Thematic Maps and Map Construction

Selecting the Right Map Type

Trying to map out local crime statistics using a world atlas is like trying to perform surgery with a butter knife. In geography, cartographic selection relies on a strict decision-making protocol based on your data's characteristics.

First, you must determine if your data is quantitative (numerical) or qualitative (descriptive). Next, establish whether the data is discrete (applies to specific, separate units or boundaries) or continuous (exists everywhere, gradually changing).

Follow this step-by-step protocol to choose the correct map:

1. Point Data (Magnitude or Density): If your data represents varying amounts at specific sites (like city populations), choose a Proportional Symbol Map. If you need to show the density and distribution of a feature (like livestock), use a Dot Map.
2. Area Data (Rates and Percentages): If your data is discrete and bounded by administrative regions (like unemployment rates by county), select a Choropleth Map.
3. Continuous Data: If you are plotting data that changes gradually and ignores political borders (like temperature or rainfall), an Isoline Map is required.
4. Movement and Connectivity: If you are mapping the volume of traffic along a path, use a Flow Line Map. If you only need to show the direct origin and destination points, choose a Desire-line Map.

Constructing Sketch Maps and Scales

Translating a complex real-world landscape onto a blank piece of paper requires careful simplification. A Sketch Map is not a detailed tracing; it is a summary of key geographical features.

To construct a high-quality sketch map, start by drawing a frame and lightly pencilling a 3x3 grid to help you place features proportionally. You must be selective, drawing only the relevant features required by the exam task (such as meanders or settlement boundaries). Every sketch map must include four essentials: a title, a North arrow for orientation, a key, and an accurate scale.

A Linear Scale (or scale bar) is a divided line that allows you to measure distances directly on the map. When drawing one, you should mark out primary divisions (e.g., 1km segments) and subdivide the first segment into smaller, precise units (e.g., 200m blocks).

Alternatively, a Ratio Scale expresses the mathematical relationship between map units and real-world units, without relying on specific measurements like centimetres or inches.

Worked Example: Constructing a Ratio Scale

You have drawn a sketch map of a local coastline. You measure the distance between two headlands on your map as 8 cm. In reality, the distance between these headlands is 4 km. Calculate the ratio scale of your sketch map.

Step 1: State the ratio formula, ensuring both sides use the same unit (centimetres).

$$\text{Scale} = 1 : \left( \frac{\text{Real Distance in cm}}{\text{Map Distance in cm}} \right)$$

Step 2: Convert the real-world distance into centimetres. Remember that $1\text{km} = 100{,}000\text{cm}$.

• $4\text{km} \times 100{,}000 = 400{,}000\text{cm}$

Step 3: Set up the initial ratio.

• $8\text{cm} : 400{,}000\text{cm}$

Step 4: Simplify the ratio so the map distance is 1.

• $8 / 8 = 1$
• $400{,}000 / 8 = 50{,}000$
• Scale = 1:50,000

Adding Descriptive Annotations

Examiners heavily penalise students who confuse a basic label with a geographical annotation. A label simply identifies a feature (e.g., "Steep slope"). In contrast, an annotation provides descriptive, explanatory notes directly on the map that detail the geographical processes at work.

To secure high marks, use a three-part formula for your annotations:

1. Identify the feature.
2. Explain the process causing it.
3. Link the feature to a broader geographical theory or hypothesis.

• Example: "Meander loop [Identify] where lateral erosion is occurring on the outer bank due to high-velocity flow [Explain], which may eventually cut through the neck to form an ox-bow lake [Link]."

Ordnance Survey (OS) Maps

OS maps are the standard for detailed topographical mapping in the UK. They rely on a grid system where vertical lines are called Eastings (numbers increasing eastward) and horizontal lines are Northings (numbers increasing northward).

To locate features, geographers use two main systems. A 4-figure Grid Reference identifies a general $1\text{km} \times 1\text{km}$ square (e.g., 17 51). A 6-figure Grid Reference provides pinpoint accuracy, identifying a $100\text{m} \times 100\text{m}$ square by mentally dividing the 4-figure square into tenths across and tenths up (e.g., 174 517).

Feature	1:25,000 OS Map (Explorer)	1:50,000 OS Map (Landranger)
Scale Meaning	4cm on the map = 1km in reality.	2cm on the map = 1km in reality.
Level of Detail	High. Shows field boundaries and public footpaths.	Moderate. Best for regional planning; omits minor field boundaries.
Contour Intervals	Contour lines are drawn every 5 metres.	Contour lines are usually drawn every 10 metres.

Relief (the shape of the land) is shown using contour lines, where closer lines indicate a steeper gradient. Exact heights are marked with spot heights (a black dot and number) or trig pillars (a blue triangle). To measure curved distances, such as meandering rivers, use the "ticking" method by rotating the straight edge of a piece of paper along the curve and marking direction changes.

Atlas and Base Maps

An Atlas is a collection of small-scale maps (often 1:1,000,000 or smaller) used to identify global spatial distributions and patterns. Because they cover vast areas, they rely on an index of latitude and longitude coordinates rather than grid references.

A Base Map is entirely different. It is a highly simplified outline map, stripped of complex details like contours, leaving only major features such as coastlines or large rivers. Its main purpose is to serve as a blank template for fieldwork, allowing you to plot primary data directly onto it without background clutter.

Thematic Maps

Thematic maps are designed to highlight specific geographical topics or datasets.

• Choropleth Maps: These maps use increasing intensities of a single colour to represent spatial data within predefined boundaries. They are ideal for showing the Index of Multiple Deprivation (IMD) or population density, but they falsely imply an abrupt change in data at the border of each shaded region.
• Isoline Maps: These maps use continuous lines to connect points of equal value. They are vital in weather mapping, using isobars for air pressure, isotherms for temperature, and isohyets for rainfall.
• Flow Line Maps: These map the movement of people, goods, or traffic along a specific path. They use proportional arrows where the width of the line directly represents the volume of movement (e.g., a thicker line indicates more buses per day).

Movement and Influence Maps

Visualising how people interact with services and the landscape requires specialist mapping techniques.

• Desire-line Maps: These use straight, "as the crow flies" lines to connect an origin to a destination. They are strictly straight and ignore physical road networks, making them perfect for showing general migration or commuting trends.
• Route Maps: Unlike desire lines, route maps follow the exact physical path taken along a transport network. They are useful for comparing actual travel distances against straight-line distances in mountainous areas.
• Sphere of Influence Maps: These display the geographical area over which a settlement or service exerts an impact. They are often used to define a catchment area—the specific zone from which a business or school draws its customers or students.

Finally, modern geographers often use Geographic Information Systems (GIS) to layer these different map types digitally, revealing complex relationships—such as overlaying a flow line map of hurricane tracks onto a choropleth map of population density to assess hazard risk.