Ordinance Survey has only parts of Belfast and Enniskillen digitized, but
data is very good and very complete. Have been working on it since 1983,
expect to complete by 1998.

Ordinance Survey building on QUB campus contains a deeply sunk granite reference point for the Northern Ireland part of the National Grid

A grid references is a linear distance in meters East and North of a fixed reference. They are typically given as a 12 figure Grid References: 6 digits
Easting and 6 digits Northing.
   
George said that additional digits added on the left side as you divided
the squares down.

The Irish grid covers Ireland North and South; the National Grid covers England, Scotland and Wales. The two are not directly related except via their origins. Origins are linearly related.

The National Grid is based from Greenwich?
The Irish grid reference he said was  0, 53 50'?

Irish grid has a True Grid Reference and a False Grid Reference.
The TGR is linearly related to the National Grid.

The False Grid reference is shifted to 200000,200000, which puts all of Ireland
into a positive grid numbering.Thus the True Origin is 200000 meters North and
200000 meters East of the False Reference.

There is a Geographical Coordinate Transform and tables.

US runs in Universal Transverse Mercator.

GPS's give world coordinates, not local grid references.

Found a number of inconsistancies between
   the Irish and National grid equations, some of which are due to the
   Irish document being from 1953.

   The UK in a few cases puts a term after a sin term with no bracketing:
	a sin b d
   is in reality:
	ad sin b

   The Irish Grid has constants built into the equations that are unique
   to Ireland, although it is obvious where that is done. 

It also uses
   terms to keep the numbers size down. These have been dropped in the new
   UK one and make no difference anyway.

There are also some slight
   differences in where minus signs are put. The UK version cancels out
   a few minus signs in V and VI with their eventual destination in the
   equation for E.

More substantial is the addition of an extra terms, IIIA and XIIA in the UK.

The Irish grid has an extra term on the end of VI.

   It took awhile to figure out how the M equation was used. I still have
   not got the iteration figured out for transform back to world coords.
   It has an error equation that is to be used to correct a guessed value
   of phi prime. It says nothing about what a good initial guess is, so
   I made my guess the latitude of the central meridian plus a small delta.
   I have not figured out what correcting phi prime means in math terms.
   Add? subtract? whatever makes the value converge?