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Using this site, we can see that the (approximate) centre of Southampton, UK has OSGB coordinates of 441847,112279. However, looking at the UK grid, where each large square is (500 km)2, then these coordinates don't seem to follow: where does the leading number come from? The traditional 6 figure grid ref for the location is SU418122; given a 6 figure GR denotes a (100 m)2 area, this shows that the location is 41.8 km east and 12.2 km north of the SU grid square origin. Does the extra figure not denote an order of magnitude larger in distance?

enter image description here

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converter… up to 10 fig refrences – Mapperz Feb 18 '14 at 1:46
Another easy method (though not batch) to convert OSGB coordinates is to use StreetMap and find the tiny link under the map which says 'Convert', this then gives you the coordinates in 3 different formats including degrees. See Martin's answer for the correct explanation of how OSGB works and where the leading 4 and 1 come from. – MappaGnosis Feb 18 '14 at 8:02
For batch converting OS grid references to OS coordinates (and the other way) I find NG Converter very useful.… – nhopton Mar 5 '14 at 19:55
up vote 9 down vote accepted

Those leading figures you ask about are the Eastings and Northings of the SW corner of the "medium" square "U" which is inside its parent large square "S".

Each large square is 500km wide, as you state. Each large square is further subdivided into 25 "medium" squares, 100km wide. I call them medium squares because they too, as you know, are subdivided further into 100 x 100 topographic map, small grid squares, 1km wide. (Actually, there's yet another intermediate level of squares, 10km wide, between "medium" and "small"!)

Now, the labeling pattern is the same for both large and medium squares (i.e., the A, B,C ... X, Y, Z pattern also exists inside each large square), so each medium square has a unique two letter label -- one for its parent, one for itself.

The origin of the UK grid (i.e., 000000,000000), is at the very SW corner of the large square "S". Thus the 6-figure reference for the origin is SV000000. The "V" means it is in the SW corner of its parent, "S".

The following index map is from Using the National Grid, a two-page consumer guide. I added two red dots for the sample locations that follow.

From Using the National Grid

The Southampton reference of SU418122 is equivalent to 441800,112200 because the SW corner of medium square "U", is 4 squares (or 4 x 100km) east and 1 square (or 100km) north of its parent large square's SW corner.

Another example: my home town of Leigh-on-Sea, on the North of the Thames estuary, is at TQ845858. This means its

  • Easting is T + Q + 845 = 1 x 500km (T is 1 large square east of S) + 0 x 100km (Q is 0 medium squares east of its parent's origin) + 845 x 100m = 584500m
  • Northing is T + Q + 858 = 0 x 500km (T is 0 large square north of S) + 1 x 100km (Q is 1 medium squares north of its parent's origin) + 858 x 100m = 185800m

The combined graticule and grid in the map below is from A guide to coordinate systems in Great Britain, a good 43-page technical document.

Grid and graticule for UK

Notice the relationship between the "false origin", the "true origin", and the SW corner of the ref. square SU. (It should explain the honest confusion in Brad's answer.)

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Ah, thanks - I didn't know that the SW corner of SV was defined as 000000,000000 - and it doesn't seem to be stated anywhere on the OS website either as far as I can see – ChrisW Feb 18 '14 at 11:49
I didn't know either but i just figured it out after studying your question! – Martin F Feb 18 '14 at 14:55
PDF with false origin context… false origin [7.1] – Mapperz Feb 18 '14 at 18:35
Should this answer acknowledge the Crown Copyright on the source for the images? – BradHards Feb 22 '14 at 2:56
Perhaps -- how does that work? Thanks. – Martin F Feb 22 '14 at 3:46

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