Monday, 29 April 2013
I'm inclined to think that the most plausible explanations are that the "spikes" are artefacts, and that most of the effect I am seeing are due to aligning buildings so they capture the sun.
However, I'm not sure it can be as simple as that.
Here is a similar plot based on the alignment of different types of road in the British Isles. The characteristics are similar to buildings: a bias towards East-West orientation, and spikes at the main points. There also seems to be quite a strong tendency for UK mappers to draw roads from West to East, which is quite interesting, but not particularly important. You wouldn't expect the same effect for buildings of course - because they need to be traced all the way round.
We could imagine the travel of the sun affecting both roads and buildings. There might be a link between the alignment of buildings and residential roads, and some other built up roads for example. But I'm not sure this would explain why all the different road types show the same effect.
The bottom line is that I can't convince myself that the travel of the sun is sufficient explanation.
My other concern is that different map projections are having an effect on my calculation of way lengths. But at the moment I can't see what the problem could be. This time I've used ST_Distance_Sphere to calculate the distance between end points of each straight line, instead of transforming coordinates to OSGB and using ST_Length. I haven't tried ST_Distance_Spheroid yet, but perhaps I should. I understand it is more accurate, but much slower. And surely it wouldn't make enough difference to have this much effect. Or would it?
Posted by gom1 at 14:31