Statistical Graphs, Charts and Maps in Geography
Senior 5 · Geography
Statistical Graphs, Charts and Maps in Geography
Lesson 4 of 4
25 min
A bar graph can tell you that one district has more people than another. It cannot tell you where in the district they live, or how one district's pattern joins its neighbour's. For that, geographers draw statistical maps (also called quantitative maps): maps on which the data themselves are plotted. This lesson uses an invented province, Province X, with six districts (A to F), a river and a town, to show the three methods named in the syllabus: dot maps, isoline maps and choropleth maps.
| District | Population | Area (km²) | Density (people per km²) | Density class |
|---|---|---|---|---|
| A | 180,000 | 600 | 300 | 300–599 |
| B | 420,000 | 350 | 1200 | 900 and over |
| C | 250,000 | 500 | 500 | 300–599 |
| D | 90,000 | 450 | 200 | under 300 |
| E | 330,000 | 400 | 825 | 600–899 |
| F | 150,000 | 600 | 250 | under 300 |
Illustrative data.
Definition
Dot map: a map on which each dot stands for a fixed quantity (the dot value), and dots are placed where that quantity is actually found. Example: Figure 10, where one dot stands for 10,000 people.
Each district gets as many dots as its population divided by the dot value; District B, with 420,000 people, gets 42. The skill lies in placing them. Here they crowd along the river valley, where water and flat land attract farming and settlement, and around the town in District B, and thin out on the higher ground to the north and south. The district boundaries are drawn only for reference: the dot pattern ignores them.
Worked example 1: choosing a dot value
Problem. The districts of Province X range from 90,000 to 420,000 people. A pupil tries a dot value of 1,000 people, then 100,000. Judge both and choose a better one.
Solution. With 1,000 per dot, District B needs dots in an area a few centimetres across: they merge into a black blot. With 100,000 per dot, District D gets , about one dot, and its pattern disappears. A value of 10,000 gives 9 dots for D and 42 for B: few enough to separate, enough to show a pattern. Choose the value so that the densest area is crowded but the dots do not quite touch.
Definition
Isoline: a line joining places with the same value of a quantity. Isolines are named by what they show: contours (height), isotherms (temperature), isohyets (rainfall), isobars (air pressure). The difference between neighbouring lines is the interval. Example: in Figure 11 the isohyets are drawn every 200 mm.
Rain gauges measure rainfall only at points. To draw isohyets, you estimate where each chosen value lies between the stations, a process called interpolation, then join those points with smooth curves. In Figure 11 the result is a set of rings around Mount S: rainfall is heaviest on the mountain and falls away in every direction, faster to the north and south, where the rings are closer together.
Worked example 2: interpolating an isohyet
Problem. In Figure 11, station P (1,680 mm) and station Q (1,400 mm) lie on a straight line 70 map units apart. Where does the 1,600 mm isohyet cross that line?
Solution. Assume rainfall changes evenly between the stations. The 1,600 mm value lies
of the way from P to Q, so units east of P. On the map, P is at 330 units and the isohyet crosses at 350: exactly that point. The 1,400 mm isohyet passes through Q itself.
Linear interpolation assumes a steady change between stations. Where a hill or valley lies between them and is not sampled, the real line may be elsewhere, so isoline maps are only as good as the network of stations.
Definition
Choropleth map: a map in which areas (districts, provinces, countries) are shaded according to the value of a ratio or density for each area, using one colour that gets darker as the value increases. Example: Figure 12, population density by district.
To draw Figure 12:
Read the map: density is highest in District B (1,200 people per km², with the town) and then E (825). The southern districts D and F and the north-western district A are lightest. District A, at exactly 300, falls in the class 300–599.
Worked example 3: from data to shading
Problem. District E has 330,000 people on 400 km². Find its density and its shade.
Solution. people per km², which falls in the class 600–899, the third shade.
| Method | Advantages | Disadvantages |
|---|---|---|
| Dot map | Shows the real distribution within areas; numbers can be recovered by counting dots | Placing dots needs local knowledge; dense areas merge into a blot; dot value is hard to choose |
| Isoline map | Shows gradual change across space with no boundaries; values between stations can be estimated | Only for quantities that vary continuously (rain, height, temperature); accuracy depends on how many stations there are |
| Choropleth | Easy to draw from district statistics; shows regional differences clearly | Implies the whole area has one value and that it changes suddenly at the boundary; the map changes with the class limits chosen; large areas dominate the eye |
The same question decides every method in this unit: what does the reader need to see? A trend through time calls for a line graph; a comparison of categories, a bar graph; parts of a whole, a divided circle or bar; a population structure, a pyramid; and a pattern in space, a statistical map.
Warning
Common mistakes
Tip
Exam technique. Asked to "suggest a method", name it, justify it from the data ("density is a ratio for areas, so a choropleth suits it"), and give one weakness. Asked to "describe the distribution" on a statistical map, name where values are highest and lowest, quote figures, and describe the overall pattern (clustered, linear along a river, decreasing outwards).
Heimler's History: "Introduction to MAPS!" (AP Human Geography review). Watch the part on thematic maps and note which map types he uses for continuous data and which for data reported by area, then compare with the table above.
Practice
Summary
Further reading
Sources and credits
Written by Ganzaa. Province X, its districts and all data in Figures 10 to 12 are illustrative, invented for teaching. All figures were drawn by Ganzaa.
Video: "Introduction to MAPS! [AP Human Geography Review—Unit 1 Topic 1]", Heimler's History, embedded from YouTube.
