Ordnance survey boundaries

The UK Ordnance Survey has generously released some of its data for unlimited use. One of the data sets is about boundaries and included in it are parish boundaries. Boundaries are not painted on the ground so adding them to OSM is hard. Some people have added some parish boundaries, or bits of them, from the out-of-copyright NPE maps from OS, but a lot has changed in over sixty years.

The OS boundary data comes in the form of ESRI: shape files, about 179MB to make up the parish boundaries. I loaded it in QGIS to take a look at it - there where about 14,000 polygons. Dealing with this is not going to be a small task.

I dug about T'Internet to work out how shape files work and after a bit of work I've extracted a list of the polygons in the shape file, then a method to create an OSM file for each polygon suitable for loading into JOSM.

I loaded the polygon for my village and at first sight it looks good, but when I compared it to a real OS map, the parish boundary doesn't quite match - I've not transformed the shape file from its original OS projection to the OSM projection correctly.

This is  not the only problem of course. If I load a single parish boundary that's fairly easy. If I want to load an adjacent boundary some of the nodes need to be shared nodes along parts of the boundary. In addition the parish boundaries will share nodes with the county boundaries too. There are district boundaries available too, so these will also share nodes. On top of this there is the problem of dealing with the existing data that people have already loaded which might need to be merged, deleted or tagged as an historic boundary. This alone makes it important that each boundary is processed by someone who can make decisions about what to merge etc.

I'm going to sort out the projection issue, then import a single boundary via JOSM and see what that looks like. If that goes well I'll import an adjacent boundary and see what it takes to merge the shared nodes.

If I can make this work well, I'll write up a process and consider how to offer parish boundary files to other people for them to import in their local area, but supplying 14,000 files might be too big a task. Then there are the other boundaries types too ...