Base Scales of the Ordnance Survey Map
Ordnance Survey - Large Scale Mapping
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.
The map scale affects the relative size and accuracy of features, as demonstrated below:
The Ordnance Survey basemap detail used by RoS is captured and designed for display at three standard survey scales.
1:1250 | 1:2500 | 1:10,000 |
|---|---|---|
Urban 1:1250 scale is primarily used for urban areas, new developments, and places where there is a high concentration of man-made features such as cities or towns. The map data will have been created using survey instruments, gps, etc, and as a result the geometric accuracy is high and consistent. Real world width of line on map when viewed at base scale – 0.3m | Rural 1:2500 scale is used in more rural areas where there is still a reasonable level of man-made features. These areas can include smaller towns, villages and properties surrounded by larger tracts of land or farmland. The map data will have been captured over time using a number of different data capture methods that has resulted in a wider accuracy range. Real world width of line on map when viewed at base scale – 0.6m | Mountain & Moorland 1:10,000 scale is used in more remote areas with fewer man-made features. These areas include mountains and moorland, isolated farms and estates, and areas that are largely uninhabited. As with the 1:2500 scale mapping, 1:10,000 scale maps and their data have been produced by a number of different processes over a period of time. However, more generalisation of features has been carried out at this scale, and the accuracy range is wider again. Real world width of line on map when viewed at base scale – 2.4m |
|
|
|
OS large scale mapping - scale of capture
Digital mapping packages and systems such as the Plan Creator, Plan Viewer and ScotLIS enable the viewer to zoom in and out on the map data as much as they like. This means that you cannot say that the map data is always displayed at a particular scale, and we have no control over the way users choose to view it. However, all the topographic data displayed on these maps has a scale of capture – that is the scale that the source data for the map was collected or measured at. It is important to understand the source or base 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.
Coverage
In the past, the OS applied the same survey scale to whole map tiles in LandLine (map tiles were square areas based on the National Grid and measured 1km x 1km at 1:2,500 scale), meaning that the whole square of map had the same levels of detail and accuracy. This has changed since the transition to the OS MasterMap product, and now new map data is captured in “spec regions” where the most appropriate survey scale is used. The result is that there may be a blend of features within the same area that were captured at different survey scales.
The Plan Creator’s Check Information tool allows the plans officer to check the survey capture detail of each feature on the OS base map. The Accuracy figuren given allows us to quickly work out the capture scale of the features in that area of the map. This is very useful information as we often have to make mapping decisions based around the features on the OS map. The accuracy, survey scale, size, age and type of feature can all have an impact on our decisions.
Generalisation
Cartographic generalisation is the process of simplifying and improving the legibility of maps. The amount of detail depicted within each base scale is very much dependent on the ability of the surveyor and cartographer to capture and then represent distinct features on the map. Too many features portrayed at a smaller 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.
Burns & streams
On OS maps, rivers, streams and burns will by shown as either a double-banked feature or as a single-line feature, depending on the width of the water feature and the base scale of the map.
1:1,250 scale map At this scale, burns and streams wider than approximately 1m are shown as double-banks features, and are surveyed in as much detail and with the same degree of accuracy as the man-made features on the map. |
| |
1:2,500 scale map At this scale, burns and streams will be shown as double-banked features when they are approximately 2m wide or more, while this changes and is simplified to a single line feature when they are narrower than 2m. |
|
|
1:10,000 scale map At this smaller scale, the depiction of streams and burns is subject to more generalisation and simplification, but should still comply with the stated accuracy of the base map. Here, the burn is shown as a single line feature and it does not show every small twist and turn of the watercourse as these would be too small to see when viewed at 1:10,000. |
|
|
Parallel features
When we have parallel features that lie close together, these will be represented differently at the three base scales. To try to show parallel features separately in smaller scale maps would sometimes involve displacing the features on either side to “make room for them” and this is generally avoided where possible. Instead, the map is simplified or generalised, with only one or two of the physical features represented on the map.
Physical features in the real world - a stream with a fence to the north and a stone wall to the south. |
|
1:1,250 scale map While the physical features are quite close together, at 1:1,250 scale it is still possible to represent all three of them separately, and also to show the double banks of the stream. |
|
1:2,500 scale map At this scale, the stream is shown as a single line feature as it is too narrow to show both banks clearly. In addition, the stream and fence to the north are too close together to show separately so the map has been simplified or generalised so that it shows only one feature, the stream. |
|
1:10,000 scale map At this smaller scale, the stream is shown as a single line feature. Neither the fence or stone wall can be shown clearly so the map has been simplified or generalised so that it shows only one feature, the stream. |
|
Showing buildings
Again, at the different base scales, there are differences in the way buildings are represented on the OS map. At the larger scales, more detail on building shape can be shown and seen, but at smaller scales the building shape has to be simplified as the small detail can’t be seen when viewed at base scale. Wherever possible, the OS retain the orientation and position of the building.
1:1,250 scale map At this larger scale, buildings are shown in quite a lot of detail, with smaller juts, indents and window bays often shown. Where parallel boundary features are more than 1m away from the building, these will be shown as separate features. | |
|
1:2,500 scale map At this scale, the shape of buildings is shown accurately but without the smaller juts or indents. Parallel boundary features that are less than 2m away will likely be generalised or simplified into a single feature. |
|
|
1:10,000 scale map At this smaller scale, the depiction of buildings is subject to more generalisation and simplification, but should still comply with the stated accuracy of the base map. Here, buildings are shown more indicatively, often as rectangles as juts and indents will normally be within the tolerances of the survey scale and would not be visible when viewed at 1:10,000. | |
|