Grid references

You knew that I couldn't resist trying again!

I clicked the 'switch to numeric squares IDs' in the Great Britain National Grid box and it does the business. I still can't understand why the authors of the study did not opt for 2 digit NGR prefixes.

Now to try deciphering some of their locations again.....

It's not as simple as I first thought. The numeric grid and the aphabetic modern NGR have their origin about 55 miles WSW of the Scilly isles. This is thus SV 0000 0000, which should also be 000000 000000. However the converter gives this as 100000 100000. Now feeding 1xxxxx 1yyyyy into the converter also produces SW, SR, and HW depending on where you are in the UK.

I've added the numerics to the map below and you can see the problem. The bottom two rows have the same figures, and this gets repeated to the north of Scotland.



We get the same effect in the leftmost two columns. All this comes down to the fact that: The rule is simple - stick to the modern NGR with two letters and two sets of numbers and all is well. Use the older system and open the can of worms. The converter often throws its toys out of the pram when feeding in ambiguous references. Beginning to see why a Javascript program might be needed.

This is as bad as the UK TV Digital Switchover. Hang on it is a digital switchover.

PNK;67075 wrote:

I never really worked out how the "flat" grid worked on a curved surface.

Remember reading about this recently. To get minimum distortion the UK should have its grid origin in the centre of the UK - which on the map above puts it in the middle of square 'NT' - which is near Coldstream in Northumberland. Maximum distortion due to curvature then occurs equally around the edge of the map.

The problem with this is that points south and west of Coldstream will have negative coordinates. To prevent this they now shift the origin to the most south-westerly point off the Scilly Isles. This is the meaning of 'False Origin' in the term 'War Office, False Origin' as in Cassini WOFO projections.

And that is the end of my knowledge on projections - got to be worth a 'B' in GCSE Geography. Furthermore I would have thought that the centre of the UK would be 10 Downing Street.

Another thought on the numerical GRs mentioned above. Why do we allow these to be still used when they are ambiguous? OK one can differentiate between similar NGRs when one is on the English south coast, and another is a Scottish island. However Horton in Glos. and Morton in Dorset will have similar numerical GRs since squares 'SY' and 'ST' both decode to '31'.

Graham

Carnaby;67084 wrote:

Remember reading about this recently. To get minimum distortion the UK should have its grid origin in the centre of the UK - which on the map above puts it in the middle of square 'NT' - which is near Coldstream in Northumberland. Maximum distortion due to curvature then occurs equally around the edge of the map.

The problem with this is that points south and west of Coldstream will have negative coordinates. To prevent this they now shift the origin to the most south-westerly point off the Scilly Isles. This is the meaning of 'False Origin' in the term 'War Office, False Origin' as in Cassini WOFO projections.

And that is the end of my knowledge on projections - got to be worth a 'B' in GCSE Geography. Furthermore I would have thought that the centre of the UK would be 10 Downing Street.

Another thought on the numerical GRs mentioned above. Why do we allow these to be still used when they are ambiguous? OK one can differentiate between similar NGRs when one is on the English south coast, and another is a Scottish island. However Horton in Glos. and Morton in Dorset will have similar numerical GRs since squares 'SY' and 'ST' both decode to '31'.

Graham

The numerical references are not ambiguous - they are based on exactly the same false datum and projection as the system that uses the grid letters (both represent the number of meters east and north of the false datum)

Pad the easting and northing with leading zeros to give eeeeee,nnnnnnn (ie. easting has six numbers and northing has seven).
For the easting take the first number to give you the first letter (500km square) and for the northing take the first two numbers to give the second letter (100km square).

So
000000,0000000 would be the south west corner of 100km square designated SV = SV0000000000 (our false datum)
352939,0428916 would be SD5293928916
400000,1100000 would be HU0000000000 (and shows why I suggest padding northing to seven digits)

SY is '30' not '31'
ST is '31'
- if you arrived at them being the same from the conversion software you've mentioned then that software is not doing the conversion correctly

PNK, having read tigger's post, my head also hurts! But, I think that I understand. They key seems to be to remember that the FO is six and seven digits respectively. I shall leave Carnaby to trial it!

Of course, all you GPS users out there will be wondering what the heck we are talking and worrying about but then, we didn't have calculators at school (unless you include an abacus)

tigger;67129 wrote:

if you arrived at them being the same from the conversion software you've mentioned then that software is not doing the conversion correctly

Yes thanks for the confirmation Tigger.

Just done some testing and there are two problems with the converter: Thus the correct grid now becomes:



I guess that does it.

Graham

Graham...minor point, your example NT123456 987654 would actually be 312345.6,698765.4
The decimal point becomes necessary as your reference shows more precision than metres. As I said before, easting is six figures and northing is seven figures (for H- and J- 500km squares, six is ok for S-, T-, N- and O-) as the metre is the standard unit for the OSGB grid system.

This report (see link) gives 6 digit grid references with no prefix. Presumably, they relate to the Military Grid Refrence System (MGRS).

How can I relate these to a National Grid Reference (NGR)

I know the location of all these sites except Kilmarnock so perhaps that should be my question?

www.scribd.com/doc/60530845/MoD-Briefing...lean-Sweep-July-2011