Flow Map
A map showing movement of quantities between locations using lines of varying width — thicker lines mean larger flows. The classic way to visualize trade, migration, and transportation patterns on a geographic surface.
// 01 — The chart
What it looks like
A flow map showing trade volume between four regions. Thicker lines represent larger bilateral trade flows, with arrows indicating direction.
// 02 — Definition
What is a flow map?
A flow map is a thematic map that shows the movement of quantities between geographic locations using lines (often curved arcs) whose width is proportional to the volume being moved. The result is a visual network of flows layered on top of a geographic base.
Flow maps encode three pieces of information simultaneously: origin (where the flow starts), destination (where it ends), and magnitude (how much is flowing). Arrows or tapered lines can also indicate direction, distinguishing imports from exports or inbound from outbound migration.
The technique dates back to 1869, when Charles Joseph Minard created his famous map of Napoleon’s 1812 Russian campaign — widely considered one of the greatest data visualizations ever made. Modern flow maps are used to visualize trade, migration, commuting patterns, airline routes, and internet traffic.
Classic example: Minard’s 1869 map of Napoleon’s Russian campaign encoded six variables on a single 2D surface: army size, location (latitude and longitude), direction of travel, temperature, and date. Edward Tufte called it “the best statistical graphic ever drawn.”
// 03 — Anatomy
Parts of a flow map
// 04 — Usage
When to use it — and when not to
- You want to show movement between specific geographic locations
- Your data has origin-destination pairs with associated quantities
- Revealing dominant trade routes, migration corridors, or traffic patterns
- Direction matters — you need to distinguish inbound from outbound
- You have a manageable number of flows (under ~50) to avoid visual clutter
- Your audience understands the geography being shown
- You have hundreds of origin-destination pairs — lines will overlap into an unreadable tangle
- Your data lacks geographic coordinates — use a Sankey diagram or chord diagram instead
- You only need to show values at locations, not movement between them — use a bubble or spike map
- All flows are similar in magnitude — line widths will be indistinguishable
- Your audience is unfamiliar with the geographic region being mapped
- You need precise numeric comparisons — line width is hard to read exactly
// 05 — Reading guide
How to read a flow map
Follow these steps whenever you encounter a flow map.
Identify what is flowing and between where
Read the title and legend. Is this trade, migration, commuters, data traffic? Understanding the variable helps you interpret why certain routes are thicker than others.
Find the thickest lines
These represent the dominant flows — the highest-volume routes. In a trade map, the thickest line might connect the two largest trading partners.
Check the direction arrows
Are flows one-directional or bidirectional? Arrows or tapered lines indicate direction. Without them, you only know two places are connected, not which way goods or people move.
Look for hub-and-spoke patterns
Does one location have many thick lines radiating from it? This indicates a hub — a dominant origin or destination that anchors the network.
Compare line widths across routes
Are most flows concentrated on a few routes, or spread evenly? Concentrated flows suggest dependency; distributed flows suggest a resilient, diversified network.
// 06 — Pitfalls
Common mistakes
×Too many flows creating visual spaghetti
Fix: Filter to the top N flows or use edge bundling to merge similar routes. A flow map with 200 lines is unreadable — show only the most significant connections.
×No direction indicators
Fix: Always use arrowheads, tapered widths, or color gradients to show direction. Without direction, a trade flow from A to B looks identical to B to A.
×Straight lines that cross and obscure each other
Fix: Use curved (arc) lines. Curves create visual separation between overlapping routes and are easier to trace from origin to destination.
×Inconsistent width scaling
Fix: Use a consistent, linear or sqrt width scale. If you manually adjust line widths for aesthetics, you destroy the quantitative comparison that makes flow maps valuable.
×Ignoring bidirectional flows
Fix: If flows go both ways between two locations, show both lines (offset slightly) or use a net-flow approach. A single line hides whether trade is balanced or one-sided.
// 07 — In the wild
Real-world examples
Minard’s map of Napoleon’s Russian campaign (1869)
The most famous flow map ever made. It shows the French army’s size shrinking from 422,000 to 10,000 soldiers as it marched to Moscow and back, layered with temperature data. Edward Tufte called it the best statistical graphic ever drawn.
Global shipping routes
Organizations like the World Bank and maritime analytics firms use flow maps to show container shipping volumes between ports. The thick lines connecting Shanghai, Rotterdam, and Los Angeles reveal the arteries of global trade.
US interstate migration
The Census Bureau’s migration flow maps show which states people are moving between. Thick flows from California and New York to Texas and Florida have become a defining story of American demographics.
// 08 — Quick reference
Key facts
// 09 — Variations
Types of flow maps
Flow maps come in several forms depending on how lines are styled and what kind of movement is being shown.
Arc flow map
Uses curved arcs between locations. The curvature separates overlapping lines and makes direction easier to read.
Tapered flow map
Lines taper from wide at the origin to narrow at the destination. Direction is implicit — no arrows needed.
Radial (hub) flow map
All flows radiate from a single hub location. Common for showing a city’s trade partners or an airline’s route network.
Route flow map
Shows flow along a specific path with waypoints. Used for supply chains, journey mapping, and historical campaigns.
// 10 — FAQs
Frequently asked questions
What is a flow map?+
A flow map is a thematic map that shows the movement of quantities between geographic locations using lines (often curved arcs) whose width is proportional to the volume being moved. The result is a visual network of flows layered on top of a geographic base.
When should you use a flow map?+
Use a flow map when you want to show movement between specific geographic locations. It also works well when your data has origin-destination pairs with associated quantities, and when revealing dominant trade routes, migration corridors, or traffic patterns.
When should you avoid a flow map?+
Avoid a flow map when you have hundreds of origin-destination pairs — lines will overlap into an unreadable tangle. It is also a poor fit when your data lacks geographic coordinates — use a Sankey diagram or chord diagram instead, or when you only need to show values at locations, not movement between them — use a bubble or spike map.
Is a flow map suitable for dashboards?+
Yes — a flow map can work well in dashboards as long as the panel is large enough for readers to perceive the encoded values, has a clear title, and includes the legend or axis labels needed to interpret it.
What category of chart is a flow map?+
Flow Map belongs to the Geospatial family of charts. Charts in that family are designed to answer the same kind of question, so they often work as alternatives when one doesn't quite fit your data.
How do you read a flow map?+
Start with the axis labels and legend, then look at the overall shape before zooming into individual marks. Compare prominent features against the rest of the data, and verify any conclusion against the underlying numbers when precision matters.