Christchurch radius map: 5, 10, 25 and 50 kilometre circles

The inner circle has no GeoNames name anywhere on it, which tells you something about how Christchurch is built. Everything in between scales from those two.

Read the distances as point to point. GeoNames gives every settlement one coordinate, roughly central, and these figures are the gaps between those coordinates. A sprawling suburb whose centre lands at 10 km might well start at half that. Where the boundary actually matters, draw the circle and look at where the line crosses.

Place names, coordinates and populations come from GeoNames, which the site uses under CC BY 4.0.

Worth pausing on the areas. 79 km² inside 5 km. 314 inside 10. 7,854 by the time you reach 50. Each step looks modest on screen and is anything but on the ground, because the maths squares the radius while your eye just reads a line that got a bit wider.

Set the areas next to the place counts and Christchurch looks sparse, which is partly real and partly the dataset. 1,963 km² inside the 25 km circle, 5,890 km² more in the band beyond it, and almost no names attached to any of it. Use the circles for the ground they cover. For who lives on that ground, the table here can only undercount.

Straight line, not drive time, and the difference bites hardest on the big ring. Wellington comes out at 305.5 km from here as the crow flies, and nobody has ever driven it in a straight line. Use the circle as an outer bound. Treat anything sitting near the edge as optimistic, because real travel distance beats the radius every time and the gap grows with whatever the terrain throws in.

If travel time is genuinely what you need, use these rings to narrow the field first and check the survivors properly afterwards. The circle is cheap, instant, and honest about what it measures. It also can’t tell you that the 10 km to the north crosses water while the same 10 km to the south is dual carriageway the whole way.

Where it all comes from. Names, coordinates and populations are GeoNames cities15000, which covers settlements of roughly 15,000 people and up. Anything smaller is simply absent. City subdivisions get stripped out on purpose, so a page about Christchurch doesn’t fill with its own neighbourhoods instead of its neighbours. Distances are haversine on a 6,371.0088 km sphere, areas are spherical caps.

All of it was computed when the site was built, then written into a file that ships with the page. Nothing here is fetched while you read. That’s why the tables are in the HTML the server sends rather than appearing a second later, and it’s why the figures can’t drift: rerun the build and every number on all fifty of these pages is recalculated from the same dump.

Most of the traffic to a page like this is practical. Somebody wants to know which towns near Christchurch fall inside a service area, or how many people live within 25 kilometres of a site they’re weighing up. The table answers the first one outright. For the second, take the ring populations as a floor, since everything under 15,000 people is missing from them.