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A. Global Settlement: Patterns and Sustainability · Global Settlement: Patterns and Sustainability

Reading and Making Choropleth Maps

A3.7 A2.3 A2.4About 50 minutes
Learning goals
  • I can explain what a choropleth map is and the conventions it follows.
  • I can read a choropleth map's legend and describe the pattern it shows.
  • I can calculate rates such as population density, sort them into classes and construct a choropleth map.
  • I can explain the strengths and limitations of choropleth maps.

Words to know

choropleth map
A thematic map in which areas (such as countries, provinces or townships) are shaded lighter or darker to show the value of a statistic in each area.
class
A range of values that share one shade on a choropleth map, such as "10 to 99 people per km²".
legend
The key that explains what each shade or symbol on a map means.
rate
A number compared with something else, such as people per km² or a percentage. Choropleth maps should show rates, not raw totals.
raw total
A plain count, such as the total number of people, not compared with area or population.
sequential colour scheme
Shades of one colour from light to dark, used to show low to high values.
spatial technologies
Tools such as GIS, digital maps and satellite images that help us collect, map and analyse information about places.

Remember from Grade 7

In Grade 7 you met several kinds of thematic maps, including the choropleth map, where areas are shaded lighter or darker to show amounts, and you used maps and GIS to investigate resources. In Grade 8 you'll learn the rules for choropleth maps, how to build one yourself from data, and how to judge whether a choropleth map is telling a fair story.

Let's start with a question

Why might a pattern be easier to understand on a map than in a table? Look at a table of 13 provinces and territories with their population densities: it takes a while to see a pattern. Shade the same numbers on a map, and in a second you can see that the most crowded areas are in the south-east and the least crowded are in the North. That is the power of a choropleth map.

Conventions (rules) for choropleth maps

  1. Show rates, not raw totals. Use people per km², percentages or amounts per person. A big area with a large total isn't necessarily crowded.
  2. Use one colour, light to dark (a sequential colour scheme): light = low values, dark = high values. Readers understand this right away.
  3. Use a small number of classes, usually 3 to 7. Too many shades are hard to tell apart.
  4. Classes must not overlap and must cover every value. For example: under 10, 10 to 99, 100 or more. (Not "0 to 10" and "10 to 100", where 10 fits in two classes.)
  5. Include a title, legend, data source and year, and usually a north arrow and scale (or "not to scale" for a sketch).
  6. Show "no data" differently (for example, with a pattern) so missing data isn't mistaken for a low value.
ABCDEFlakepeople per km²under 1010 to 99100 or morepretend Lakeland County, not to scale
A choropleth map of pretend Lakeland County. Townships C and F, beside the lake, have the strongest shading (100 or more people per km²). A and D have medium shading (10 to 99). B and E have the faintest shading (under 10). Pretend data; not to scale.
Worked example

Example 1: Let's construct a choropleth map

Pretend data for Lakeland County:

  • A: 12 000 people, 300 km²
  • B: 2 400 people, 400 km²
  • C: 45 000 people, 150 km²
  • D: 5 000 people, 250 km²
  • E: 900 people, 450 km²
  • F: 30 000 people, 200 km²

Calculate each township's population density, sort into three classes (under 10, 10 to 99, 100 or more) and describe the pattern.

  1. Calculate the rate (density = population ÷ area): A = 12 000 ÷ 300 = 40; B = 2 400 ÷ 400 = 6; C = 45 000 ÷ 150 = 300; D = 5 000 ÷ 250 = 20; E = 900 ÷ 450 = 2; F = 30 000 ÷ 200 = 150.
  2. Check the classes cover every value with no overlaps: under 10, 10 to 99, 100 or more. ✓
  3. Sort: under 10 → B (6), E (2). 10 to 99 → A (40), D (20). 100 or more → C (300), F (150).
  4. Shade: lightest for B and E, medium for A and D, darkest for C and F. Add a title, legend and "pretend data" source.
  5. Describe the pattern: the densest townships are beside the lake (C and F); the middle townships (B and E) are the least dense.

Classes: B, E = under 10; A, D = 10 to 99; C, F = 100 or more. Pattern: population density is highest along the lake shore and lowest in the middle of the county.

Watch out: raw totals mislead

If you shaded the map by total population instead of density, a huge township with many scattered farms could look darker than a small, crowded town. Always ask: per what? Per km², per person, or per 100 people?

Shaded by TOTAL people(misleading)Big Township10 000 people1 000 km²Town5 000 people25 km²Shaded by DENSITY (a rate)(fair)Big Township10 000 people1 000 km²Town5 000 people25 km²10 000 is more than 5 000,so the big one looks "crowded"10 per km² vs 200 per km²:the town is 20 times as crowdedpretend data, not to scale
The same two places shaded two ways. By total, the big township gets the stronger shade; by density, the small town is clearly the crowded one. Always ask: per what?

Choosing class breaks

The class breaks (where one class ends and the next begins) change how a map looks.

  • Equal intervals: each class covers the same range (0 to 99, 100 to 199, 200 to 299...). Simple, but if most values are small, most areas end up in one class.
  • Equal groups (quantiles): each class has about the same number of areas. Every shade appears, but very different values can share a class.
  • Natural breaks: breaks are put at the big gaps in the data, so similar values are grouped together.

Because class choices can change the story a map tells, a careful reader always checks the legend.

Strengths and limitations

Strengths: quick to read; shows spatial patterns and regions clearly; easy to compare places.

