So, I see the terms Irish Grid and Irish Transverse Mercator bandied about a lot on the web. What is the difference between the two? I know IG uses the Airy ellipsoid and ITM uses WGS 84, but, what the real deal here?
Ordnance Survey Ireland are migrating their mapping from ITM from IG. Right now we're in that awkward phase where both systems are in use. I think IG will take quite a few years to disappear, mainly because there is so much data out there which uses it.
As to why OSi is making the change, it's down to accuracy. As you've said ITM uses a different ellipsoid (GRS80, which was the original basis for WGS84 - WGS84 has been refined slightly since and now differs from GRS80 very slightly) which means much greater compatibility with GPS along with a number of other benefits.
You can get more background information in this PDF from OSi:
Any idea when the OSI plan to start printing maps with ITM grid lines rather than IG?
I'm looking for a gps unit for around 100 euro that specifically supports Irish (ITM) as a grid format.
Anyone got any info??