How to draw a radius on a map

What a radius on a map shows

A radius circle marks every place that lies a given distance from a centre point, measured in a straight line over the surface of the Earth. People draw one to see which towns are within 50 miles of a warehouse, how far a delivery or service area reaches, which schools are within a few kilometres of a new home, or what a radio, drone or emergency planning zone covers.

The distance is "as the crow flies". It is not a driving distance or a travel time: roads, rivers, mountains and borders are ignored. A place 20 miles away in a straight line can easily be 30 miles away by road. If you need travel time, a radius is a useful first sketch, but a route planner gives the real answer.

Draw the circle step by step

The radius map on GeoDrafter needs no account and no installation and works on a phone. Everything is calculated in your browser; the circles are kept in the page address, not on a server.

1. Open the radius map and find the centre, either by searching for a town or city or by panning and zooming the map.
2. Click or tap the map where the centre should be. A circle with the current radius appears.
3. Type the radius and choose the unit (metres, kilometres, miles, feet or nautical miles), or drag the edge of the circle until the readout shows the distance you want.
4. Click again to add more circles, for example 10, 25 and 50 miles around the same point, or one circle around each of several branches.
5. Copy the share link to send the map to someone, or download the circles as KML for Google Earth or GeoJSON for GIS software.

Why a circle on a web map is not a perfect circle

Almost every web map, including Google Maps and OpenStreetMap, uses the Web Mercator projection. It keeps shapes correct in small areas but stretches the map more and more towards the poles: at 60 degrees north, in Oslo, Helsinki or southern Alaska, one centimetre on the screen covers only half as much ground in each direction as at the equator.

A tool that draws a "flat" circle of a fixed size in map units therefore gets the radius wrong away from the equator, and the error grows with latitude. GeoDrafter instead walks the true distance out from the centre in 256 directions on the WGS84 ellipsoid, using the GeographicLib library, and joins those points. For small circles the result looks round. For very large circles, hundreds or thousands of kilometres across, it looks like an egg stretched towards the nearest pole, because that is what a true circle looks like on a Mercator map. The tool can show the flat Mercator circle next to it so you can see the difference.

How accurate is the radius?

The geodesic calculation itself is accurate to a few nanometres, far below anything a map can show. In practice the accuracy is limited by where you put the centre. A click at a low zoom level can be off by hundreds of metres, so zoom in before you place the centre, or paste exact coordinates if you have them. The circle edge is drawn as 256 straight segments, which is invisible at normal zoom levels.

The area of a circle on the curved Earth is slightly smaller than pi times the radius squared. For a 100 km radius the difference is about 0.002 percent; for a 1,000 km radius it is about 0.2 percent. GeoDrafter reports the area of the real, curved circle.

Units and conversions

A statute mile is exactly 1,609.344 metres, a nautical mile exactly 1,852 metres and an international foot exactly 0.3048 metres. So a 10 mile radius is 16.09 km and a 25 nautical mile radius is 46.3 km. Nautical miles are used at sea and in aviation; one nautical mile was originally one minute of latitude. If you need to measure an actual route rather than a radius, use the distance calculator, which adds up the legs of a line you draw.