Boston radius map: 5, 10, 25 and 50 mile circles

Numbers on their own are hard to feel. Five miles from the centre of Boston gets you about as far as Medford, which the table puts at 4.8 mi north-north-west. Stretch it to 25 and you’re out past Lawrence, 24.7 mi north-north-west. Hold those in your head and the rest of the map scales itself.

One caution before you lean on the table. A GeoNames centre is a single point, usually somewhere near the historic core, and a city isn’t a point. Cambridge is listed at 2.6 mi because that’s the gap between two reference points. Its nearer edge is closer than that, probably by a good margin.

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

Here’s the thing people get wrong about a radius. Double it and you quadruple the ground underneath. Going from 5 miles to 50 multiplies the area by a hundred, so 79 mi² becomes 7,854 mi². The outermost band on its own, the gap between the 25 and 50 mi circles, accounts for 5,890 mi². Three quarters of everything inside the big circle, in one band.

Put the two tables together and you get a crude density. Inside the 25 mi circle there are about 3,749,910 listed people spread over 1,963 mi², so roughly 1,910 per mi². Crude is the right word. It counts GeoNames settlements only, so villages and open country vanish, and it treats a harbour or a reservoir as populated ground.

Straight line, not drive time, and the difference bites hardest on the big ring. New York City comes out at 190 mi 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.

Plenty of sites draw a plain circle, label it drive time and leave it there. Those are worth ignoring. A proper isochrone needs a road graph, turn restrictions and some idea of traffic, and it changes shape by the hour. What a circle gives you instead is a bound that doesn’t move: nothing outside the 25 mi ring is within 25 miles of Boston, whatever the roads do.

Two things about the method. Distance is measured over the curve of the Earth, not across the flat map image, which is where a surprising number of simpler tools quietly go wrong. And the Earth gets treated as a sphere when it’s really a bit squashed, costing a fraction of a per cent. Fine for a catchment. No use for settling a boundary, so don’t try.

One oddity you’ll notice if you compare pages. Population counts come from GeoNames, which records what the national statistics office last published, so some are a few years old and a couple are more. They’re good for ranking and for rough sizing. The figure carried for Boston is 653,833, which for some cities means the municipality and for others the whole urban area, so compare it with care.

What people actually use these rings for, roughly in order of how often it comes up: delivery and service areas, finding somewhere to live within a sensible distance of work, sizing a catchment for a shop or a clinic, checking how far an event’s noise and traffic will carry. Straight-line distance is the right first pass for all of them.