Big picture
A table of numbers can hide patterns that a graph reveals at a glance. Graphical representation turns data into diagrams and maps, making it easier to compare values, see trends over time and understand spatial patterns. A climograph shows a place's climate in one figure; a choropleth map shows which states have high or low literacy.
This chapter explains the rules for drawing graphs and how to construct the main types of diagrams and thematic maps used in geography.
Learning goals
Explain the need for graphical representation and the general rules for drawing graphs.
Construct line graphs and polygraphs.
Construct simple, multiple and compound bar diagrams.
Construct line and bar graphs (climographs).
Construct pie diagrams by calculating angles.
Explain and construct dot, choropleth, isopleth and flow maps.
Core ideas
1. Why represent data graphically?
Graphs and maps give a visual impression that is easier to understand than numbers.
They make comparisons and trends clearer.
They help identify spatial patterns (where things are concentrated).
2. General rules for drawing graphs
Rule | Explanation |
|---|---|
Select a suitable method | Choose the method that suits the data: line graphs for continuous data over time, bar diagrams for comparing categories, pie diagrams for parts of a whole, maps for spatial distribution |
Select a suitable scale | The scale should cover the full range of data and use the available space well; it should be in round numbers |
Design | Include a title (the area, time and subject), a legend or index explaining colours and symbols, direction (for maps), and the source of data |
3. Line graphs and polygraphs
A line graph shows time-series data, such as temperature, rainfall, population growth or production over years. Time is on the x-axis and the value on the y-axis.
Construction:
Round the data to whole numbers if needed.
Draw the X and Y axes; mark time on X and the variable on Y.
Choose a suitable scale and label the axes.
Plot the values as points and join them with a free-hand line or straight segments.
A polygraph is a line graph with two or more lines showing different variables or places on the same graph, for comparison. For example, growth rates of several states over decades. Different line styles or colours distinguish them.
4. Bar diagrams
Bar diagrams (columnar diagrams) use bars of equal width with lengths proportional to the values. They are best for comparing discrete categories.
Type | Use | Example |
|---|---|---|
Simple bar diagram | One variable for different categories or times | Rainfall of a station for 12 months |
Multiple bar diagram | Two or more variables side by side for comparison | Literacy of males, females and total for several states |
Compound (sub-divided) bar diagram | One bar divided into parts showing the composition of a total | Total production of a crop by states, or electricity generation by source |
Rules: bars should have equal width and equal spacing; the scale should start from zero; bars can be vertical or horizontal; each bar or part should be labelled or shaded.
5. Line and bar graph (climograph)
A line and bar graph combines a line and bars on the same diagram, using two Y-axes. The common example is a climograph: temperature shown as a line and rainfall shown as bars, month by month.
Construction:
Draw the X-axis for the 12 months.
Draw two Y-axes: one on the left for temperature (°C) and one on the right for rainfall (mm), each with a suitable scale.
Draw bars for monthly rainfall.
Plot monthly temperature at the middle of each month's space and join the points with a smooth line.
Add title, legend and source.
A climograph lets you see at once whether a place has hot, wet summers and cool, dry winters, or another pattern.
6. Pie diagram
A pie diagram shows the composition of a whole: a circle is divided into sectors proportional to the parts.
Calculating angles: the circle has 360°, so the angle for each part = (value ÷ total) × 360°. If the data are percentages, angle = percentage × 3.6°.
Example 1. Workers in a country: agriculture 45%, industry 25%, services 30%.
Sector | Percentage | Angle |
|---|---|---|
Agriculture | 45% | 45 × 3.6 = 162° |
Industry | 25% | 25 × 3.6 = 90° |
Services | 30% | 30 × 3.6 = 108° |
Total | 100% | 360° |
Example 2. A farm produces rice 150 t, wheat 100 t, pulses 50 t and others 100 t (total 400 t). Angles: rice = 150 ÷ 400 × 360 = 135°, wheat = 90°, pulses = 45°, others = 90°.
Construction:
Calculate the angles; check that they add up to 360°.
Draw a circle of a suitable radius.
Draw a radius (usually from the 12 o'clock position) and measure angles clockwise, starting with the largest sector.
Shade or colour sectors and add a legend.
7. Thematic maps
Thematic maps show the spatial distribution of a particular theme, such as population, rainfall or literacy.
Map | What it shows | How it is made |
|---|---|---|
Dot map | Distribution of absolute values such as population, livestock or crops | Each dot represents a fixed number (for example, one dot = 10,000 people); dots are placed where the phenomenon occurs, considering relief and settlement; dots must be of uniform size |
Choropleth map | Data related to administrative units expressed as ratios, densities or percentages (population density, literacy rate, sex ratio) | Data are grouped into categories; each unit is shaded with a tone that increases in intensity with value (light for low, dark for high) |
Isopleth map | Continuous data that change gradually across space: temperature, rainfall, pressure, relief | Lines join places of equal value: isotherms (temperature), isobars (pressure), isohyets (rainfall), isohypse or contours (height) |
Flow map | Movement of goods, people or vehicles between places | Lines of width proportional to the volume of flow are drawn along routes, such as the number of trains or passengers on a route |
Interpolation (for isopleth maps): inserting intermediate values between two known values, so that lines of equal value can be drawn. For example, if two stations have rainfall of 40 cm and 60 cm, the 50 cm isohyet passes midway between them (assuming a uniform change).
A reliable approach to questions
Match the type of data to the right diagram: time series → line graph; categories → bar diagram; parts of a whole → pie diagram; distribution across space → thematic map.
For a pie diagram, calculate each angle and check the total is 360°.
For a climograph, use bars for rainfall and a line for temperature, with two Y-axes.
For maps, use dots for absolute values, choropleth shading for ratios by unit, and isopleths for continuous data.
Connections that matter
Data sources and compilation and data processing prepare the data that are graphed.
Climate chapters use climographs and isotherm maps.
Mathematics: statistics uses bar graphs and pie charts.
Common mistakes
Using a choropleth map for absolute numbers. Choropleths should show ratios, densities or percentages; dot maps show absolute values.
Showing rainfall as a line in a climograph. Rainfall is shown as bars and temperature as a line.
Forgetting that pie angles must total 360°.
Using bars of unequal width in a bar diagram.
Revision checklist
I can state the general rules for drawing graphs.
I can construct line graphs and polygraphs.
I can construct simple, multiple and compound bar diagrams.
I can construct a climograph.
I can calculate pie diagram angles and construct the diagram.
I can explain dot, choropleth, isopleth and flow maps.