Thematic Maps and Map Construction

Choosing a map for a geographical question

A map makes information geographical by showing where it occurs. The best choice depends on what you want to communicate: the distribution of a value, connections between places, access to services or the layout of an area.

A thematic map concentrates on a particular subject. Choropleth, isoline and flow-line maps are different ways of making thematic maps; they are not interchangeable. Population density is measured for areas, rainfall can vary continuously across space, and bus services connect places. Each needs a different representation.

Starting with a suitable framework

An atlas map helps place an area in its wider regional, national or global setting. Choose a page whose coverage and theme suit the question, then read its key and scale. A national map can show how a study area relates to major cities, but it would not be a suitable framework for plotting individual buildings in a street survey.

An Ordnance Survey (OS) map provides detailed information about the landscape. The two scales studied are 1:25 000 and 1:50 000. At 1:25 000, 1 cm represents 250 m; at 1:50 000, it represents 500 m. The 1:25 000 map shows the same ground distance at twice the map length, making it useful when local detail matters. For a fixed page size, 1:50 000 covers a wider area, which can help show the setting of a longer route or a group of sites.

A base map is the framework onto which you add your information. For data collected by district, it needs district boundaries; for connections between settlements, it needs their positions. Select a scale that leaves enough room for the information without losing the wider pattern. Retain the locations and boundaries needed for comparison, but avoid unnecessary clutter. For example, a survey map could retain streets and a river from an OS map, then add numbered survey sites and a key explaining them.

Shading areas: choropleth maps

A choropleth map uses shading to show values for defined areas. It suits population density, percentages and similar measures that can be compared between areas. Raw population totals answer a different question: a large district may contain more people simply because it covers more land.

To construct one, match each value to the correct area on a base map. Divide the values into clearly defined classes, then shade each area according to its class. Use an ordered sequence, usually light to dark for low to high values. The key must show the class boundaries and units.

The figure below uses population density classes of 0–19, 20–39 and 40–59 people per km². A district with 32 people per km² belongs in the middle class. The boundaries and values are fictional: they demonstrate the method, not an actual population distribution.

On a real map, use the actual administrative boundaries and dataset. Describe where high and low values occur, then support the pattern with values from the key. Remember that one shade summarises a whole district: it hides differences within that district, and a sharp colour boundary need not represent a sudden change on the ground.

Connecting equal values: isoline maps

An isoline joins locations with the same value. Unlike a choropleth boundary, it does not have to follow an administrative boundary. Contours represent equal height; other isolines can represent rainfall, temperature or air pressure.

Construction begins with values plotted at their measurement locations. Choose a consistent interval between lines. Where the required value falls between observations, estimate its position between them: this is interpolation. Join these estimated positions smoothly and label the lines with their values and units. Each line represents one value, so lines representing different values must not cross.

In the figure, a 4 mm line passes between observations of 3 mm and 5 mm. Placing it halfway assumes an even change between the sites. This is an estimate, not an additional measurement; sparse observations make the position less certain.

A rainfall example in OCR's 2020 geographical skills handbook records amounts in millimetres over 12 hours. It shows a broadly concentric pattern, with higher readings near the centre and lower readings towards the outskirts. Comparing such a map with land use could help investigate whether vegetation interception affected the amount collected in rain gauges. The rainfall pattern alone does not establish that explanation.

Showing connections: desire lines and flow lines

A desire-line map connects an origin directly to destinations. It makes direction and straight-line separation clear without tracing every bend in a road. To construct one, plot the origin and destinations, draw direct connections, and label the places. Include direction arrows where direction matters and a distance scale where distances are represented.

This can help compare access to services. OCR's town-centre example connects a central point to a bus stop, cash machine, petrol station, pub, playground and primary school. Distance rings at 200, 400, 600 and 800 m help compare their straight-line distances. These connections do not show walking distance or obstacles along the journey.

A flow-line map adds quantity through proportional line width. Choose a width key and apply it consistently: a connection representing twice the quantity should be twice as wide. Identify what is counted and its time period, and add arrows if the data distinguish directions.

OCR's 2020 example shows public transport connections from Skinningrove in North Yorkshire. The broad connection to Middlesbrough represents around 40 buses per day, whereas the narrower connections to Guisborough and Whitby represent fewer than 10 buses per day. This supports a comparison of service frequency, not passenger numbers or current timetables.

