Marine coordinates: format converter and waypoints

An online coordinate converter built for the sea. It reads any notation — decimal degrees, degrees and decimal minutes, degrees-minutes-seconds, UTM, MGRS — and returns all of them at once, starting with degrees and decimal minutes, the format that is actually on the plotter and in the receiver. Alongside it: a map with a draggable pin, a waypoint list with GPX export, distances by rhumb line and great circle, and a phonetic readout of the position for passing over VHF.

How to use it

Paste the coordinates

In any notation. Or press "I am here", or simply click the map.

Take the format you need

All formats appear at once, each with its own copy button.

Build a route

Add waypoints, read the distances by both curves, and export the GPX.

Paste a position in any notation and get every format at once

Click the map or drag the pin — the coordinates follow. The route is drawn as a great circle, so it curves: a straight line on this projection would be a rhumb line.

Waypoints

Load GPX

A supporting calculation. Distances are computed on a sphere and allow for neither the flattening of the Earth nor, far more importantly, land and shoals: the line between two points here is geometry, not a ship's track. Use a corrected chart for passage planning.

Features

The format used at sea comes first

Degrees and decimal minutes — 55°45.350′N — are what a plotter, a GPS receiver and a DSC form use. Land converters lead with decimal degrees or seconds, so the position has to be rewritten by hand before it goes into the instrument.

It reads what you paste, it does not demand a form

It understands “55.7558 37.6173”, “55°45.35′N 037°37.04′E”, “N 55 45 21 E 037 37 02”, reversed order “E037 N55”, a comma for a decimal point, and MGRS. The parser is checked against eight different notations and three thousand random points through a round trip.

Rhumb line and great circle side by side

The constant course you actually steer is always longer than the shortest path, and on an ocean passage the difference runs to tens of miles. Both are shown with the difference between them — a land "bearing" does not distinguish them at all.

UTM and MGRS verified independently

The projection was cross-checked against the Krüger series — different mathematics, same result — and agreed to the millimeter, while the meridian arc matched numerical integration to a fraction of a millimeter. The round trip over three thousand points is better than five centimeters.

Waypoints and GPX both ways

A list you can rename and reorder, export to GPX for a plotter or receiver, and import back from someone else's file. The great circle is drawn point by point on the map: a straight line on a flat chart is a rhumb line, and showing it as the shortest path would simply be untrue.

A position ready for the radio

Spelled out in the international phonetic alphabet, including the maritime pronunciation of the digits. A position given letter by letter survives interference; one read out in ordinary words does not.

What you can do

Convert coordinates between every marine and land format
Parse a position written any way round, including reversed order
Find distance and course by rhumb line and by great circle
Build a route and export it as GPX for a plotter
Import someone else's GPX and read its distances
Get a position ready to pass over VHF

Why degrees, minutes and seconds are not used at sea

Because a minute of latitude is exactly one nautical mile, and a second is thirty meters — and both are awkward to reckon with. A navigator needs to judge distance in their head: twelve minutes of latitude is twelve miles, and that is obvious at a glance. With seconds the same sum needs a division by sixty. That is why the receiver, the plotter and the distress form all use degrees and decimal minutes. There is a second reason too: entry. On a plotter keypad you type exactly what is written, and one more conversion in your head, on a bridge, at night, in a seaway, is precisely where errors in position come from.

Frequently asked questions

What is the difference between a rhumb line and a great circle?

A rhumb line is a line of constant course: you steer one heading and arrive. A great circle is the shortest path across the surface, but the course along it changes continuously. The rhumb line is always longer; on a short passage the difference vanishes, on an ocean crossing it reaches tens of miles. That is why great-circle sailing is broken into a series of rhumb-line legs.

Why is the route line on the map curved?

Because it is a great circle and the chart is a Mercator projection. On it the shortest path appears as an arc, while a straight line is a rhumb line. Drawing the route straight and calling it the shortest path would simply be wrong.

What is MGRS and why is it on a marine tool?

A military grid based on UTM: a zone letter, two letters for a 100 km square and digits within it. It is not used at sea, but it turns up when working with shore authorities, aircraft and rescue services, and the position sometimes has to be converted in a hurry.

How accurate are UTM and MGRS here?

The forward and reverse conversions agree to better than five centimeters over three thousand random points worldwide. The projection was additionally cross-checked against an independent Krüger-series implementation and matched to the millimeter. An MGRS reference with five digits gives a meter, with three digits a hundred meters.

Where are the waypoints stored?

In this browser on this device, so the list survives a page reload. To get them onto a plotter, export the GPX — a standard format read by receivers, chartplotters and passage-planning software alike.

What is the phonetic version of the position for?

It is the letters and digits of the position spoken in the international phonetic alphabet. The digits have their own forms too: not "five" but "pantafive". Spoken that way a position carries through interference and through an accent, whereas ordinary digits read out on the air are confused constantly.

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Who is this tool for?

⚓ Sailors and yacht crews

Published 01 Sep 2026 Updated 06 Sep 2026 Author: The Supreme