Map legends
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Ordnance SurveyData
OS Data HubAn important part of most maps is a legend (or key) which provides an explanation of the symbols on the map and helps the map user to understand and interpret the map. A well-designed, logically presented legend will help achieve effective map communication.
It is really important when putting together a legend that the size, shape, colour and orientation of symbols in the legend are identical to those on the map. In addition, the text explaining the symbol in the legend must be kept close to the symbol itself (or linked using a series of dots or a line) so that there is no doubt which symbol the descriptive text relates to.
Which symbols to include in the legend will depend on the purpose of the map. On general purpose topographic maps, such as the Ordnance Survey 1:25 000 and 1:50 000 series maps, all features are included in the legend. On thematic maps however, self-explanatory symbols forming part of the basemap can often be omitted from the legend. This might include omitting features such as rivers, roads and the coastline if they just provide background context and their meaning is clear and does not require further explanation. If you have any overlays of data, it is imperative that all symbology describing these are included in the legend to aid user understanding.
Think about the user and what they are going to use your map for. What might they want or need explained? If the user was seeing your map for the first time, would they understand what everything was? If any feature is key to the message, use or understanding of your map, it should be included in the legend.
Your legend needs to arrange logically so that it is quick and easy for a user to find the feature they are looking for. It is best practice to group related features together. For example, this might be placing all symbols related to railways together e.g. tracks, level crossings and stations or all tourist symbols together or all roads together, like we do for our 25k and 50k mapping series. Within these broad categories, symbols of the same symbol type (point, line or area) should be kept together. If text is used to describe features on a map e.g. a river name or a castle, use the same font type and colour in the legend. Headings and subheadings within the legend can be used to help users quickly locate the feature they’re looking for.
Another popular way of presenting information in a legend, beyond a standard list of features, is in the form of a natural legend where features are shown in context, in a simple map-like representation, showing the spatial relationship between features.
If your data is quantitative, ensure that the legend allows the user to relate the size or colour of features to its value. It is also important that you include the units in your legend as this is easily forgotten e.g. population per km2 or percentage of population.
Placing the legend on the page can be one of the hardest parts of map design – they can be quite extensive and take up lots of space. Often legends are included in boxes to one side or bottom of the map. However, this doesn’t have to be the case. Legends can just be placed in white space around your main map figure without a bounding box. Either way, the legend should be placed such that the map layout appears balanced. Read more about map layout here.
On web or mobile maps, where maximising map space is important due to small screen sizes, the legend may need to be hidden in a separate panel to maximise space for your map on the screen. It’s best to ensure that the legend is easy to access via a popup or side panel and that the icon or menu used to access the legend is clear as to what it’s going to do if clicked. A dynamic legend which only shows features which are visible to the user at each set zoom level could also be an option worth considering if space is limited.
