Interior Survey


To commence the survey 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 principal of expanding rates of two different metals would compensate and cancel each other out leaving the total lengths unchanged 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 base line 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.


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.


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

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


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 bench marks from the first levelling left a mark on the landscape in the form of the crows foot cut into walls buildings and bridges, This datum remained in use until it was superseded by Mean Sea Level at Malin Head 1958.