Core Skills in Graphical Construction and Presentation

Worked Example: Pie Chart

A geographer is investigating land use in a village covering a total area of 240 hectares. They find that 60 hectares of this land is covered by woodland. Calculate the angle required to represent woodland on a pie chart.

Step 1: State the formula for converting a value into degrees.

• $\text{Degrees} = (\text{Value} \div \text{Total Value}) \times 360$

Step 2: Substitute your values into the equation.

• $\text{Degrees} = (60 \div 240) \times 360$

Step 3: Calculate the final angle.

• $\text{Degrees} = 0.25 \times 360 = 90^\circ$

Presenting Data Clearly

A perfectly plotted graph is useless if the reader does not know what it shows. Clear communication relies on several essential features:

• Titles: A geographical title must follow the "What, Where, When" rule. It needs to be highly specific (e.g., "Figure 3: Line graph showing changes in river depth ($\text{cm}$) downstream along the River Severn").
• Axes: The horizontal X-axis displays the independent variable (the factor you chose to change, like distance), while the vertical Y-axis shows the dependent variable (the outcome you measured, like velocity). Both axes must have clear labels and units.
• Scales: Your axes must use a linear scale with equal, predictable intervals (such as going up in steps of 2, 5, or 10).
• Keys (Legends): If you are using different colors or shading—such as in a divided bar chart—you must include a key. The shading in the key must perfectly match the shading on the graph to allow easy comparisons.

Scatter Graphs and Proportional Symbols

When you need to investigate the relationship between two sets of numerical data—known as bivariate data—you should select a scatter graph.

• If both variables increase together, this is a positive correlation.
• If one increases while the other decreases, it is a negative correlation.
• To highlight the relationship, draw a line of best fit (LOBF). This must be a single, straight line drawn with a ruler that balances the points equally on either side, ideally passing through the mean point of the data.
• You can use the LOBF to estimate missing values within your data range (interpolation) or predict values beyond your collected data (extrapolation).

If you need to show the size or frequency of a variable at specific map locations, use proportional symbols.

• The area of the symbol (usually a circle) must represent the data value.
• While these provide excellent visual impact, a major negative feature is that large symbols often overlap on a map, causing clutter that hides smaller symbols underneath.

Worked Example: Constructing Proportional Circles

To ensure the area of a circle correctly represents the value, you must calculate the radius using a scale factor.

Step 1: Use the formula $\text{Radius} = \sqrt{\text{Data Value}} \times \text{Scale Factor}$. Step 2: If mapping a population of $4,000,000$ where your scale factor is $1$, find the square root of the value (expressed in the same units as your scale, e.g., millions).

• $\sqrt{4} = 2$ Step 3: Multiply by the scale factor to find the radius to draw.
• $2 \times 1 = 2\text{ cm radius}$.

Describing and Annotating Data

To achieve full marks when interpreting graphs, you must move beyond simple descriptions. Adding an annotation—a descriptive text box linked to a specific part of the graph with a leader line—is the best way to communicate findings.

When writing an annotation or answering a description question, use the TEA method:

1. Trend: Identify the overall pattern or general direction of change (e.g., "There is a steady decrease in pebble size downstream").
2. Example: Quote specific data points to prove the trend, always including the correct units (e.g., "Falling from $15.5\text{ cm}$ at Site 1 to $4.2\text{ cm}$ at Site 5").
3. Anomaly: Spot any data points that do not fit the established pattern (e.g., "However, Site 3 is an anomaly, as pebble size briefly increases to $12.1\text{ cm}$").