Interior Survey

The Lough Foyle Baseline Measurement

To commence the survey of Ireland one side of one triangle was measured. Accuracy of the baseline was essential as the entire survey depended on it. The instrument used to measure the baseline used the principle that expanding rates of two different metals would compensate and cancel each other out leaving the total lengths unchanged, this was Colby’s Bars. Observed in 1827-28 under tenting to minimise fluctuations in temperature the length (7.89 miles approx) took 60 days of measurement.

In 1960 the baseline was re-measured using Tellurometer and the difference in the measurement using Colby’s bars and the Tellurometer was about 1 inch over an 8-mile length a testimony to the high standards and accuracy of the 1827 measurement.

Principle Triangulation

The survey was based on a network of triangles. The original Theodolites used were Ramsden and Throughton and Simms. There was a three-foot and a 2-foot Theodolite.

The first task was to locate the framework of points on which the triangles would be based. Some of the triangles had sides of 150 km.

How to achieve a Detail Survey

District commanders then used smaller Theodolites to observe the secondary and tertiary trigonometric network. Chain Lines were run between the tertiary stations giving a check on the trigonometrical computed distances and facilitating the subdivision of the triangle for detailed chain surveys of every road and track.

Wall and hedge, river and stream, house and barn were surveyed and mapped.

The Irish Datum point at Poolbeg, Dublin

1837 Irish datum point fixed on Poolbeg Lighthouse in Dublin bay for primary Levelling and contouring of Ireland. The Primary network was completed in 1843.

The network of benchmarks from the first leveling left a mark on the landscape in the form of the crow's foot cut into walls buildings and bridges, This datum remained in use until it was superseded by Mean Sea Level at Malin Head 1958.