920
December 2016
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
Lough Foyle Base was remeasured in 1960 using electronic
distance measuring equipment, with a difference found to
Colby’s original measurement of only 1 inch!
The One Inch Map (1” = 1 mile) of Ireland was cast on the
ellipsoidal Bonne projection in 1850 which was the projection
in vogue throughout Europe at the time. Also used for the ½
inch series and some 9 inch sheets, the Latitude of Origin (
j
o
)
= 53° 30´ North, and the Central Meridian (
l
o
) = 8° 00´ West
of Greenwich. The Airy 1830 ellipsoid was used where a =
6,377,563.396 meters and
1
/
f
= 299.3. The radius of the mean
parallel used for the Bonne was 13,361,612.2 feet.
The topographic surveying demanded greater accuracy
than the methods used for the One Inch Map. Colby issued
an instruction (specifications) in what became known as the
“Colonel’s Blue Book.” Note that in the United States, the
current (1999) specifications for note keeping for acceptance
of data by the Federal Geodetic Control Committee is based
on the “Blue Book.” However, Colby’s annual reports were
also termed the same. In 1838, with the Irish survey on a firm
footing, Colby returned to England and turned his attention
to the survey of Great Britain. Sir Thomas A. Larcom, KCB
was the Officer in Charge at Mountjoy, Dublin from 1828
to 1846. The final cost was £820,000 (more than twice the
original estimate of time and money), but the Survey of
Ireland served as a model for the remainder of Great Britain.
In 1858, Captain Alexander Ross Clarke (the same fellow
that computed his ellipsoids of 1858, 1866, and 1880), selected
the observations to be used in the adjustment. Clarke’s
interlocking network of “well conditioned” triangles is now
known as the “Principal Triangulation of Ireland (1824-
1832).” Clarke rigorously adjusted the observations by the
method of least squares in 21 independently computed but
connected blocks with the aid of an average of 8 computers
(persons). Note that in 1881, Colonel A. R. Clarke, R.E.
received a forced retirement from the Ordnance Survey
of Great Britain rather than accept a post to the island of
Mauritius which was this column’s topic last month. Clarke
was 52 years old at the time. For the remaining 30 years of
his life, he published no further scientific work.
In the meantime, some form of framework was required on
which to control the new mapping at the scale of six inches to
one mile. Six InchMap control was therefore based on a network
of secondary and tertiary blocks of triangulation, begun in 1832
and completed in 1841, just ahead of the chain survey teams who
were surveying the detail. One Inch Map control began in 1852,
and 25 Inch Map control began in 1888. Although these lower
order control points included five of Clarke’s points, they were
based only on provisional coordinates. In Northern Ireland, the
coordinates of the origins of Counties Down and Armagh were in
general agreement with the values deduced from the Principal
Triangulation. In the other four counties they were about 80 feet
too far south and 25 feet too far west. Unlike the aphylactic sheet
system used in the United States with the Polyconic Projection,
each Irish County had its own central meridian as a true “Grid”
about a single point of origin. There were 26 County Cassini-
Soldner Grids in what is now the Republic of Ireland, and an
additional 6 were in Northern Ireland. Of these coordinate
origins, 16 were based on church towers and spires, two were
observatories, two were “round towers” (ancient defensive
stone structures from various wars), and a number were on
mountains and at monuments. Printed sheets had neither grid
nor graticule. This system of local (county) Cassini-Soldner
coordinates was also used in England, and was later adopted
for the “Meridional Circuits” of New Zealand. An observation on
the practicality of this seemingly haphazard method of mapping
stated:
“When the map of Ireland is picked up and shaken, it
is only the mathematician who hears the rattle.”
This system
of provisional coordinates resulted in the County Coordinates
being in active use for over one hundred years. The Irish Grid,
based on a single Cassini-Soldner projection for the entire island
was used from 1936 to 1956.
The Ordnance Survey of Northern Ireland (U.K.) was set
up in 1921, and the Irish Free State occupying five sixths
of the number of counties was established in 1922. The
boundary between the two was settled in 1925. Eire declared
itself completely independent in 1937, and was renamed the
Republic of Ireland in 1949. The funds for first and second
order re-triangulations of Northern Ireland were authorized
in 1947 and the survey was completed in 1956. The first order
re-triangulation of Ireland was carried out between 1962
and 1964. Ordnance Survey of Northern Ireland (Belfast)
did not adopt the 1965 adjustment values for subsequent
mapping, whereas Ordnance Survey of Ireland (Dublin)
did. A fascinating result of the adjustments was the “Airy
Modified” ellipsoid where the semi-major axis was changed
to 6,377,340.189 meters! This was done to best accommodate
previous published maps and coordinates, and, thus, the
ellipsoid was reduced by 35 parts per million. In the days
when Grid-to-Graticule Tables were used, this was very
important. The Gauss-Krüger TransverseMercator projection
Irish National Grid was adopted as a replacement for the old
Cassini-Soldner Irish Grid. The defining parameters include
No and 8o being the same as the old Bonne (and Cassini-
Soldner) Grids, False Easting = 200 km., False Northing
= 250 km., and the Scale Factor at Origin (m
o
) = 1.000035.
(Notice the “35” in the scale factor.)
A subsequent adjustment was performed in 1975 which
was designed to provide a common coordinate system for the
whole of Ireland for mapping purposes. The Northern Ireland
primary stations were held completely fixed. The Ireland
Datum of 1975 origin is at Slieve Donard (new) where: Φ
o
= 54° 10´ 48.262˝ North, and Λ
o
= 05° 55´ 11.898˝ West of
Greenwich. Various other datums used for scientific purposes
include: the Ordnance Survey of Great Britain Scientific
Network of 1970 (OGSB SN 1970), OGSB (SN) 80, ED50,
ED87, and WGS84. The latest major campaign, IRENET 95,
provides European Terrestrial Reference Frame (ETRF 89)
coordinates for a network throughout Ireland.
Compatibility between the existing Irish National Grid
and ETRF 89 has been developed in three levels. The first
level will provide a simple Easting and Northing coordinate
shift between the old Grid and the new Grid and have an
accuracy of better than ±5 meters relative to the new geodetic
framework. The second level will provide a 7-parameter
