Common and Specialized Geographical Graph Types

Step 1: Calculate the difference between the new value and the original value.

• $54{,}000 - 45{,}000 = 9{,}000$

Step 2: Divide the difference by the original value.

• $9{,}000 \div 45{,}000 = 0.2$

Step 3: Multiply by $100$ to find the percentage.

• $0.2 \times 100 = 20\%$

Line Graphs and Climate Graphs

Every time you watch a weather forecast, you are likely interpreting complex line graphs without even realising it. Line graphs are specifically designed to illustrate how a variable changes over a continuous scale, such as time or distance.

A climate graph is a specialised combination chart that shows a specific location's average monthly weather. It features the twelve months of the year on the x-axis and uses two separate y-axes.

Precipitation is always plotted as a bar graph and read from the right-hand axis (measured in $\text{mm}$). Temperature is plotted as a line graph and read from the left-hand axis (measured in $\text{°C}$).

When constructing the temperature line, the points must be plotted in the exact centre of each month's column. These should be joined with a smooth, freehand curve rather than a ruler to reflect natural cycles.

When interpreting these graphs, always look for the general trend while watching out for an anomaly — a data point that breaks the established pattern. A reliable way to describe these graphs is using the TEA (Trend, Example, Anomaly) method.

Pie Charts and Proportional Symbols

Did you know that scaling circles by their width instead of their overall area completely distorts the visual data? When constructing proportional symbols on a map, the area of the shape must be mathematically scaled to match the data value.

This allows geographers to illustrate differences in magnitude across many different locations simultaneously. However, a major drawback is occlusion, where large symbols overlap and obscure smaller symbols or underlying map features.

Pie charts are similarly used to show proportions of a whole, where the total area of the circle represents $100\%$. To draw a pie chart, you must convert your data into angles out of $360^{\circ}$ using the formula: $\text{Angle} = (\text{Value} \div \text{Total}) \times 360^{\circ}$

These pie charts can even be used as proportional symbols on a map. This allows you to show both the total size of a variable and its internal breakdown at different locations.

Worked Example

A city generates $144\text{ megawatts}$ of solar power. Calculate the radius required for a proportional circle if the scale factor is $1\text{ unit of the square root} = 2\text{ mm}$.

Step 1: Find the square root of the data value.

• $\sqrt{144} = 12$

Step 2: Multiply the result by the scale factor to find the radius.

• $12 \times 2\text{ mm} = 24\text{ mm}$

Scatter Graphs and Dispersion Graphs

Understanding how two variables connect helps geographers predict future events, such as estimating flood risks based on rainfall. Scatter graphs are used to plot bivariate data (two different variables) to see if a relationship, or correlation, exists. The independent variable goes on the x-axis, and the dependent variable goes on the y-axis.

To interpret scatter graphs, geographers look for the type of correlation:

• A positive correlation shows a trend from the bottom-left to the top-right, meaning both variables increase together.

• A negative correlation trends from the top-left to the bottom-right, indicating as one variable increases, the other decreases.

• If the points are scattered randomly with no discernible pattern, there is no correlation.

If a clear pattern emerges, you can draw a line of best fit by eye, ensuring an equal number of points fall on either side of a straight, ruled line.

In contrast, dispersion graphs are used to plot univariate data (a single variable) to observe its spread and distribution. The data points are plotted as individual dots along a single vertical axis.

Because dispersion graphs only look at one variable, you must never draw a line of best fit through them. Instead, geographers analyse the spread by calculating the interquartile range (IQR).

Worked Example

Calculate the interquartile range (IQR) for the following pebble sizes (in $\text{cm}$): $2, 8, 4, 6, 10, 5, 9$.

Step 1: Rank the data in numerical order.

• $2, 4, 5, 6, 8, 9, 10$

Step 2: Find the median (middle value).

• $\text{Median} = 6$

Step 3: Find the lower quartile (median of bottom half) and upper quartile (median of top half).

• $\text{Lower Quartile (LQ)} = 4$

• $\text{Upper Quartile (UQ)} = 9$

Step 4: Subtract the LQ from the UQ.

• $IQR = 9 - 4 = 5\text{ cm}$

Population Pyramids, Radial Graphs and Rose Charts

While most graphs measure simple numbers, some are uniquely shaped to show demographics or environmental scores. Population pyramids consist of two back-to-back horizontal bar charts showing age groups for males on the left and females on the right. A wide base indicates a high birth rate.

Radial graphs (often called radar charts) feature multiple axes radiating from a central zero point. They are excellent for displaying bi-polar assessment scores from fieldwork, where variables like noise and litter are scored from negative to positive.

Rose charts are circular histograms used specifically for directional data. The length of a bar pointing outwards from a compass point indicates the frequency of an event, such as the prevailing wind direction.

Worked Example

A country has $25\%$ of its population aged $0$-$14$, $60\%$ aged $15$-$64$, and $15\%$ aged $65+$. Calculate the dependency ratio using the formula below:

Step 1: Add the percentages of the dependent age groups.

• $25\% + 15\% = 40\%$

Step 2: Divide by the percentage of the economically active population ($15$-$64$).

• $40 \div 60 = 0.666...$

Step 3: Multiply by $100$ to find the final figure.

• $0.666... \times 100 = 66.7$

Cross-sections, Transects and Pictograms

Have you ever wondered how mapmakers turn a flat piece of paper into a 3D visual of a hill? Cross-sections are side-profile views showing the relief of the land between two points on a map. They are constructed using a paper strip laid across contour lines.

Because hills would look completely flat on a standard graph scale, geographers use vertical exaggeration (VE) to stretch the y-axis. This makes slopes more visible. The formula for VE is:

Cross-sections are vital for determining intervisibility (whether one location can be seen from another). They also help identify areas of dead ground hidden behind slopes.

To gather data across these landscapes, geographers use transects — straight lines along which measurements are taken at set intervals. Finally, to present simple discrete data, geographers might use pictograms. These use thematic icons to represent numerical units, making data visually memorable.