The National Library of Scotland (NLS) is one of the depository libraries of the United Kingdom. The NLS is in the process of releasing some of its stock of OS maps, stitched together into continuous maps.
Here are some predefined combinations of old-and-newer maps. You can make your own selections within the NLS site, and zoom-and-pan across the whole of Great Britain. The links open into new tabs.
1937 1:25,000 + OpenTopo map
1885 vs 1920 1 inch-1 mile
1892-1905 1:2,500 + Bing Satellite
OS maps are subject to Crown Copyright, which is for 50 years. The NLS has partnered with a Swiss company Klokan Technologies to publish the out-of-copyright maps online. Coverage is not yet complete, but progress is quite swift.
Watchpoints
- Maps are not snapshots of a single point in time. A given layer is a mosaic of sheets covering a range of years, each of which can contain areas with different revision dates
- You may find small errors in the geolocation with some of their earlier attempts – only a few hundred metres at worst. The mathematics involved is formidable and relies on using the correct datum (see Geolocation below).
Comparison layers
A small selection of the many online map providers is available to choose a comparator from. Providers may added or removed if their licencing models change.
- Bing (Microsoft) is a commercial provider, but free-to-use
- OpenStreetMap is the crowd-sourced mapping project, with some 250,000 people contributing globally. It is usually the most up-to-date map because it relies on people on the ground submitting changes. For instance, Stafferton Way was already showing on its opening day
- OpenTopoMap renders OpenStreetMap data in a very pleasing way, together with relief data released by NASA.
Geolocation
The conversion of paper maps is complex. Sheets need to be scanned to a high resolution, stripped of their borders, reprojected and joined into a continuous layer. Reprojection is required for two reasons:
- Both the OS and modern online maps use the Mercator system, but the OS maps use transverse Mercator with an artificial equator just south of Great Britain. The stretch is reduced as you move northwards – important because the OS is aiming to present a constant scale (e.g. 1 inch on the paper = 1 mile on the ground). This is the basis for the British National Grid
- Online maps use the WGS84 datum whereas the OS uses the OSGB36 datum. The datum is a model that adjusts for the fact that the Earth is an ellipsoid rather than a true sphere. It means that latitude/longitude coordinates can be several hundred metres different. GPS signals use the WGS84 datum, and it makes sense for online mappers to comply, but OSGB36 is well established for the small area of Great Britain (Ireland has its own datum and grid system). The OS updated its datum in 1936, and it will have taken time for the revisions to feed through.
