When looking at a map, probably the most important thing to bear in mind is the map scale. This is the relationship between the dimensions on the paper or on the screen to the real distance on the ground. Multiplying the distance on the map by the scale factor gives you the real world dimension. For example, if a building is 13 m long in the real world and a map depicts this length as 13 mm, the scale is 1:1000. Multiplying the distance on the map by the scale factor gives you the real world dimension

The Ordnance Survey maps Britain at three base scales to produce the MasterMap Topography Layer which is used as the basemap for the cadastral map. The three base scales are -

1:1250

1:2500

1:10,000

Urban

1:1250 scale maps and the data derived from them have all been created using instrumental control and as a result the geometric accuracy is high and consistent.

Line width – 0.3m

Rural

The 1:2500 scale mapping has been produced over time in a number of different ways that have resulted in different accuracy within the series.

Line width – 0.6m

Mountain & Moorland

As for the 1:2500 scale mapping, 1:10,000 scale maps and their data have been produced by a number of different processes. However, more generalisation has been carried out at this scale.

Line width – 2.4m

OS large scale mapping coverage - scale of capture

Computerised mapping packages such as the Plan Creator enable you to zoom in and out on the map data as much as you like. This means that you cannot say that the map data has a particular scale. However, all topographic data has a scale of capture – that is the source data for the map was captured at a particular scale.

It is important to understand the source scale of your map data as each of the three large scale maps produced by OS have specific levels of accuracy.

See: https://getoutside.ordnancesurvey.co.uk/guides/understanding-map-scales/ for more information.

Generalisation

Cartographic Generalisation is the process of simplifying and improving the legibility of cartographic maps aiming to improve map legibility. Detail depicted within each scale is very much dependent on the ability of the surveyor to capture and represent distinct features on the map. Too many features portrayed at a small scale would clutter the map, making the final product unintelligible and impossible to use. To avoid such confusion, a series of rules were established by the OS to accompany each scale of mapping.

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