Translating coordinates from old Irish grid system

JMcL

Not sure if here's the best place to ask this, though since I want to put the result into a GPS it's the closest I can think of!

I have an excellent book by Peter Harbison on monuments around Ireland, each of which has a map reference. The problem is, it was originally published in the early 70s, so all the references are to the old Irish OS maps, and are along the lines of Map 8 M 18; OS1/2" 23S.54.06. I've been trying to find a converter to convert this to latitude/longitude, with no joy. The Ordinance Survey site wasn't much help. Anybody know of any converters?

Alun

That format doesn't look at all familiar. Standard Irish Grid map references are of the form X NNN NNN, where X is a letter indicating which grid square the reference is in, and NNN NNN are the easting (x-axis) and northing (y-axis). These are accurate down to 100m, referring to the SW corner of a 100mx100m square that contains the target. You can quote more digits as well, but for most navigational purposes, 6-digit refrences are used. The current system has been in use for many years, and would have been used in the early 70's too I'd have imagined.

I don't have access to any old 1/2" maps, but maybe if you could somehow get hold of one it might give some clues as to the coordinate system used.

robinph

Not sure if this is a correct match of Map 8 M 18; OS1/2" 23S.54.06 but it didn't throw any errors and seems to correclty guess that it was an Irish OS reference at least.

> Converts to 53.76177 -9.36581 (WGS84) or 53.76180 -9.36485 (Irish Datum)

Conversion done from here.

Edit: The original page that I just copied and pasted your reference into was this one but it looks like they actually have a form for correctly entering Irish map references here.

Alun

It seems to have somehow interpreted that string as M 10000 80000 and used that to convert to lat/long. You can actually just enter M18 and get the same result, so I don't think it's as clever as you're giving it credit for.

coylemj

I'm old enough to have used the OS 1/2" maps for orienteering in the boy scouts back in the 60s and those maps and the newer Discovery (1:50,000)series all use the Irish Grid (IG) described by Alun above.

A grid reference consists of a letter which indicates a 100km square and an even number of digits typically 6, 8 or 10 which describes the SW corner of a 100m, 10m or 1m square respectively within that 100 km square so the greater the number of digits the greater the precision.

I have a couple of 1/2" maps at home, can you quote the reference from the book for a monument in Co. Meath and we'll see if we can make sense of it.

robinph

Yeah it does look like it tried to convert the wrong bit there actually. I just put in a bunch of random numbers and got presented back with another bunch of random numbers that were somewhere over Ireland so just assumed it was close enough.

Alun

OK, I'm making a bit of a guess here, but I think it might be as follows ...

Your 23S.54.06 could be sheet number 23 with either a six-figure grid reference of S 054 006 (i.e. he's missed off the leading zero's) or possibly it's only a 4 figure reference, i.e. accurate only to 1km, which would be S 540 060 as a standard 6 figure reference. Try putting these into the converter and seeing if they show as being anywhere near the point in question.

coylemj

I have MapSource on my laptop and can switch from Irish Grid to Lat/Lon so I just checked out Clonmacnoise in Co. Offaly and it's Irish Grid reference is N 00834 30438, can you see that in your book? If not, give me some other prominent monument and I'll see if I can find it.

JMcL

Thanks for the replies. I've just found an explanation to his map references, and I've discovered I was a bit overeager with the bits I was quoting. The "Map 8 M 18" bit refers to a small scale map section at the back of the book, the rest refers to the OS half inch maps, as you quite rightly say. Sorry about that

robinph: The result looked promising, but unfortunately the website was getting confused by the misplaced "Map" bit, the actual coordinates for that one should be around 52.10N, 7.11W

coylemj: He's got Newgrange as 13O.00.72 (that's 13"oh".zero zero), Tara at 13N.92.60, or a bit further west again, Loughcrew at 13N57.77 (all OS 1/2")

coylemj

Ok so he's gone for low precision and as Alun said that '13' must have been the sheet number of the 1/2" map though the grid reference doesn't require sheet numbers, that's just for convenience so you can walk into the map shop and ask for that sheet number, the Discovery maps also have numbers but they're not used within grid references and only cause confusion when they are jumbled in with the grid reference as has happened here.

Garmin MapSource has a symbol for Newgrange located at 'O 00712 72593' so your man giving 'O 00 72' means that you have to make do with a 1 km square and figure out where within that square the momument is located whereas a 10 digit reference as above narrows it down to a 1m square. Note that if you're using a converter which requires a fixed number of digits, you need to pad the numbers from your book on the right and not on the left.

I checked out Newgrange on Google Earth and it's 1.65 NE of that grid reference (effectively O 00000 72000) in your book.

Alun

I make Newgrange to be at O 00749 72745 (10 figure ref, accurate to 1m), so it looks like my hunch was right, they're 4 figure grid references, i.e. only accurate to 1km. It's not so bad really ... in effect what they refer to is the grid square on a standard 1:50k Discovery map (the blue lines) that happen to be on a 1kmx1km grid.

EDIT: Damn! must learn to type quicker

JMcL

Cheers for that, that makes sense! I'll probably rely on the Discovery maps rather than my shiny new toy to find them, as they've generally got the various monuments marked, so it's usually fairly obvious where it is as soon's you have the right square.

The converter on nearby.org.uk seems to not be quite getting it right. It does recognise the 4 digit reference (eg S5406) as an Irish grid reference, but the resulting latitude/longitude are wrong. At least I now understand the actual references, so I can play about with it.

Thanks again for the help, it's saved much head scratching!

coylemj

Have just checked my 1/2" map and it's Sheet 16 which includes Dublin city in the top right corner. Sheet 13 is immediately to the north and looks like it includes most of Co. Meath.

Over Leinster there was no overlap between 1/2" sheets so the bottom left (SW) corner of sheet 13 which is also the top left (NW) corner of sheet 16 is grid reference N 49695 41575 or Long/Lat N53.42216 W7.25327

On the 1/2" maps they gave the exact grid reference (to the sq.m) in each corner so those values above are exact.

ivanthehunter

Try Grid Inquest its software from the osi. I thinks it freeware as i never paid for it:p