Radius map
Draw a true circle of any radius around any point. Computed in your browser; the map itself is optional.
Set a centre and a radius and this draws the circle, measures the ground it covers, and gives you the four compass points on its edge.
What "radius" means on a map
A circle drawn on a normal web map is not a circle on the ground. Web maps stretch east–west as you move away from the equator — by a factor of about two at 60° north — so a shape that looks round on screen covers far less ground east–west than it appears to.
This draws the real thing: every point on the edge is genuinely the distance you asked for from the centre, measured across the curve of the Earth. On a map that far north it will look like an ellipse, and that is what a true circle looks like there.
What you get
- The area covered, as a spherical cap rather than a flat disc. At neighbourhood scale the two agree; at 3,000 miles the flat formula is out by about 5%.
- The bounding box — and it is not the four compass points. The east–west extreme of a circle on a sphere sits slightly nearer the pole than the centre's own latitude, so a map fitted to the compass points alone crops the shape it is showing.
- The four compass points on the edge, with coordinates you can paste anywhere.
How accurate this is
The geometry is great-circle on a sphere of the Earth's mean radius. Against a rigorous ellipsoidal calculation it is out by up to about half a percent — roughly 80 metres on a ten-mile radius.
That is far smaller than the uncertainty in the centre. A browser location fix is good to tens of metres at best and hundreds on a desktop, and a pin you drop by hand is worse. A more elaborate formula would add precision the input does not have, so the model is stated rather than hidden.
Questions
What does a radius on a map actually mean?
It is the set of points a fixed distance from a centre, measured across the curve of the Earth. That is not the same as a circle drawn on the screen of a web map, which is stretched east-west more and more the further you are from the equator.
Does it use my location?
Only if you press "Use my location", and only to fill in the centre coordinates. They stay in the page — the circle is computed here, not on a server, so your position is never part of any request.
Do I have to load the map?
No. The circle, the area, the bounding box and the compass points are all computed and drawn without it, and the diagram above loads nothing from anywhere. The tiled map is an extra behind its own button, and if it fails to load everything else keeps working.
Why does the circle look like an oval on the map?
Because it is a real circle. Web maps stretch east-west as you move away from the equator — at 60° north by about a factor of two — so a shape that covers equal ground in every direction has to be drawn wider than it is tall. A shape that looks round that far north is covering much less ground east-west than it appears to.