Interpreting Population Pyramids and Thematic Maps

Three ways to reveal geographical patterns

A population pyramid shows who makes up a population. A choropleth map shows how a value varies between areas. A flow-line map shows movement between places and its size. Interpreting them means reading their values accurately, identifying patterns and using the evidence to reach a geographical conclusion.

Start with the title and date: what is being shown, where, and when? Then check the axes or key and the units. The same colour or bar length can mean very different things on different diagrams.

Reading a population pyramid

A population pyramid shows a population divided by age and sex. Age groups run up the vertical axis, normally with the youngest at the bottom. Horizontal bars extend to either side, commonly males on the left and females on the right; check the labels rather than assuming this arrangement.

Each bar represents one sex within one age group. Its length shows either an absolute figure, meaning the actual number of people, or a percentage. Longer bars mean more people or a larger share of the population. Read the horizontal scale outwards from the centre on each side.

Suppose a town has 1,000 residents, divided into the groups shown below. The two pyramids represent exactly the same population, but one uses numbers and the other percentages of all residents.

There are 150 males and 140 females aged 20–39, making 290 people altogether. Since the town has 1,000 residents, this group represents 29% of its population. On the percentage pyramid, the two bars read 15% and 14%, which also total 29%.

Absolute figures are useful for judging the number of people who may need a service. Percentages are useful for comparing population structures where places have different total populations. A large country can have more older people than a small country but a smaller percentage of older people. Before comparing pyramids, check that their age groups, units and percentage bases match.

Interpreting the shape of a pyramid

Look at the whole shape before selecting individual bars. Compare the youngest groups, the middle age groups and the oldest groups, then look for differences between males and females.

A broad base shows a large younger population. This can suggest relatively high recent birth rates. A narrower base than the groups above it shows that younger groups are smaller, which may suggest declining births. A substantial older population suggests that many people survive to older ages and is consistent with higher life expectancy.

Bulges and gaps show unusually large or small age groups. Birth rates, deaths and migration can all affect these shapes: for example, migration of working-age adults may enlarge the middle of a pyramid. However, a pyramid alone does not prove which process caused a particular bulge.

Connect an observation to its possible significance. A large younger population may create demand for schools; a large older population may increase demand for healthcare and care services. These are reasoned implications, not services directly measured by the pyramid.

Reading a choropleth map

A choropleth map uses shading or colour within defined boundaries to represent a variable. Each shade usually represents a range of values. Variables might include population density, life expectancy or an indicator of economic development such as income.

Read the key first, including the units and the boundaries between categories. A shaded area normally tells you its value range, not its exact value. The darkest shade is not automatically the ‘best’ or ‘worst’: its meaning depends on the variable and the key.

The next figure uses fictional districts to demonstrate reading shading categories and commuter flows. The income panel lists colour assignments rather than geographic positions; the income values and commuter movements are separate datasets.

Districts A and B fall in the £15,000 to under £25,000 category, C falls in the £25,000 to under £35,000 category, and D falls in the £35,000 to under £50,000 category. A supported comparison is therefore: District D has a higher mean annual income than District C, and both have higher mean annual incomes than A and B. A and B share a category, but their exact incomes need not be equal. The order of rows does not tell us where the districts lie.

On a choropleth map, these colours fill the districts within their geographic boundaries. After reading the categories, examine where similar shades occur: neighbouring areas with similar values form a cluster, while a change in shading across a region may reveal a directional contrast. Use the map's orientation to establish compass directions, not the order of categories in its key. Support the overall pattern with named places and value ranges, then identify any area that differs from its neighbours.

Remember that shading summarises an area. It does not show every household's income or differences within the district. Also, a large shaded district occupies more map space without necessarily containing more people. Income can indicate one aspect of development, but it does not measure all aspects, such as health or education.

Reading a flow-line map

A flow-line map uses lines to connect origins and destinations. Line width represents the quantity moving: a wider line means a larger flow. Where arrows are included, they show direction. The key identifies both the quantity and its unit, such as migrants per year, tourists per month or vehicles per day.

In the commuter panel, arrows run from three districts towards a central workplace. The widest line represents 400 commuters per working day, compared with 200 on the medium line and 100 on the narrow line. The largest flow is therefore twice the medium flow and four times the smallest flow. These comparisons concern numbers of commuters, not journey lengths.

A real transport example is Skinningrove in North Yorkshire. A public-transport flow-line map in a 2020 geographical skills handbook shows around 40 buses per day connecting it with Middlesbrough, but fewer than 10 per day with Guisborough or Whitby. This indicates a stronger bus connection with Middlesbrough; it does not establish how many passengers use those buses.

To interpret another flow-line map, locate the origins and destinations, use the width key to compare quantities, and identify the dominant connections. Lines may simplify movement rather than trace the actual road or travel route. A strong connection can suggest an important relationship between places, but the map alone may not explain why people travel.

Population pyramids

• Age groups are vertical; bar lengths show numbers or percentages for each sex.
• Add both sexes to find an age-group total.
• Absolute figures compare numbers; percentages compare population structure.
• Broad base: large younger population. Substantial older groups: many people surviving to older ages.
• Bulges or gaps may reflect births, deaths or migration; shape alone does not prove the cause.

Choropleth maps

• Read the variable, units and shading ranges from the key.
• Describe the overall spatial pattern, support it with places and values, and identify exceptions.