Scale
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Ordnance SurveyData
OS Data HubAll maps are reduced (scaled down) versions of the real world, but what scale to use will depend on the purpose of the map and the user’s requirements, relating to what features need to be included, the size of the area of interest and the paper (or screen) size. Also, not all maps need, or should have, a scale bar as we’ll discuss below.
Scale is represented as a ratio of the map distance to the ground distance. For example, a scale of 1:100 means that one unit on the map represents 100 of the same units on the ground. Let’s say a road is 10,000cm long (100m) and you want to represent the same road as 10cm on the map, the scale can be calculated by dividing the real-world distance by the map distance. In this case giving 1,000. The scale is therefore 1:1,000 with 1cm on the map representing 1,000cm on the ground.
1:10,000 where 1cm on the map = 10,000cm on the ground (100m)
1:25,000 where 1cm on the map = 25,000cm on the ground (250m)
1:50,000 where 1cm on the map = 50,000cm on the ground (500m)
The scale of a map is often represented by a ratio (e.g. 1:10,000) or as a statement in words such as ‘one inch to one mile’ but is also often represented by a scale bar, allowing users to visually assess distances on the map. A statement in words or ratio only hold true for a printed map if the print size is controlled. Any map that is enlarged or shrunk when printed will render the statement and ratio incorrect.
The choice of map scale is intrinsically linked to the purpose of the map and what it needs to show, to meet the users’ requirements. For example, if you are planning a driving route around Scotland, you’ll want a map which shows a large area (possibly the whole of Scotland) but don’t need details of every building, bus stop, and café. This is known as a small-scale map. On the other hand, if you are making a map for a walking tour around a village, you would want a map of a small area but with lots of detail. This is known as a large-scale map.
To explain small scale and large scale a bit better and help you get your head around it:
Small scale maps require a small sheet of paper to show an area with a smaller amount of detail.
Large scale maps require a larger sheet of paper for the same area and show a much larger amount of detail.
As mentioned earlier, scale has a huge impact on what features we show and how the same feature might be depicted differently at different scales. For example, take an airport: on a large-scale map, you would show the airport boundary, runway detail, buildings, car parks and more. However, on a small-scale map, you would perhaps only show a single point symbol. Linked to scale is the process of generalisation which is the process a cartographer (or more recently, a computer algorithm) undertakes to alter the way in which features are represented in order to maintain legibility as the map scale decreases.
Explore map scales further here: A beginners guide to understanding map scales
Not all maps need a scale bar, particularly if it is of a familiar area or is at the continent or global scale. The distortion to distance caused by a map projection means a scale bar (or ratio, or statement in words) placed on a map covering large regions or countries may only be correct for one line on the map. On maps at these scales, scale bars are either omitted, a statement is made that the stated scale is only correct along a certain latitude or longitude (such as the equator), or a variable scale bar could be used which shows graphically how scale varies across the map. Scale bars are also sometimes omitted from thematic maps where scale is not important to the message or theme of the map. As a general rule however, scale should be indicated on most large scale maps to aid user understanding.
Digital zoomable maps are slightly different as the scale varies dependent on the level of zoom. You also don’t know what screen size your user will be viewing your map on – it could be a phone or it could be a wide-screen monitor. As such, a statement that 1cm on the map = 25,000cm on the ground won’t work. As such, on digital maps, scale bars are the best way to indicate scale. On a slightly separate note, when designing a zoomable map, think carefully about what features show at each zoom level, taking care not to clutter your map.
Scale bars should be designed so that they are simple and easy to read and understand. Their design, in terms of the look and feel and complexity, should be in keeping with the design of your overall map. For example, a simple locator map probably only needs a simple scale bar. It is up to the cartographer as to whether the scale bar is metric or imperial, or includes both distance conventions. It is general practice to place the distance values above the scale bar, ensuring that the maximum illustrated values are round e.g. 1000m to allow easy halving and distance estimation. A distance of 973m is hard work with – what’s half of 973? Or even worse….. 753.3m. Including various intervals can also help users to estimate distances.
Similar to north arrows and data sources, scale bars should be subtle and should not attract too much attention. They should appear relatively low in the visual hierarchy of the map but should be easy to locate and use if required by the user. This can be achieved by keeping the linework relatively fine and the text relatively small (but still readable). It is best practice to place the scale bar somewhere close to the mapped area (but not a hard-fast rule), where the user can easily use it to relate the scale bar to distances on the map. Generally, it is placed below the mapped area, where the user is likely to be accustomed to finding it but is often also placed with the title or the legend. Take care when placing a scale bar if you’ve got inset maps, making sure that the scale bar doesn’t look too much like it relates to the inset map. Your inset map may need its own scale bar.
