The chart illustrates the alignment of buildings in the OSM database, for the British Isles. The trace of each building was divided into line segments, and the orientation of each segment to due north was calculated. Then the lengths of segment were totalled according to their orientation, and the result plotted as shown above.
If buildings were arranged randomly you would expect the plot to be more-or-less circular: with roughly equal lengths of the building perimeter heading in every direction. Clearly life isn't as simple as that.
What the chart suggests is:
- A relatively high proportion of buildings are oriented along the main points of the compass: North-South and East-West. Hence the spikes at the top and bottom, left and right.
- There is a second, smaller, cluster of walls that run roughly 15 degrees (anti-clockwise) off the main points of the compass. Lets say EENE, WWSW
- There is a tendency to align buildings so that longer walls run roughly East-West and shorter walls run roughly North - South. Hence the oval shape in the chart, with the major axis running horizontally, and the minor axis running vertically.
The question is, why?
Specifically: I loaded an extract of OSM data covering the British Isles into a Postgres / Postgis database. I split ways that define buildings into individual line segments (the straight lines between each node). I calculated the orientation of each line segment using the ST_Azimuth function. All of this was based on the original WGS84 projection. I calculated the length of each segment by transforming the projection to OSGB 1936, and using the ST_Length function to get the length in metres. Totals were summed in the Postgres query, and I plotted the results in Excel.
For what it's worth, I've tried this with various man-made features (roads, hedges, etc). The east-west orientation seems pretty common. So far I've only found the spikes on the main points of the compass for buildings.