UK Election maps
We had some maps from our political pundit this week. The guys in News found the first a bit big for reading into Flash, the second had the points mashed up a bit. The same problem that we had a few years ago and when we decided to correct it all manually.
I decided this time to have a look at the Governments Open Data initiative which should give us the map data free. I downloaded from https://www.ordnancesurvey.co.uk/opendatadownload/products.html where you can also get map image data and postcode information. Downloaded the Boundary Line ESRI GB data (10/2010), about 270Mb. Also looked at the OS Vector District Map which gave a raster form for each OS square. The point data was huge and with all the errors of the previous dataset so I had a look at the MapPublisher plugin from Avenza to help clean it up.
The data comes as several layers in an ESRI format. The MapPublisher software is a plugin for Illustrator to help import and manipulate these layers. They have a 14 day trial. It was quite successful but there are many layers and I eventually crashed with a lack of memory. I had to restart Illustrator and try to do one layer at a time. I simplified as I went and that function works really well.
The layers included,
- county electoral division region
- county region
- district borough unitary region
- district borough unitary ward region
- european region
- greater london const region
- high water polyline
- parish region
- scotland and wales const region
- scotland and wales region
- unitary electoral divisdion region
- westminster const region
high water polyline – basically the coastline but with any river tidal areas, so lines along rivers for quite a way inland.
Downloaded the Scottish Parliament Constituencies and Regions 2010 – really detailed even though the MapPublisher plugin said it would have to simplify it. Tried a print, but it took ages, gave up eventually. Then tried using the MapPublisher simplify lines tool, with a 1.0 parameter which took a million points down to less than 10k. Impressive. Need to check the edges to make sure that the region and constituency are the same shape. Some areas still seemed to have concentrated points though (?). Helped to do some constituencies individually. But crashed on save (it saved it though), so try again and save often.
Started again with Scottish constituencies and this time only used a value of 0.25 to get more detail, with the idea of using this as a detailed reference layer, that I can put simplified layers on top of. This should then show up any potential inaccuracies. Went from 1.9M pts to 54k, still quite detailed. Then imported the regions as a layer and did the same simplify, 1M points to 17.6k. I only use the Simple import option as they are all from the same source, but the advanced will give options to match and adjust scaling of different data. Some gaps can be seen between layers, so looks like a manual check for each constituency.
Now to retry the full UK map. Started with the Westminster layer. This seems to cover the whole UK, but not Northern Ireland. Very slow to start simplifying lines – maybe 20mins building topology. It had 3.8M points, which with a 0.25 proximity value, reduced to 41k. I decided in the end to use 0.1 and that gave 86k points. The should be good for a reference layer. The original map data has certain idiosyncracies, eg at coastal spots such as the Bristol Channel, the constituency covers part of the river area (Lundy island?) and appears as an unexpected square ended piece of land. We need to check for all these. Not a bad job though. Compares well against the MapInfo package that I’ve used before. It does need Adobe Illustrator as well though, so it’s not a cheaper option.
Unitary areas are the underlying wards. Regions are groupings of electoral constituencies. When several of these layers are visible Illustrator has problems doing a Preview on my machine – not enough memory probably. Greater London constituency regions were brought in at 0.01 proximity. County Electoral division (wards?) at 0.05. County regions are the counties so just reduce a little, 0.05.
(If you do need a copy, it would cost you just a day at my daily rate.)