Map Radius Tool

What a map radius means

A radius is one direct surface distance from a chosen centre. Every point on the generated boundary is calculated at that same distance along a WGS 84 geodesic, the shortest path over the reference ellipsoid, rather than as a fixed number of screen pixels. GeographicLib’s geodesic documentation describes the direct problem used here: start with latitude, longitude, bearing and distance, then find the destination point.

This is often described as an “as-the-crow-flies” distance, but it is not a road journey. A river, mountain, restricted site or sparse road network can make a destination inside the circle much farther away in practice.

Mapbox’s isochrone documentation illustrates the distinction: isochrones use driving, walking or cycling profiles to model reachable areas. This tool does not perform routing or traffic analysis.

How the boundary is calculated

The entered distance is first converted to metres:

metres = entered metres
metres = kilometres × 1,000
metres = miles × 1,609.344
metres = feet × 0.3048

For successive bearings around the centre, the tool solves the WGS 84 direct geodesic problem:

boundary point θ = DirectWGS84(centre latitude, centre longitude, bearing θ, radius in metres)
θ = 0° … 360°

The final point closes the ring. GeoJSON uses WGS 84 decimal degrees and stores each position as longitude, latitude, as specified by IETF RFC 7946. That order differs from the latitude-then-longitude labels commonly shown in forms, so always check exported coordinates before using them elsewhere.

The exact international conversion is 1 mile = 1.609344 kilometres, documented by NIST. A 10-mile radius is therefore 16.09344 km, or 16,093.44 m; the boundary is 20 miles across through its centre. That still does not promise a 10-mile drive to every point inside it.

Accuracy, projection and responsible use

The boundary calculation follows the WGS 84 ellipsoid, while any web map is a flat picture of the curved Earth. The shape can therefore look stretched depending on latitude, zoom and projection. Coordinates from browser geolocation also carry real-world uncertainty and may be less precise indoors or when GPS is unavailable.

Use the result as a planning aid, not as:

• a drive-distance or travel-time guarantee;
• a legal service, school, voting or administrative boundary;
• a cadastral survey, property line or safety exclusion zone;
• proof that a location is accessible or eligible;
• a substitute for current routing, traffic or official geographic data.

For decisions with legal, safety or financial consequences, verify the location against the relevant authority or a qualified surveyor.

Privacy, sharing and map providers

Your centre and radius remain in storage for the current browser tab while you work. Device location is requested only after you choose the location action, and the browser controls the permission; the W3C Geolocation specification treats location as privacy-sensitive information.

The default schematic does not need third-party map tiles. If you choose Show OpenStreetMap background, the browser requests the visible tiles from OpenStreetMap infrastructure. Those requests disclose normal network information and the viewed tile area to that provider, and the map must retain visible OpenStreetMap attribution under the OpenStreetMap tile usage policy.

GeoJSON export creates a file containing the centre-related boundary coordinates. Sharing happens only after explicit confirmation and reveals the encoded centre and radius to the recipient and to any service used to transmit the link. Do not export or share confidential locations unless that disclosure is appropriate.