Coordinate Converter

How the Coordinate Converter works, and what it assumes

Three notations describe the same angle. Decimal degrees write it as one number. Degrees and decimal minutes split it into whole degrees and a decimal count of the sixty minutes in each, which is the form GPS receivers and marine charts use. Degrees, minutes and seconds go one step further and split each minute into sixty seconds. Converting between them is arithmetic and exact; the only places to go wrong are the sign and the order.

Both are handled explicitly here. A hemisphere letter always wins over the Input order setting, so W 0 07 39 N 51 30 26 is read correctly even though the longitude comes first. Without letters, the setting decides. A value carrying both a minus sign and a hemisphere letter is rejected rather than guessed at, because -51S contradicts itself.

The two projections are exact transformations on the WGS 84 ellipsoid, with no datum shift anywhere. Web Mercator (EPSG:3857) is the projection web map tiles are drawn in; it applies spherical formulas to the WGS 84 semi-major axis, which is why it cuts off at 85.0511 degrees, the latitude that makes the world an exact square. UTM uses the standard Transverse Mercator series, accurate to about a centimetre inside a zone, and the zone is chosen for you including the widened zone 32 over southern Norway and the four Svalbard exceptions.

Using the Coordinate Converter

• Turning a coordinate quoted in a report as degrees, minutes and seconds into the decimal degrees an API wants.
• Converting a position from a GPS unit or a marine chart, which use degrees and decimal minutes, into something a map library accepts.
• Working out which UTM zone a site falls in before choosing a projected CRS for a survey.
• Getting the Web Mercator metres behind a latitude and longitude when debugging tile or pixel arithmetic.

A worked example

Entering 51°30'26"N 0°07'39"W gives:

Decimal degrees      51.507222, -0.1275
GeoJSON position     [-0.1275, 51.507222]
Degrees and minutes  51° 30.4333′ N, 000° 07.6500′ W
Web Mercator         -14193.24, 6711510.68
UTM                  30U 699337.83E 5710144.81N  (EPSG:32630)

The UTM easting sits near 700,000 rather than 500,000 because London is two degrees east of zone 30's central meridian at 3° W. An easting of exactly 500,000 always means a point on its zone's central meridian, which is a quick way to sanity-check a projected coordinate.

Where this comes up in GIS work

1. Type or paste a position, for example 51.5072, -0.1276 or 51°30'26"N 0°07'39"W.
2. If your pair is written longitude first, change Input order to match; a hemisphere letter overrides the setting.
3. Choose how many decimal places the decimal degree values should show.
4. Read the notations in the first table and the projected values in the second.
5. Select Copy on any row to put that value on the clipboard.

When the output looks wrong

Latitude must be between -90 and 90 degrees

The two values are almost certainly swapped. Latitude never exceeds 90; longitude runs to 180. Either switch Input order to longitude first, or write hemisphere letters, which settle the question whichever order they appear in.

This value has both a minus sign and a hemisphere letter

They contradict each other: -51 and 51S mean the same thing, so -51S is ambiguous. Use the sign or the letter, not both.

Web Mercator is undefined beyond 85.0511 degrees of latitude

Mercator stretches the poles to infinity, so every web map stops just short of them. That cut-off is not arbitrary: it is the latitude at which the world becomes an exact square, which is what makes the tile scheme work.

UTM covers latitudes from 80 degrees south to 84 degrees north

Outside that band UTM is not defined, and the Universal Polar Stereographic systems are used instead: EPSG:32661 in the north and EPSG:32761 in the south.

Questions about the Coordinate Converter