Month: March 2010

The provision of free-for-any-use spatial data for the UK may be changing dramatically on April 1 with the anticipated release of some Ordnance Survey mid/small-scale mapping and data for unrestricted use.

Until then, the only Ordnance Survey maps that have been able to be used to derive data from for the OpenStreetMap project in its bid to complete coverage in the UK, are those that are more than 50 years old and therefore not protected by Crown Copyright.

Scans of such maps, generally the New Popular Edition from the 1940s/50s, have been available for some time now – the scans were originally done for the NPEmap project (gathering copyright-free postcodes by asking people to geolocate them to the maps) and then were orthogonalised and reprojected from the British National Grid into the “Web Mercator” projection used by Google Maps, the “standard” OpenStreetMap website and numerous other online “slippy” maps.

Scotland wasn’t included at the time, partly because the maps were sourced differently. After consultation with the owners of the scans, I’ve taken the imagery, rectified, reprojected, stitched and tiled it using TimSC’s warp-gbos tools, so that it can now be used to trace Scottish data.

The source maps are at 1:63360 scale (one inch to a mile) and typically there remains an error of around 10m – up to 30m in some places, and of course more where the source map was drawn slightly incorrectly. So the imagery isn’t particularly useful for detailed mapping – particularly in cities which have seen significant growth in the last 50-80 years since the maps were made. However they are very useful for completing the rural road network, town and village names and natural features (hills, rivers) that are unlikely to have changed significantly recently – the low-res Landsat imagery can be used to show the presence of more recent forests or reservoirs.

In the above example, it would not be unreasonable to assume the path here still exists, and add it in – citing the source. You could also add the cliff, and reshape the lochs slightly from their current jagged Landsat-derived image. Adding in names for the hills and the lochs are also a definite benefit, although these old maps tend to have the Anglicised versions of names – newer editions of OS mapping for the Highlands of Scotland tend to use more authentic Scottish-Gaelic names. (Not an issue in this particular example, just north of Slioch in the Great Wilderness, as all the names on the map are in Gaelic.)

You can see the map tiles while editing OpenStreetMap data in Potlatch for Scotland, by selecting “UK Historic: Scotland” in the Potlatch preferences pane. They are available to zoom 15, although zooms 14/15 are simply smoothly interpolating pixels. The tiles are hosted on a computer under my desk here at UCL. There are better quality and higher resolution scans of more recent maps for some parts of Scotland, information about these is . The main advantage of the Scottish Popular Edition tiles over using just these they offer substantially complete coverage of Scotland.

(Strictly, some of the areas are using newer mapping which is still more than 50-years old, but is part of more recent editions than the Scottish Popular Edition).

So, the main benefit – having a map with full coverage of Scotland which can be traced from for OpenStreetMap – may be diminished, if a useable equivalent appears shortly which is only a few years out of date (i.e. Ordnance Survey Landranger/Explorer), but for now, they are still a useful source of base data.

If you just want to look at the mapping rather than editing them, you can see them here on my own site, or here on the NPEmap project.

The OSM wiki has a list of UK Mapping Priority areas, where there are a presumed substantial number of missing roads which need to be mapped. These can then get mapped at a mapping party. The probable introduction of a freely-derivable “complete” set of Ordnance Survey road data in ten days time may mitigate the need to do such first-step mapping, but for now there are many areas in the UK which could do with some on-the-ground data gathering.

As an alternative way of identifying such areas, I developed the “Lonely Buses” map. As an extension to this, I’ve now created a “top 50” list of areas that have the most number of bus-stops away from roads, i.e. areas with probable missing roads. You can see this list here. A basic form of reverse geo-coding is used to assign a name to each tile.

The list is automatically updated weekly. It is flawed in that it considers each “tile” exclusively, so if a town is split in half by tile edges, it will appear twice and lower down the list, compared to a similarly incomplete town being in the middle of its respective tile.

Looks like the Newcastle area is the No. 1 target area (within areas which have the government bus-stop dataset imported.)

Andover has a mapping party planned for mid-May and is currently No. 39 on the list. If you are in the area, sign up!

Contributors to OpenStreetMap in the UK have been gradually importing in the NaPTAN dataset of the locations of bus stops (amongst other things).

Many, but not all, of the local authority areas, have now had their bus stops imported in to the OpenStreetMap database in this way. These, and the other areas, also have bus stops shown that have been manually added by contributors based on ground observations.

In areas where the roads have not been added into OSM (and there are still plenty such areas, where there is no accurate aerial imagery to trace from, or contributors on the ground with GPS) the bus stops will appear disconnected from any road on the map. Large numbers of these “orphaned” bus stops in an area are indicative that key roads or even whole villages and towns are missing from the map.

To aid discovery of such roads and towns, I’ve created a simple visualisation. Bus stops appear as green dots if they are within around 20m of any highway (this does include cycle paths, paths and railways, but in general if these features are present, nearby roads would be too). Stops that are further away are shown as red dots.

The visualisation can be seen here. The mapnik stylesheet used is here (XML) – Mapnik 0.7.0+ is required because of the !bbox! usage, and the generic entity stylesheets used in OSM/Mapnik are also referenced.

The tiles are generated directly from my copy of the OpenStreetMap “planet” database, using Mapnik to call PostGIS with a spatial query. The process of calculating the bus stop statuses is reasonably CPU-intensive – for each bus stop, a buffer around the stop is created and then line-in-polygon tests are done on all roads in the area using a spatial index. PostGIS makes the process straightforward – ST_DWithin is used for the buffering, and ST_Intersects is used separately on the stops and highways to restrict the search to the local area.

To reduce the complexity of the query, the cut-off distance actually varies slightly – the coordinate reference system used is “metres” in EPSG 900913 – the so-called “Google” projection. The “metres” are only 1m long at the equator – away from it, they are shortened by a factor of cosine of the latitude, so for UK latitudes, 30 EPSG 900913 metres are equivalent to between 14 and 20m, decreasing as you travel north.

The results show some key towns in need of more roads, such as Cleethorpes, Kidderminster and Thetford, while much of the town-to-town network in SE England is pretty good. The process does assume the bus stop locations are correct, which is not necessarily the case, and also for very wide roads, the bus stops in real life may be further away than 20m from the road centreline, and so incorrectly shown on the visualisation.

Basingstoke is looking pretty good:

But nearby Andover needs more mapping:

In places, the density and linearity of the orphaned bus-stops means that, in theory, a road could be added to OSM based solely on the pattern of stops in these places.

Try it out here [now offline].