I’m quite interested in the debate going on in Auckland at the moment about whether population growth should be accommodated by more “sprawl” or increased density. In particular, people claim Auckland currently has quite a lot of sprawl and the vision set out in the Auckland Plan (and implemented in the unitary plan) is that we will move towards a “compact city” (i.e. increased density). A related discussion is what type of transport investments we should prioritize to accommodate that growth (i.e. roading vs public transport/cycling infrastructure).
I was therefore interested to see the New Zealand Institute attempting to debunk the myth that Auckland has a lot of sprawl by posting the following graph, in their very useful “graph of the week” series
On it’s face, this is a bit of a home run against the argument that Auckland is a sprawling city. Though having been to a few of these cities, the relative rankings started to worry me a bit. For example, Auckland has the same density as Los Angeles and both are apparently more dense than New York.
This illustrates why you always need to look at how statistics are defined, so that you know whether the number is actually answering the question you are asking (which brings us back to the transport/cost of living debate). Which is why I found this follow up post by the NZI very interesting. They note that for New York, the US census figures give a density five times higher than the Demographia figures (used for the graph), but the discrepancy can be explained as follows
Indeed, the 2010 US census put New York’s population density at 26,953 people per square mile. That’s five times denser that the statistics published by Demographia.
The difference is easily explained. The US census measures population density inside New York’s metropolitan urban limit – the formal boundaries of the bustling and chaotic City that Never Sleeps. This is an area of 303 square miles (488 square kilometres).
Demographia measures New York’s population density over a much larger area – an area of 4,406 square miles (7,091 square kilometres). It uses techniques like satellite photos of suburban lights on a clear night to assess the extent of urban sprawl beyond the metropolitan limit.
They then go on to argue that the Demogrpahia measure is more appropriate for assessing “sprawl”
If the purpose is to assess the extent of urban sprawl, it does not make sense to stop at the city limit if this is not where the sprawl ends. Using photos of the lit-up areas at night to gauge where the built-up areas end makes sense.
That is why the Graph of the Week focused on Demographia’s measure of density. It shows that Auckland’s population density must be greater in the outskirts of Auckland than in the outskirts of New York’s metropolitan limit [emphasis added]
Which brings me back to the point of what question we are trying to answer…what is “sprawl” and why do we care about it? What the two numbers illustrate to me is that New York has a very uneven distribution of population density (i.e. a very concentrated “core”). Whereas Auckland might have a more uniform distribution of population density (I don’t think I can make any firm conclusions on this without more data).
So the question is, when considering “sprawl” in the context of “an ever-growing population, increasing fuel costs and environmental concerns” as the NZI note in the opening paragraph of the linked article, what is the right measure? Both probably matter, but my hunch is that the distribution of density may be more important than the aggregate average, since it tell us what the “core” of the city looks like. A big city (geographic and population wise) with a concentrated core and very few people living on the periphery will have very different problems to a city of the same size that distributes its population evenly over the same area.
Update: The very knowledgeable folks over at Auckland Transport Blog have pointed out the concept of “weighted density” via twitter. This measures captures what I was talking about above. If you’re interested, check out a post they have done on the topic here as well as some Aussie stuff here and here. I also found this article in my follow googling quite useful.