Flow map

A flow map shows things that move.

It can show many things. It can show planes in the sky. It can show water in the sea. It can even show wind.

These maps use lines to show paths. Lines can be thick or thin. A thick line means a lot is moving. A thin line means only a little is moving.

Some maps show where things start and end. These can use curved lines.

Flow maps help us see how the world moves.

A flow map is a special kind of map. It shows how things move from one place to another.

These maps use lines to show movement. The lines can be thick or thin. A thick line shows that a lot of something is moving. A thin line shows that only a little is moving. This helps us see the amount of movement at a glance.

Flow maps can show many different things. They can show people moving to new homes. They can show trade goods like wine moving between cities.

Some maps show the exact path things take. These are called network route maps. A subway map is a good example.

Other maps show movement that does not follow a single path. For example, ocean currents move in large masses of water.

Early maps like the 1838 Ireland map showed cargo on roads and canals.

Today, we use computers to make these maps. They help us understand how the world works.

A flow map is a special kind of thematic map. It shows how things move between different places. Instead of just showing where a city is, it shows movement. This movement can be anything at all. It might be people traveling to new homes. It could be trade goods like wine moving between ports.

It can even be water in an ocean current or data moving through wires. These maps help us see how the world is connected.

These maps work by using lines to show paths. A very important part is the thickness of the line. A thick line shows a large amount of movement. A thin line shows a small amount of movement. This is called weight scaling.

Some maps also use arrows to show direction. This tells us if something is moving north or south. Other maps might use different colors to show speed. A map can show many of these details at once.

People have been making these maps for a long time. In 1838, Henry Drury Harness made maps about cargo in Ireland. These maps showed movement on roads and canals.

Later, a man named Charles Joseph Minard became very famous for them. He made forty-two flow maps in the 1850s and 1860s. His 1869 map of the French invasion of Russia is very special. Some people call it the best statistical graphic ever drawn.

There are different ways to design a flow map. An origin-destination map shows a connection between two places. These often use curved lines to look nice.

A network route map focuses more on the paths themselves. A subway map is a great example of this.

Some maps show movement that does not follow a path. These are called mass flow maps. They show big masses of things like wind or water.

Today, we use computers to help us create these maps. In the 1980s, a man named Waldo Tobler did experiments with computers. Early computer maps were not as good as Minard's work. Now, we have special software called GIS to make them better. These maps help us understand many things. They show us how trade, weather, and people work together. Even without a map, we can see flow in our own bodies.

A diagram can even show how blood moves through your organs.

A flow map is a specific type of thematic map. It uses linear symbols to represent movement between various locations. You might think of it as a hybrid between a traditional map and a flow diagram. While a standard map shows where things are, a flow map shows how things move. This movement can involve many different subjects. It can represent people, highway traffic, or trade goods. It can also map water, ideas, or telecommunications data.

To understand how these maps work, we must look at the specific aspects of movement they track. Researcher Eduard Imhof identified several key elements of flow phenomena. First, a map can show the origin and destination. This answers the question of where something is moving from and to. Second, it can show the route. This is the path the movement takes. The route might be precise or a simple, generalized line. Third, it identifies the type of movement. This describes what is actually being moved, such as a specific product or water temperature.

Flow maps also track the amount of movement. This is a non-negative ratio variable. It could be the number of vehicles per day or total migrants. Some maps even show flow capacity, like telecommunications bandwidth. Other details include the direction of movement, often shown with arrows. Finally, maps can show flow velocity. This is the speed at which something moves, such as wind velocity or mean vehicle speed. A single flow map can portray all of these different aspects at the same time.

There are several distinct categories of flow maps used by cartographers. The first type is the origin-destination map. Its primary goal is to show a connection between two places. These maps often use curved lines, or circular arcs, to look more pleasing. They also help avoid overlapping lines. A common example is an airline route map.

The second type is the distribution map. These show a balanced focus on the origin, the route, and the volume. In these maps, paths leave an origin with a width proportional to the total destinations. They then divide as they distribute toward each location.

A third type is the network route map. This type focuses more on the routes within a network than on the specific nodes. It can be very precise or highly generalized. A subway or transit map is a classic example.

The fourth type is the continuous or mass flow map. This is used when movement does not follow a specific linear network. Instead, two- or three-dimensional masses like air or water flow. These can be modeled as a vector field. This field measures the magnitude and direction of movement at any point in space.

The history of these maps is quite long. The earliest known maps representing flow volume were created by Henry Drury Harness. He published two maps in 1838. These maps showed cargo traffic by road and canal in Ireland.

Later, Charles Joseph Minard mastered this technique. During the 1850s and 1860s, he published forty-two flow maps. His 1869 map of the French invasion of Russia is considered one of the best statistical graphics ever drawn.

In the 1980s, Waldo Tobler conducted experiments to create these maps using computers.

To make these maps visually clear, cartographers use weight scaling. This is the most common method to show the amount or speed of flow. Most often, designers use line weight, measured in points or millimeters. If a line is twice as thick as another, it represents twice the value. This helps readers make intuitive judgments about ratios. However, if the difference between values is too high, it can be difficult to draw. If values vary by more than 25:1, thick lines might overwhelm the map. In those cases, designers might use linear interpolation or simple ordinal weights like thick, medium, and thin.

Flow maps connect to many different fields of study. In human geography, they are used for migration, trade, and public utilities like electricity or water. In physical geography, they help us understand streamflow, wind, and wildlife migration. Even outside of geography, the concept of flow is useful. For example, a non-cartographic flow map can show how blood moves through the human body.

This helps us visualize the complex systems that keep us alive.