Mapping influence and routes

A sphere of influence map shows the surrounding area affected or served by a place in a particular context. A large out-of-town shopping centre may draw customers from a wider area than a village shop. The context must be stated: a shopping catchment and a travel-time area are not necessarily the same thing.

One way to represent accessibility is to map travel-time bands. OCR's Guildford example shows 10-, 20- and 30-minute driving-time zones from the town centre. Read the key before comparing the zones. Their irregular outlines show that equal travel time does not produce a simple circle of equal distance. Such zones help explore potential access to Guildford; they do not prove that everyone within them uses its services. Constructing a travel-time map requires travel-time information, not just circles drawn at equal distances from the centre.

A route map, by contrast, shows the path followed between places. It answers ‘Which way should I go?’ rather than merely showing a direct connection between endpoints. Start with a map that shows the relevant roads or paths, identify the start and finish, and trace the connected route between them. Mark important junctions and landmarks so the reader can follow the sequence.

For a walking route, a description might be ‘start at the car park, follow the path to the bridge, cross the river, then turn towards the survey site’. A route map would show these connected sections and label the bridge and turning point. On an actual map, check that the paths and crossing exist rather than drawing a direct line across the river. Keep the route visually distinct from other roads or paths and state whether the map is drawn to scale.

Constructing a clear sketch map

A sketch map simplifies an area for a purpose. It can be drawn from an OS map extract or a photograph, but it is not a replica: it selects the features needed to communicate the geographical question.

Begin by deciding which features matter. For a river investigation, the river channel and measurement sites are essential; nearby roads, tributaries or land use may help explain their setting. Unrelated buildings and place names can usually be left out.

Draw a frame with proportions matching the original. If you enlarge or reduce it, keep the change consistent in both directions. Lightly copy useful gridlines from a map, or divide a photograph and your drawing into four matching sections. These guides help you place features accurately: a site near the upper-right corner of one section should remain there in the sketch.

Draw the main framework first, then add the selected features in their correct relative positions. Use simple lines and symbols rather than elaborate detail. Finish with explanatory annotations, a precise title, orientation and a key for any symbols or shading. Where distances are represented accurately, include the appropriate scale. If the sketch is not to scale, say so rather than implying that distances can be measured from it.

Selecting features for a river fieldwork sketch

OCR's River Syfynwy example investigates downstream change, using measurements of channel depth, channel width and flow velocity at three sites. Site 1 is 1.15 km from the source, Site 2 is 1.24 km from the source, and Site 3 is 3.75 km from the source. Site 1 is furthest upstream.

A useful sketch would retain the river's course and place the three sites in the order 1 → 2 → 3 downstream. Sites 1 and 2 are close together compared with the gap to Site 3, so spacing them equally would misrepresent the investigation. The distances given are distances from the source, not straight-line distances to measure across the sketch.

Numbered site symbols could be accompanied by an annotation such as ‘Sites 1 and 2 provide closely spaced upstream measurements; Site 3 provides a comparison farther downstream’. This explains why the locations matter without inventing measurement results. On an actual OS extract, use the mapped channel and site positions to reproduce the layout; the site distances alone are not enough to reconstruct the river's bends.

Annotations should connect any map to its geographical meaning. ‘Widest line: most frequent bus connection’ explains a pattern; ‘bus route’ merely names a feature. A successful map lets the reader understand both the representation and the information it communicates.

Match the map to the purpose

• Atlas: wider geographical setting.
• OS: detailed landscape information; study 1:25 000 and 1:50 000.
• Base: framework for adding data.
• Thematic: one geographical subject.
• Choropleth: area values; ordered shading and a class key.
• Isoline: equal values joined by lines; interpolate between observations.
• Desire-line: direct connections and straight-line separation.
• Flow-line: connections plus quantity represented by proportional width.
• Sphere of influence: extent of influence or accessibility for a stated purpose.
• Route: connected path followed between places.
• Sketch: simplified layout of selected features.

Construction checks

Choose a suitable base and scale → plot information accurately → apply symbols, classes or widths consistently → add title, key, units, orientation and explanatory .