“Whereas
you have represented unto Us that it will be advantageous to OUR Service to raise an additional Company
of Royal Sappers and Miners to be employed in the operation of the Survey in Ireland…”
Out
of this extract from George IV’s Royal Warrant was born the Irish Ordnance Survey in Ireland.
Born
of the need for accurate land measurement for valuation purposes, the Irish Ordnance Survey, under Lt-Col
Thomas Colby (pictured left), completed the world’s first large-scale mapping of an entire country by
1846. The survey, at a scale of 6 inches to one mile, was primarily a townland survey and not detailed
enough for a proper valuation, so a more comprehensive survey was completed. By 1867, from Fair Head
to Mizen Head every road, track, hedge, fence and stone wall, every river and stream, every house and
barn had been surveyed so that Sir Richard Griffith and his team could plot their valuations.
Surveying
an entire country was a long and complicated task. Firstly Colby and his team had to build a framework
of points upon which the mapping could be based. This was done by triangulation and by creating a series
of primary triangles. Sightings were taken between mountaintops using theodolites. Much of this work
was done at night and, with some of the sightings over 150kms, Colby needed more powerful light than
that provided by the Argand Lamps he was using. Colby’s assistant, Thomas Drummond, who displayed unique talents as an inventor and mathematician, invented
the Limelight. Drummond was also instrumental in improving the heliostat reflector which was used for
daylight observations.
In order to complete the network
of triangles, the length of one leg of one triangle had to be measured. The leg chosen was along the
shores of Lough Foyle and this measured distance is known as the Baseline.
The
accuracy of the measurement of the Baseline was paramount as it set the scale for the whole survey.
Colby, not satisfied with previous methods of measuring baselines, asked Drummond to develop a method
based on the compensation principle, using parallel bars of two different metals (iron and brass). Drummond’s
design allowed the bars to be connected in the middle with steel tongues fixed across the ends, leaving
them free to shift according to the expansion or contraction of each bar and allow the coefficient of
expansion of metal to be used in the calculation of distance.
Measurement
of the baseline began in 1827 and was carried out under tenting to avoid fluctuations in temperature.
The bars were mounted on tripods and the total distance of 7.89 miles, which included crossing the River
Roe, was completed in November 1828 after 60 days of measurement by 70 men. The accuracy achieved is
still marvelled at today.
The first 6-inch survey
was followed in time by further editions and by larger and smaller scale maps, accepted by all as the
authoritative representation of the topography in Ireland.
