On 11/10/23 16:22, Andrew Davidson wrote:
The first thing to keep in mind is how concentrated the AU population
is. Sydney and Melbourne both have 20% of the population living in
them. If we add on Brisbane we reach the 50% mark, which means the
majority of people live in one of three cities. As a result there is
not much to go around for the rest.
If we adopted 50,000 as the cutoff for a city we're going to more than
halve the number of currently mapped cities. 50,000 might work for the
US (and is also the value the UN has adopted for global comparisons)
but it's too big for AU. At the other end 15,000 is too small, we'd
end up creating an additional 25% of cities.
I would suggest that we adopt the ABS's threshold of 20,000. This is
the population level at which they consider a settlement starts to
have "gravity" and pulls in surrounding urban areas. It used to be
30,000 back in the early days of their methodology but I assume they
think people are more mobile so the "pull in" starts earlier now.
20,000 also has the benefit of not changing the number of cities we
have by much. 10 currently mapped cities would become towns and 13
current towns would become cities.
For towns the US threshold of 10,000 is way too crazy high. There are
1,000+ things currently mapped as towns. If we adopted 10,0000 this
would drop to 101. Even 5,000 would only get that to 198.
I was thinking that we would just use the ABS's UCL list. This divides
settlements into urban centres and urban localities. If a settlement
is on the urban centres list and its population is over 20,000, then
that's a city, otherwise it would be a town. In effect this is a
cutoff of 1,000, which the ABS has used for more than 50 years
suggesting that it's getting relatively smaller over time.
The urban localities would be villages (a lower cutoff of ~200) and
settlements not on the list hamlets.
Agree with the basics of the above...
The bigger shifts are going to be in the towns and villages. The UCL
has (using the rules above):
72 cities
657 towns
1080 villages
but we currently have 1,000+ towns and 1,800+ villages. It is hard to
be very precise, as these will include place nodes nested inside other
urban centres and localities.
I looked at the ratio of CTVs from the US/CA/NZ on the assumption that
being new world settlements the ratios should be similar. The 9 towns
for each city in AU is similar to the others 7/9/8. What is different
is the ratio of villages to towns. AU is 1.6 the others 2.4/4.0/2.3,
which suggests:
1. There are a lot of villages in CA
2. Settlements in AU are more thinly spread.
I'd expect some similar thin spread of settlements in CA too... but I
have not looked.
3. 200 might be too high. The problem being it is a lot of work to get
population numbers for places too small to register on the UCL.
I think that the population figures can be the 'best guide' but where
the population figure is not in OSM then a fall back on to the
facilities measurement would be a guide lacking anything else... however
unfortunate that maybe. If cities and towns all have OSM population
figures than it is only towns that need this fall back guide?
Oh .. hamlets ... I'd not tag a farm as a hamlet no matter how large it
may be. Farm populations fluctuate with activities, I'd just leave them
as farms.
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