An isoline map turns a continuous surface (elevation, pressure, temperature, travel time) into a set of lines, each one connecting points where the underlying variable is the same. It is one of the oldest tricks in cartography and still the cleanest way to read a smoothly varying quantity at a glance.
This guide explains what an isoline map is, the most common types, how the lines are computed, and how MapAtlas serves the most commercially important type: travel-time isochrones.
The One-Sentence Definition
An isoline map is a thematic map that uses lines, called isolines, to connect points of equal value of a continuous variable across geographic space.
The word isoline comes from the Greek isos (equal) and Latin linea (line). The same idea has many names depending on what is being measured.
The Most Common Types of Isolines
- Contour lines connect points at the same elevation above sea level. The basis of every topographic map.
- Isobars connect points of equal atmospheric pressure. The lines you see swirling on weather charts.
- Isotherms connect points of equal temperature. Used for climate and weather visualisation.
- Isohyets connect points that received equal rainfall over a period.
- Isochrones connect points reachable within the same travel time from a chosen origin. The most important type for modern logistics, retail, and real estate.
- Isodapanes connect points of equal total transport cost in classical location economics.
All of them share the same logical structure: pick a variable that varies continuously across space, choose a set of threshold values, and draw the curve where the variable crosses each threshold.
How Isolines Are Computed
For a gridded variable (elevation from a digital elevation model, pressure from a numerical weather model) the standard algorithm is marching squares. The grid is walked one cell at a time. For each cell, the algorithm checks which of its four corners are above and below each chosen threshold, and draws a line segment crossing the cell accordingly. The segments link up into closed or open isolines.
For travel-time isochrones the computation is different. The input is a road network, not a regular grid. An isochrone algorithm starts at an origin, runs a shortest-path expansion (Dijkstra or contraction hierarchies) across the road graph, and marks every reachable node with its arrival time. At the end, the algorithm walks the boundary of all nodes reachable within each time threshold (5 min, 10 min, 15 min, etc.) and stitches them into a polygon.
The output is usually a GeoJSON MultiPolygon per time band, ready to drop onto a map as a coloured layer.
Isoline vs Isochrone
A common point of confusion. Every isochrone is an isoline. Most isolines are not isochrones.
Use isoline when you are talking about generic level curves: contours, isobars, isotherms. Use isochrone when you specifically mean travel-time curves.
The two are computed differently (marching squares for general isolines on grids, network expansion for isochrones on road graphs). They serve different products. The MapAtlas Isochrone API does the second.
What Travel-Time Isochrones Replace
Before isochrone APIs were affordable, location analysis used simple distance buffers ("everything within 5 km of the store"). Buffers are wrong for almost every real question:
- A 5 km circle around a store in central Amsterdam covers tens of thousands of households. The same 5 km circle in rural Sweden might cover a few hundred.
- A 5 km circle does not know about rivers, motorways, one-way streets, or the actual road network.
- A 5 km circle gives the same number for car, bike, and walking analysis, which is wrong for any real use case.
A 15-minute drive-time isochrone solves all three. It respects the road network. It accounts for one-way streets and turn restrictions. It comes back different per travel mode. It scales naturally for retail catchment, delivery feasibility, real-estate searches, and emergency response coverage.
Reading a Contour Map
Even with travel-time isochrones dominating commercial use, the classic contour map is still the right tool for terrain. A few reading conventions to keep in mind:
- Closely spaced contours mean a steep slope. Widely spaced contours mean a gentle slope.
- Closed loops mean a peak or a pit. Hatched (tickmarked) contours indicate the pit case (a depression).
- The contour interval (vertical distance between adjacent lines) is fixed across the map. A finer interval shows more detail but clutters the rendering.
- Index contours are heavier lines drawn every 5th or 10th line, labelled with their elevation for orientation.
Where Isoline Maps Are Used
- Logistics: delivery zones, last-mile feasibility, depot placement.
- Retail and real estate: store and amenity catchments, "homes within X minutes of a school".
- Emergency services: response coverage modelling, identifying underserved areas.
- Field service: technician dispatch routing, service-level guarantees.
- Meteorology: pressure systems and temperature analysis on every weather forecast.
- Earth sciences: terrain rendering, geological mapping.
- Public transit planning: accessibility analysis by transit mode.
The commercial centre of gravity is firmly on travel-time isochrones.
Where MapAtlas Fits
The Isochrone API returns travel-time isochrones as GeoJSON polygons for any origin point, any time bands, and any travel mode (driving, cycling, walking, transit where available). Pair it with the Matrix API when you need point-to-point times across many origins and destinations, or with the Directions API when you need turn-by-turn routes rather than catchment boundaries.
For the wider thematic map context, see the Thematic Map Guide and the companion glossary post on choropleth maps.
Frequently Asked Questions
What is an isoline map?
An isoline map is a thematic map that uses lines to connect points of equal value across a geographic area. The most familiar example is a contour map, where each isoline (called a contour line) connects points at the same elevation. Other examples include weather charts where isobars connect points of equal pressure, isotherms connect points of equal temperature, and isochrones connect points reachable within the same travel time from an origin.
What is the difference between an isoline and an isochrone?
An isoline is the general term for a line connecting points of equal value of any variable. An isochrone is one specific type of isoline: it connects points reachable within the same travel time from a chosen origin. Every isochrone is an isoline, but most isolines (contours, isobars, isotherms) are not isochrones. If you are building delivery zones, store catchments, or travel-time analyses, you want an isochrone API. If you want elevation contours or weather lines, you want a generic isoline data source.
What are common types of isoline maps?
The most common types are: contour maps (lines of equal elevation), isobaric maps (lines of equal atmospheric pressure on weather charts), isothermal maps (lines of equal temperature), isohyets (lines of equal rainfall), isochrones (lines of equal travel time from an origin), and isodapanes (lines of equal transport cost in economic geography). All share the same underlying idea: collapse a continuous surface into a set of lines that show its level structure.
How are isoline maps generated?
Isoline maps are computed from a grid or point sample of the underlying variable. The standard algorithm is marching squares (for 2D) or marching cubes (for 3D volumes), which traces lines along cells where the value crosses each chosen threshold. For travel-time isochrones, the input is a road network: the algorithm runs a shortest-path expansion from the origin and traces the boundary of all reachable cells at each time threshold. Modern APIs return isochrones as GeoJSON polygons ready to overlay on a map.
What are isoline maps used for in business?
Logistics teams use travel-time isochrones to define delivery zones and assess last-mile feasibility. Retail and real estate use isochrones to define store catchments (e.g. all households within a 15-minute drive). Emergency services use them to model response coverage. Field service uses them for technician dispatch. In meteorology, isobaric and isothermal maps remain the standard for forecasting. In geology and earth science, contour maps are still the primary way to render terrain on paper and screen.
