Google maps and OSGB grid references

[Will]

I have a list of rides, done as well as planning to do, which have their starting point recorded in the form of an OSGB grid reference - for example, one starting in nearby Chew Magna has the following entry: 357550,163170

I'd not encountered the all-numeric format for OS grid references before. Your reference is in the 100*100km square 3, 1, which is usually called ST. So, the eight-figure reference would usually be written as ST575163 (because ST represents the first digit of each of the easting and northing).

You can enter either that conventional format or your OS eastings and northings into the Grid Reference Finder. That will give you links to Google Maps and several alternatives for the specified points.

The all-numeric format is the national grid coordinates, and in the past was mostly used by professionals. The (false) origin (0,0) is to the SW of the Scilly Isles.

The method using two letters and six digits was created to make it easier for amateurs using paper maps. Sometimes the letters are replaced by the OS Landranger sheet number, so ST575631 becomes 172/575631 or 182/575631 (as this point is found on Landranger sheets 172 and 182).

With the six digit number (575631), you only have to find the intersection of easting 57 and northing 63 on the paper map to find the 1km square containing the point. Conveniently, the eastings and northings are shown every 10km on the Landranger map, so you don't have to unfold the map to see the numbers on the edge. If the national grid coordinates were used with a paper map, then it would be too easy to select the the wrong 1km square i.e. easting 35 and/or northing 16 in this example (since the coordinates are 357550, 163170).

Will

[drossall]

Yes, I'm a Scout leader, so well used to using grid references and aware of the issue of repeating every 100km. I started on 1-inch maps

There's always something new to learn though!

[andrew_s]

Officially, OS grid refs truncate rather than round, with a grid ref like ST575631 identifying the 100 m grid square in which the object of interest lies, by its SW corner coordinates.

ST is a 100 km grid square
ST56 is a 10 km grid square
ST56SE is a 5 km square (you used to buy 1:10,000 map sheets using this identifier)
ST5763 is a 1 km grid square

So if you have a grid ref like ST575631, and dial it in to your GPS as ST 57500 63100 (mine demands 5 digits) and go there, you could be up to 140 m away from the place the grid ref is for.

Having said that, many people do mistakenly round rather than truncate.

[drossall]

And some people, even using roamers, struggle to get enough accuracy to know which they are doing...