Wednesday 12 May 2010

More OS data

I have written up notes about using OS shape files in the wiki here. I hope it's useful for anyone else who wants to use them.

I thought I'd take a look at a few of the other features that might be useful from the VectorMap data set. One thing that I glanced at before was the Settlement_Line. It seems to be electricity transmission lines, at least all the ones I've seen so far only contain this. Loading these power lines could be useful, but in OSM we mark the pylons as well as the lines. 50k OS maps mark the lines but they don't show the locations of the pylons, so you can only be sure there is one at corners and junctions, so I thought these might be the same.

I did the usual re-projection of a shape file for one of the 10x10 km areas close to home. I then modified a python script to extract all of the polylines in a shape file and ran it. When I loaded the osm file in JOSM I realised there are at least two types of power line. One way is in longish sections which are not straight. There is also another type; this is made of single sections between two nodes with a gap of more than ten metres between each section.

I think these are different types of power line. The continuous lines are lower voltage lines, with the gaps marking where the pylons are in a high voltage line. I need to go and look at the lines I have identified locally and see what I find. If I'm right it will take more work to join the sections of line together, since OSM only uses a single node to mark a pylon. It doesn't make the location of power poles on the low voltage clear yet either.

Power lines, especially high voltage power lines, can run large distances from power stations to towns and cities and the 10km squares that the data is supplied in breaks up the lines into annoying sections too.

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