Flow maps

Detail

What a flow map is

• A flow map (also called a flow chart) is part graph and part map.
• The map part shows where things move: the route between an origin (the starting point) and a destination (the end point).

• The graph part shows how much moves: each route is drawn as a band (a line or strip), and its width is proportional to volume. Twice the traffic gives a band twice as wide.

• It shows the movement of goods, people or vehicles.

• NCERT calls it a "dynamic map" (exercise 1(iv)).
• A static map shows things that stay in one place, such as relief or the spread of population.

• A dynamic map shows things that move, such as traffic, trade or migration.

• Each part of the drawing carries one kind of information:

• width shows quantity
• arrow-heads (where used) show direction

• colour or pattern can separate categories, such as goods and passengers

• Flow maps are widely used for traffic, migration, trade, commuting and water flow in rivers.

Two kinds of data shown

Data type	What it measures	Example
Number and frequency of vehicles, by direction of movement	How many trains, buses or trucks run on a route, and how often	Trains per day on Delhi rail routes
Number of passengers or quantity of goods carried	What the vehicles actually carry	Tonnes of cargo, passengers carried, cusecs of river water

Requirements for drawing a flow map

1. A route map that shows the routes and the stations (nodes) that connect them.
2. Flow data (for example, the number of trains, passengers or tonnes) with its origins and destinations.
3. A scale for line width, for example 1 cm = 50 trains.

Step by step: how to draw one

• Step 1. Trace the base route map and mark the nodal stations. A nodal station is a junction where several routes meet.
• Step 2. Put the flow data in a table, route by route.
• Step 3. Choose a width scale. It should make the busiest route clearly visible without making it so wide that it covers the map.
• Step 4. Work out each band's width: width = value ÷ scale value.
• Step 5. Draw each band along its route, centred on the line, at the width you worked out.
• Step 6. Add a terraced scale as the legend, plus symbols for the nodal stations.

NCERT example 1: trains on Delhi rail routes (Table 3.8, Fig. 3.9–3.10)

• Data: the number of trains on the different rail routes of Delhi.
• Busiest route: Old Delhi–New Delhi, with 50 trains.

• Least busy route: Udyog Nagar–Tikri Kalan, with 6 trains.

• Scale: 1 cm = 50 trains.

• 50 trains → 50 ÷ 50 = 1 cm = 10 mm band.

• 6 trains → 6 ÷ 50 = 0.12 cm = 1.2 mm band.

• Legend: a terraced scale. This is a stepped key of band widths, like a staircase, where each step is labelled with the number of trains it stands for. The reader matches a band on the map to a step to read its value.

• Nodal stations get their own point symbols.
• What the map shows: the thickest bands meet at the central Old Delhi–New Delhi area, and the bands get thinner towards the outer stations. You can see at a glance that rail traffic is concentrated at the centre of the city.

NCERT example 2: water flow in the Ganga basin (Fig. 3.11–3.12)

• Scale: 1 cm band width = 50,000 cusecs.

• A cusec is one cubic foot of water passing a point every second. It is a unit of discharge, which means the volume of water a river carries.

• The bands are drawn along the river channels, so a wider band means more water at that stretch.

• Use: it lets you compare how much water different stretches of the basin carry, in one picture.

The first master of the flow map: Charles Joseph Minard

• Charles Joseph Minard, a French cartographer of the 19th century, is widely seen as the first cartographer to master the flow map. His maps showed both the direction and the amount of movement between an origin and a destination [2].
• His width scales:
• 1862 emigration map: 1 mm of band = 1,500 emigrants. It showed migration in 1858 from Europe, Africa, China and South Asia to colonial destinations [2].
• 1866 cotton map: 1 mm = 5,000 tonnes of cotton. It compared cotton flows to Europe in 1858, 1864 and 1865 [2].

• 1862 cotton maps: compared 1858 with 1861 to show how the American Civil War cut cotton supplies [2].

• 1869 map of Napoleon's Russian campaign (1812–13):

• The band of the army gets thinner as it advances and retreats, showing the men it lost [3].
• A temperature scale runs along the bottom and is linked to the retreat [3].
• So one map shows six things together: army size, route, direction, location, time and temperature.

Beyond NCERT: present-day uses

• Migration flow maps (Census):
• Census 2011 records migrants by place of last residence: elsewhere in the same district, other districts of the same state, or other states of India. These are the D-series tables, for example D-02, D-03 and D-05 [4].
• This is origin–destination data, so it can be drawn straight onto a flow map, for example inter-state flows from Bihar and Uttar Pradesh to NCT of Delhi and Maharashtra.

• Scale: India had about 454 million migrants in Census 2011, up from 315 million in Census 2001 [5].

• Trade flow maps: bands from exporting to importing countries along sea and air routes (see pointers world-trade, india-international-trade).

• Commuting flow maps: daily journeys from suburbs to city centres, used for planning metro and suburban rail.

Desire-line maps and route-based flow maps

• A desire-line map shows straight lines from each origin to each destination, whatever the real route. The line's width still shows volume.
• It shows where people want to go. Planners use it to decide where new roads or rail lines are needed.

Feature	Route-based flow map	Desire-line map
Line path	Follows the actual road, rail or river	Straight from origin to destination
Base map needed	A route map with stations	Only the origin and destination points
Main use	Traffic on existing networks	Demand for movement; planning new routes
Example	Delhi trains (NCERT)	Origin–destination survey of commuters, Census migration

Prelims Hooks

• NCERT calls a flow map a "dynamic map" because it shows movement.
• In a flow map, the width of the band is proportional to the volume of flow, not to the length of the route.
• The three requirements are a route map with stations, flow data with origins and destinations, and a scale for line width.
• The legend of a flow map is a terraced scale, a stepped key of band widths.
• Delhi train example: Old Delhi–New Delhi (50 trains) is the busiest route and Udyog Nagar–Tikri Kalan (6 trains) the least busy. At 1 cm = 50 trains, their bands are 10 mm and 1.2 mm.
• Ganga basin water flow map: 1 cm = 50,000 cusecs. A cusec is one cubic foot per second.
• Desire lines are straight origin–destination links. Route-based flow lines follow the actual network.
• Charles Joseph Minard drew the 1869 flow map of Napoleon's 1812 Russian campaign [3][2].
• Census 2011 migration tables (D-series) classify migrants by place of last residence [4].

Mains Points

• Flow maps help planning:
• They show where traffic is heaviest (the busy core of the Delhi network) → this points to routes that need more lines, more frequent services or bypasses.

• Desire-line maps show demand where no route exists yet → they help in planning new metro, road and freight corridors.

• Migration and regional imbalance (GS-I):

• Flow maps drawn from Census migration data (about 454 million migrants in 2011 [5]) show movement from poorer source regions to richer destination states.

• They help explain urban overcrowding, remittance economies, and the stress on cities that receive migrants.

• Water resources (GS-III):

• Discharge flow maps, like the Ganga basin map in cusecs, show how water volume is spread along a river.

• This helps with flood management, river-linking proposals and sharing water between states.

• Limits of flow maps:

• Very wide bands can crowd the map and hide smaller flows.
• The data needs origins and destinations, which is harder to collect than simple totals.
• Desire lines can hide physical barriers on the real route.