Limitations:

  • Each area gets one shade, which hides differences inside it (a township with one crowded village and lots of empty forest looks evenly shaded).
  • Large areas catch the eye more than small ones, even when few people live there.
  • Boundaries are political lines, not natural edges of the pattern.
  • Different class breaks or colours can make the same data look very different.

Spatial technologies

GIS software and online mapping tools can calculate rates, choose class breaks and shade maps automatically. Statistics Canada publishes census data that can be mapped this way. But you still need to check that the map shows a rate, uses sensible classes and has a clear legend.

Worked example

Example 2: Let's read a choropleth map

Look at the Lakeland County map above. A student says: "Township E has the fewest people." Is that a correct reading of the map?

  1. Check the legend: the shading shows people per km², which is density, not total population.
  2. E has the lightest shade, so it has the lowest density (under 10 per km²).
  3. But E is also the largest township (450 km²). From the data, E has 900 people and B has 2 400, so E does have the fewest, but the map alone can't tell you that. A low density could come from a small population or a large area.

The student happens to be right, but the reasoning is wrong. A choropleth of density shows how crowded places are, not how many people live there. You need the data table or a different map to know totals.

Hands-on activity

Make your own choropleth map

You need: graph paper or plain paper, a ruler, a pencil and one coloured pencil (or just a pencil, pressing lightly to heavily).

Pretend data: Pinecrest County has 9 districts, each 100 km², arranged in 3 rows of 3. There is a lake along the east edge.

  • Row 1 (north), west to east: District 1: 500 people; District 2: 1 200; District 3: 8 000
  • Row 2: District 4: 900; District 5: 4 500; District 6: 15 000
  • Row 3 (south): District 7: 300; District 8: 2 000; District 9: 22 000
  1. Draw a 3 × 3 grid of squares and a lake along the right-hand (east) side. Number the districts.
  2. Calculate each district's density (population ÷ 100).
  3. Make three classes: under 10, 10 to 99, 100 or more people per km².
  4. Shade light, medium or dark. Add a title, legend, north arrow, and "pretend data, not to scale".
  5. Write two sentences describing the pattern.

What to look for: densities 5, 12, 80, 9, 45, 150, 3, 20 and 220. Under 10: districts 1, 4, 7 (the west column). 10 to 99: districts 2, 3, 5, 8. 100 or more: districts 6 and 9. Pattern: density increases from west to east, and is highest near the lake, especially in the south-east.

Just for fun

Why did the choropleth map need a vacation?

It had too many boundary issues. (Fun fact: choropleth comes from Greek words meaning "area" and "multitude".)

Your turn

1

What is a choropleth map?

2

Why should choropleth maps show rates instead of raw totals?

3

On a sequential colour scheme, what does a darker shade usually mean?

4

What is wrong with these classes: 0 to 50, 50 to 100, 100 to 150?

5

Pretend data: a township has 6 000 people on 40 km². Which class does it go in: under 10, 10 to 99, or 100 or more?

6

Give one limitation of choropleth maps.

7

Name four things every choropleth map should include.

More practice: warm-up

1

What part of a map explains what each shade means?

2

About how many classes do most choropleth maps use?

3

Is "people per km²" a rate or a raw total?

4

On a choropleth map, what does the lightest shade usually show?

5

Name one spatial technology that can make choropleth maps.

More practice: on your own

1

Pretend data: three regions. X: 18 000 people, 600 km². Y: 4 000 people, 50 km². Z: 1 500 people, 500 km². Calculate densities and sort into: under 10, 10 to 99, 100 or more.

2

A map of Canada's provinces is shaded by total population. Ontario and Quebec are darkest. Why might this mislead someone about how crowded those provinces are?

3

Why would a choropleth map of Nunavut's density be shaded evenly even though people live in 25 clustered communities?

4

Values for 6 areas are 2, 3, 4, 5, 6 and 95. Using equal intervals of 0 to 32, 33 to 65, 66 to 98, which classes get used? What's the problem?

5

Why should "no data" be shown with a different pattern instead of the lightest shade?

Challenge

1

Using the Pinecrest County data from the activity, make a new set of classes: under 20, 20 to 99, 100 or more. Which districts change class, and how does the map's message change?

2

You want to map the percentage of land used for farming in each township of a county. Explain why this is a good choropleth topic and describe the legend you'd use.

3

A choropleth map shows that Township C (300 people per km²) is beside a lake. A student concludes, "The lake causes high density." Evaluate this conclusion.

Check your understanding

Quiz

1.

What does a choropleth map use to show values?

2.

Which statistic is best for a choropleth map?

3.

Which set of classes is correct?

4.

A township has 9 000 people on 60 km². What is its density?

5.

What is one limitation of a choropleth map?

6.

On a sequential colour scheme, darker shading usually means:

7.

Why should you always read the legend carefully?

For parents and teachers

Big idea: A3.7 asks students to demonstrate the ability to analyse and construct choropleth maps on topics related to human settlement. The lesson also practises A2.3 (constructing and analysing maps) and A2.4 (interpreting data using tools and spatial technologies). All data in this lesson are pretend data, clearly marked.

Common difficulties:

  • Mapping raw totals. Ask, "Per what?" every time.
  • Overlapping classes. Check that every value fits into exactly one class.
  • Reading density as total population. The legend's units tell you what the shading means.
  • Arithmetic. Estimate first: 45 000 ÷ 150 is about 300.

How to help: Do the Pinecrest County activity together and check the answers in the activity box. If your child enjoys it, explore Statistics Canada's online census maps together and look at how the legend is built.