits, or signup — ideal for one-off analysis on proprietary networks or for pedagogical use.

Performance Considerations

Isochrone generation on a network of 10,000 edges takes a few seconds in the browser. Very large networks (millions of edges) are impractical for browser-based routing; use a server-side engine instead.

Data Privacy and Browser-Based Processing

Your network data and origin points stay in your browser. No third-party routing service is contacted.

Related GIS Concepts

Isodistance uses distance instead of time as the threshold. Service area is the ArcGIS term for the same concept. Reach polygon is a more generic label. Travelshed is commonly used in transportation planning. Walkshed and bikeshed are mode-specific variants. Catchment area is the business/planning term for the same thing. Gravity models extend isochrone analysis with distance-decay weighting.

Frequently Asked Questions

How do I create a drive time map?

Load a road network, click an origin point, set time thresholds, and click Generate.

Can I generate isochrones for walking or cycling?

Yes — pick the appropriate mode and the default speeds will be set accordingly.

Do I need to provide my own network?

Yes — this tool uses a network you supply, so you can route on custom or private data.

Why does my isochrone look lumpy?

The polygon is the concave hull of reached nodes. Lumpy outlines reflect uneven network density.

Can I run the tool for multiple origins at once?

Currently one origin at a time. For multi-origin catchment analysis see Catchment Analysis.

Related Tools on gis.tools

• Route Along Network
• Catchment Analysis
• Coverage Gap Finder
• GIS Buffer Tool
• Topology Helpers
• Origin-Destination Matrix
• Snap Points to Line
• Nearest Neighbor (kNN)