Some wonderful news for urban stat lovers! Ever since I've joined these forums, people have wondered whether the London metropolitan area was larger than the Paris metropolitan area or vice versa. Until now it was impossible to answer this question, because we didn't have enough data. Well, this is no more true.
ONS, the English statistical office, has published detailed cross-tabulations of commuting flow data at a very detailed level (the census tract level, or 'Output Area' in ONS parlance). It is thus now possible to calculate precisely the extent of the London metropolitan area by using INSEE's definition of metropolitan areas (INSEE, the French statistical office, is the only statistical office in Europe which has defined a rigorous scientific method to calculate metropolitan areas; like any definition, it is arbitrary, but at least it exists, and can be applied with a rigorous method).
First, you have to take the urban area, that is the contiguously built-up area with no more than 200-meter gaps between buildings. Then you add the communes (municipalities) which lie beyond the urban area but where more than 40% of the residents in employment commute to work in the urban area, then you add the communes where more than 40% of the residents in employment commute to work in the urban area or in the communes that you've just added, and you run several iterations until no more communes can be added. In the end you get the metropolitan area: an urban area surrounded by a commuter belt where more than 40% of the residents in employment commute to work either in the urban area or in the communes that are attracted by the urban area.
ONS uses the same definition of built-up areas as INSEE, i.e. no gaps larger than 200 meters (it's a UN recommendation). The problem is ONS uses the very small Output Areas as its grid for the built-up areas, whereas INSEE uses the French communes as the grid. As a result, ONS's built-up areas tend to be much more tightly delineated than INSEE's urban areas, because they do not include the unbuilt areas that are included in the French communes.
In order to apply INSEE's definition of metropolitan areas, we must use a grid in the UK which is about the same as in France. Thankfully... there is one. It's the so-called Middle Layer Super Output Area (MSOA). An MSOA is an aggregate of several Output Areas. Its size is roughly the same as that of a French commune.
So now we have everything to work on this.
What I've done so far (it's pretty time consuming) is I've defined the urban area of London. I've checked in which MSOAs lay the built-up area of London. If more than 50% of the population in the MSOA live inside Output Areas part of the built-up area of London, then that MSOA belongs to the urban area of London. INSEE does exactly the same with French communes to determine whether a commune belongs to a given urban area.
The results are already very, very interesting.
This is the built-up area of London (defined at the Output Area level):
- land area: 1,738 km²
- population (March 27, 2011 census): 9,787,426
And this is the London urban area (defined at the MSOA level, which is equal to French communes):
- land area: 2,928 km²
- population (March 27, 2011 census): 9,869,043
For comparison, this is the Paris urban area (defined at the commune level):
- land area: 2,845 km²
- population (January 1, 2011 census): 10,516,110
Several observations: first, we can see that the urban area of London is much larger than the built-up area, but its population is barely larger. This is not surprising, since the built-up area excludes all unbuilt areas, whereas the urban area includes the unbuilt areas within the MSOAs, just as is the case with French communes. The population of the urban area is marginally larger than that of the built-up area because a few built-up areas lying inside the MSOAs but which do not belong to the London built-up area now get integrated in the urban area (the same happens in the case of French urban areas).
#2: we can notice that despite the so-called "green belt", the urban area of London is actually slightly larger than the urban area of Paris. Interesting finding! In fact, having now checked the map in detail for hours as I've done, I've realized that this "green belt" is imperfect and exists only in places. In Hertfordshire, and above all in Surrey, the green belt seems not to exist, or at least to have been created AFTER sprawl had already entered these two counties (the green belt could prevent urbanization, but it could not de-urbanize what was already urbanized). In the end, it's fascinating to note that despite a "green belt", London's urban area sprawls as much as Paris which has no green belt. I can't really explain why Paris doesn't sprawl more compared to London when I think about it, since no green belt ever stopped urbanization. It's as if the central part of the Paris urban area was so dense already that it prevented the outer areas to sprawl too much.
#3: despite a slightly larger land area than Paris, London's urban area has a smaller population. Those of you who follow this thread already know that on a surface of 1,572 km² (the size of Greater London), Paris has more inhabitants than London. Now the interesting discovery is that on a surface of 2,900 km², Paris still has more inhabitants than London. This shows that the population density beyond those 1,572 km² in London is not so much higher than in Paris. In fact I've made the calculation: in London, outside of Greater London (almost all of Greater London is inside the urban area of London, except the village of Harefield), the population density of the rest of the urban area was 1,242 inh. per km² in 2011, whereas in Paris, outside of the central 1,572 km², the population density of the rest of the urban area was 1,069 inh. per km² in 2011. I would frankly not have expected that the density in those distant suburbs of Paris would be so close to the density in the distant suburbs of London!
A commonly held idea is that density drops dramatically in Paris the further out you go compared to London where it remains more equal, but it appears that even as far as 2,900 km² get you, Paris keeps a density almost as high as London. The Paris density drop probably occurs much further away (the real drop compared to London must take place 40 km from Paris, where you land in rural French countryside at 100 inh. per km² or less, whereas 40 km from London you land in dense England at 500 inh. per km²).
I will in the coming days run the several iterations needed to get the London metropolitan area. It's a long process as I need to cross-tabulate hundreds of MSOAs, pinpoint them on a map, etc. What I can say for now, having checked a few samples here and there, is that commuting rates in England are incredibly low. It seems many MSOAs will have a very hard time reaching the 40% threshold. For example Thurrock, which borders Greater London and lies just outside the London urban area, looks like it won't even make it in the metropolitan area (but I can't say yet for sure, because I'll know only after several iterations). So it's not certain at all that the London metropolitan area will be more populated than the Paris metropolitan area, contrary to what many people imagine, but we'll see!
In general, checking all these tables, I'm surprised to see that the English people seem to commute much less long distances than French people. This was confirmed to me today by two maps drawn to my attention by Minato Ku. James Cheshire has made a wonderful map showing the commuting flows between all the MSOAs of England & Wales at the 2011 census. Mathieu Garnier then used the same software and code to make a map showing the commuting flows between all the communes of Metropolitan France. I've scaled the maps at the same scale and pasted them together. The result is below. Absolutely fascinating!!
France is really a country of long-distance commuters. Probably because the country is much less dense than England, so the job market nodes are further apart, and people need to travel longer to reach them. High-speed TGV trains and planes explain some of the longest commutes (personally, when I was at the Sorbonne in Paris, one of my professors was living in Toulouse and commuted to Paris by plane during the week, using Air France's excellent air shuttle between Toulouse and Paris; there are more than 40 flights every day between Paris and Toulouse, from 6am to midnight).
For example at the 2011 census there were 5,310 residents from the Rhône department (Lyon metro area) who commuted to work in the Paris Region (Central Lyon and Central Paris are 393 km apart as the crow flies, and exactly 2 hours apart by high-speed train), i.e. 0.71% of the Rhône residents in employment. In comparison, if we take Newcastle, which lies 398 km from Central London as the crow flies, there were only 2,284 residents from the Tyne and Wear county (Newcastle metro area) who commuted to work in the London urban area at the 2011 census (0.46% of Tyne and Wear's residents in employment).
