Coordinate Converter

How the Coordinate Converter works

A coordinate is only useful if the person or device receiving it reads it the same way you wrote it. Search and rescue teams, surveyors, pilots, sailors, geocachers and anyone filling in a GIS spreadsheet run into the same problem: one source gives 51°30'27"N, the next wants 51.5075, and a printed topographic map wants a grid reference. A digit in the wrong place or a dropped minus sign moves the point kilometres, sometimes continents. All the formats below except UTM are the same latitude and longitude written differently; UTM is a genuinely different grid.

The four formats

Decimal degrees (DD) is what GPS apps, GeoJSON, KML, Google Maps URLs and spreadsheets use: latitude first, negative for south and west. London is 51.507400, -0.127800.

Degrees, minutes, seconds (DMS) is the traditional form on marine and aviation charts, land titles and survey plans. A minute is a sixtieth of a degree and a second a sixtieth of a minute. To convert by hand: decimal = degrees + minutes/60 + seconds/3600, then apply the sign from the hemisphere letter. Going the other way, 0.5074° × 60 = 30.444′, and 0.444′ × 60 = 26.64″, so London is 51° 30' 26.64" N 0° 7' 40.08" W.

Degrees and decimal minutes (DDM) is the default on many handheld GPS units, marine plotters and geocaching sites: 51° 30.4440' N 0° 7.6680' W. It keeps whole degrees but avoids the extra seconds step.

UTM (Universal Transverse Mercator) cuts the world into 60 zones, each 6° of longitude wide, and gives a position as metres east and north within its zone, so distances can be measured with a ruler on a map. London is 30U 699316 E 5710164 N: zone 30, latitude band U, then easting and northing. Eastings are offset by 500,000 m from the zone's central meridian so they are always positive, and southern-hemisphere northings are offset by 10,000,000 m. UTM covers 80°S to 84°N; outside that the converter reports "Outside UTM range", because the polar regions use the separate UPS grid.

How precise is each format?

One degree of latitude is about 111 km everywhere. A degree of longitude is 111 km at the equator and shrinks with the cosine of latitude, to about 69 km at London. The table shows what each decimal place of a decimal-degree coordinate is worth at the equator.

The converter outputs six decimal places for DD, two decimal places of seconds for DMS (about 30 cm), four decimal places of minutes for DDM (about 20 cm) and whole metres for UTM. A consumer GPS is accurate to around 3 to 5 m in open sky, so anything past the fifth decimal is describing the device's noise rather than the ground.

How the coordinate converter works

The DMS and DDM formats are straight arithmetic on the decimal value. UTM is a transverse Mercator projection on the WGS84 ellipsoid, the datum GPS reports, using the standard series expansion with a scale factor of 0.9996 at the central meridian. Zones are worked out from longitude, with the two official exceptions: south-west Norway is widened into zone 32V, and Svalbard uses zones 31X, 33X, 35X and 37X. Reverse conversions use the inverse series, so a UTM reference you paste in comes back as latitude and longitude, then fans out into every other format.

Expert tips and pitfalls

Latitude first. Almost every format and app expects latitude then longitude. GeoJSON and many programming libraries are the exception: they store [longitude, latitude]. Swap London's pair by mistake and the point lands in the Indian Ocean off Somalia. The converter only swaps automatically when the first half ends in E or W and the second in N or S.

Watch the sign. Drop the minus from New York's longitude, 40.7128, 74.0060, and you are in Central Asia. Western and southern coordinates always need either a minus or a W or S letter.

Check the datum. Older maps use local datums such as OSGB36 in Britain or NAD27 in North America. The same numbers can sit around 100 metres or more from the WGS84 position. The datum is printed in the map margin.

UTM band letters. The letter after the zone number is a latitude band, C to X, skipping I and O. Bands C to M are south of the equator and N to X are north. That means a reference such as 33S is read as band S, about 32° to 40° north, not as "southern hemisphere". For a southern point, use its band letter, for example 56H for Sydney.

Paste back what you copy. The UTM output, with its E and N markers, is accepted as input, so you can copy a result, edit a digit and paste it back to see where it moves.

How to read the result

Each conversion lists decimal degrees, DMS, DDM, UTM and a geo: URI, the RFC 5870 format phones open in their default map app. Below that are OpenStreetMap and Google Maps links for the same point. The pin on the map is the real check: if it is not where you expected, look at the order and the signs before trusting any of the numbers. "Copy all formats" puts every line on the clipboard, ready to paste into an email, a trip plan or a rescue report.

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