dd mm.mmm

 dd mm ss.ss


 OS Grid (numeric)

 OS Grid (trad)




 Accuracy (metres)


Type or paste into the appropriate box and tap the button to the right of it. Examples:

dd mm.mmm ... N51 42.729 W000 24.853
dd mm ss.ss ... N51 42 35.26 W000 24 54.82
dd.ddddd ... 51.75639 -0.43286

E & N are +ve, W & S are -ve. Use N, S, E, W, + and - prefixes or suffixes to specify the hemisphere (not required for N or E).

In the DM & DMS boxes, degree, minute & second symbols are accepted, but not required. For Degrees, use a comma or space separator, or a comma followed by a space.

OS Grid Ref numeric format is 4 to 6 digits each for eastings and northings (7 for Northings in the north of Scotland). You will be warned about low resolution if you use only 4 or 5. Traditional format is 2 letters followed by 3 to 5 digits each. Use a space to separate Eastings & Northings.

You can convert to or from what3words. This requires an API call to w3w and they are limited to 1000/day, so this is off by default - enable this each time you want to use it.

Alternative output format adds degree, minute & second symbols to the DM & DMS outputs, and removes the comma separator from the D output.

This program can use the location services provided by your browser to populate the boxes. If you are asked to permit the web page to use your location you should, of course, allow it to do so. You should also use the https protocol instead of http, though the program will attempt to redirect you should you use http.

Location services on mobile browsers vary significantly in their behaviour. Some may return a low accuracy fix if a high accuracy fix is not available, even if high accuracy is requested. The reported accuracy is therefore displayed. If you enter co-ordinates manually, the accuracy is shown as 0 metres.

The accuracy shown is that reported by the phone's API. This is usually expressed as the 95% CEP (circular error probable) figure, i.e. the distance to which the location is accurate 95% of the time. This should be the same measure used by the other apps on the same device.

The time taken to return the location is also variable. If the GPSr is already powered up and in use by a native application, the browser should return immediately.

Two slightly different UTM formats are supported, and selected by the checkbox.

The most common format uses latitude bands that range from C in the far south to X in the far north. This method has some redundancy because the latitude also partly specifies the band. If you supply a band that conflicts with the latitude, the program will use the latitude band only to determine the N/S hemishpere, and will warn you about the error.

The alternative is to not use latitude bands but to specify the hemisphere using N/S. These examples of each format are for Nelson's Column in London ...

Using latitude band - 30U 699312‍ 5710154
Using hemisphere - 30N 699312‍ 5710154

If the co-ordinates are in the UK and the County checkbox is enabled, the page will display the nearest county.

County boundaries change periodically. The program uses the 1974 boundaries because these are the only ones for which I have been able to obtain appropriate polygon data. I believe they are the same as those used by GSAK, but they are not the same as used by Project GC.

Sources of error include the resolution of the polygons and inaccuracies in the supplied location. The program will look beyond the immediate location to see if you are near a county boundary, and if so will warn you, as the result is less reliable. How near is "near" depends on the accuracy.

If you are using what3words, the API may also report the county of the nearest town.

Dumfries and GallowayDurham
DyfedEast Sussex
Greater LondonGreater Manchester
HampshireHereford and Worcester
HumbersideIsle of Man
Isle of WightKent
MerseysideMid Glamorgan
NorfolkNorth Yorkshire
NottinghamshireOrkney Islands
Scottish BordersShetland
South GlamorganSouth Yorkshire
TaysideTyne and Wear
WarwickshireWest Glamorgan
West MidlandsWest Sussex
West YorkshireWestern Isles