# Symbology
Maps are graphic or symbolised representations of the real world. No map can show everything in the real world, as it would be really cluttered and confusing. As such, maps show just a **selection of features** dependent on the **purpose of the map** and the **intended audience**.
Once the data has been selected, based on the users’ requirements, symbolisation is the way in which that geographical data is classified and represented on a map. It is extremely important to the overall visual impact of the map, and how good the map is at meeting the needs of the user.
Geographical features and phenomena can generally be represented as **points**, **lines,** or **polygons**. **Text** may be included in the form of a label. Together, these are known as the ‘graphic elements.’ It’s a cartographer’s job to decide how each feature is represented. This will likely depend on the purpose of the map, and the map scale. For example, a river might be shown as a polygon on a map of a town centre, but as a simple line on a national topographic map. Similarly, a city might be shown as an area with a boundary full of buildings, car parks, and greenspaces, but in an atlas shown as a single point (more on generalisation can be found[ here](https://docs.os.uk/more-than-maps/geographic-data-visualisation/guide-to-cartography/generalisation)).
In order to make maps clear and legible, it’s important that the symbols are styled in a sensible way to make the map easy to understand. This is done by changing what is known in cartography as the ‘graphic variables.’ These include changing the **size**, **shape**, **colour**, **lightness**, **orientation,** and **pattern** of points, lines, and polygons. They can be applied in isolation, or used in combination depending on what you are trying to show. Text is then used to add meaning or context to the map, if required.
These graphic variables can be manipulated to help create a visual hierarchy within the map, ensuring that the key information (figure) stands out, whilst other information recedes down the visual plane and into the background (ground). This is commonly referred to as the figure-ground relationship. More information on creating an effective visual hierarchy and the figure-ground relationship can be found [here](https://docs.os.uk/more-than-maps/geographic-data-visualisation/guide-to-data-visualisation/geodataviz-design-principles).
## Points, Line and Polygons
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When designing point symbols, it is important to think about what they are representing. Points can be represented simply by a **geometric** shape such as a circle, square, or triangle. Or points can be represented by **conventional** symbols such as a red cross for a hospital or an ‘*i*’ for information point. Alternatively point symbols can be **mimetic** i.e. they look like the object they are representing, such as an aeroplane for an airport or a picnic bench for a picnic site. Conventional and mimetic symbols can be used to enhance a map and give it meaning, making certain features immediately identifiable to the user without them needing to refer to the legend. On OS maps, we commonly use conventional and mimetic point symbols.
Another thing to think about with point symbols is how their size can be varied to show the relative importance or size of a feature. For example, when using a point for a place, the symbol for the city could be made larger than that of the same symbol for a village. On thematic maps, points are often used to represent the occurrence of something – for example on a Dot Density map or a Proportional Symbol map. On the latter, the size of the symbol is varied in relation to a value. Read more about [thematic mapping techniques](https://docs.os.uk/more-than-maps/geographic-data-visualisation/guide-to-data-visualisation/thematic-mapping-techniques) here.
Extract of Ordnance Survey 1:25 000 Explorer Legend
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Lines can be varied in many ways, such as being **dotted** or dashed, and are often quite complex. Roads, for example, can be made up of a road casing with a coloured fill. Using colour can help add meaning to your map, particularly if conventional colours such as blue for water are used. Lines representing the same feature class (e.g. roads) can be varied in **colour** and **size** to reflect their relative importance. For example, a motorway would be shown to be wider than a minor road. These principles can be applied to thematic maps too, where lines can be varied in their colour, thickness or form (dotted, dashed etc.) to represent varying phenomena.
Ordnance Survey 1:25 000 Explorer Legend
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Polygons often form the background to a map but may be important symbols in their own right. Polygons can be filled with a solid colour or with a texture or pattern. The **colour** of polygons can be altered to give meaning to a map, particularity by using conventional colours, such as green for woodland, brown for mud, yellow for sand, and blue for water. **Patterns** such as trees or marsh symbols can be added to polygons to further aid user understanding and improve legibility for those who perhaps suffer from forms of colour vision deficiency. The **lightness** of polygons can also be altered to give a visual order to ordered or quantitative data, such as in a choropleth map where darker colours indicate a higher value or occurrence of something.
Ordnance Survey 1:25 000 Explorer Legend
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## **Text**
Text is often categorised as another type of map symbol and, if used effectively, can **add meaning** to a map and help users understand what they are looking at. However, poor use of text can detract from the map, making it cluttered and difficult to read.
More information about the effective use of text on maps and data visualisations can be found [here](https://docs.os.uk/more-than-maps/geographic-data-visualisation/guide-to-cartography/text-on-maps).
## **Making your map legible**
Designing your symbols in isolation may be easy but what makes the map effective is how all the symbols work together. Your map must be legible for effective communication to the user; this is one of our core [design principles](https://docs.os.uk/more-than-maps/geographic-data-visualisation/guide-to-data-visualisation/geodataviz-design-principles). This can be achieved by ensuring sufficient contrast between features, such as between text or points over a background fill colour. Insufficient contrast will make the points and text difficult to read. Contrast between features of the same type might also be important, such as making sure the colours used in a choropleth map are perceptually different. You can read more about use of colour and legibility [here](https://docs.os.uk/more-than-maps/geographic-data-visualisation/guide-to-data-visualisation/geodataviz-design-principles). Finally, features, particularly point and line symbols should be large enough for the user to see with ease but not so large that they begin to overpower the map or overlap with other features. For really effective communication and exploration of your map, it should be visually appealing with colours which work in harmony with each other (not jarring to look at) and a good visual hierarchy.
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