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Open Water Endpoint Distance Calculator

Estimate spherical distance between two latitude and longitude endpoints, with coordinate checks, explicit model limits and saved distance reports.

Use this result well

Inputs that matter
Start latitude, Start longitude, End latitude, End longitude
Output to expect
Spherical Endpoint Distance
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

Spherical surface distance

Coordinates become unit vectors u and v. The central angle is atan2 of the cross-product magnitude and dot product; multiplying by the fixed 6,371,000 m radius gives the shorter arc. This is not a chord through Earth or an ellipsoidal survey calculation.

Coordinates and display

Latitude ranges from −90 to 90 degrees; longitude from −180 to 180. Enter complete decimal degrees, with south and west negative. Outputs show up to three decimals; very small positive values use scientific notation. Input uncertainty remains separate from display precision.

Model reference

PROJ distinguishes spherical great-circle geodesics from ellipsoidal calculations: https://proj.org/en/stable/geodesic.html. This tool uses its own fixed-radius spherical arithmetic and does not call PROJ.

Updated: September 2026

Example Scenarios

Estimate the separation of two source coordinates using an explicit spherical model.

Check an equatorial arc, date-line crossing or coincident endpoints without treating them as a swimming course.

Common Mistakes to Avoid

Entering degrees, minutes and seconds in a decimal-degree field

Convert the coordinate notation first and preserve its hemisphere sign.

Using zero endpoint distance as a completed swim length

A route that returns to its start has zero endpoint distance; retain the actual track or accepted route distance separately.

FAQ

No. Two endpoints do not reconstruct a track or route. A swim returning to its start has zero endpoint distance even when the swimmer travelled a long way.

Endpoints at latitude 0 and longitudes 0 and 0.01 give about 1,111.949 m on the fixed 6,371 km sphere. The example is arithmetic, not a suggested swim.

Yes. Valid longitudes near +180 and −180 use the shorter spherical arc, not almost a full circumference.

A sphere and an ellipsoid are different models. Coordinate sources and route definitions may also differ. This output does not certify a course or survey.

Click Save endpoint distance to keep the valid result in this browser. Copy Markdown Report includes endpoints, model and distance details. Old automatic records remain separate from the versioned manual history.

About Open Water Endpoint Distance Calculator

Calculate the shorter spherical surface arc between two entered endpoints. Review the fixed-radius model and coordinate units before saving a result; two points cannot reconstruct a swimming route or GPS track.