Interesting Maps

In the 19th Century, Lt-Col Thomas Colby of the Ordnance Survey completed the first survey of a country which led to Griffith's valuations becoming available. This was the six inch maps which are still in use today.

A series of triangulation points were necessary to be able to accurately measure distances. Each triangulation point needed to be visible by at least two other trig points. Colby's assistant, Thomas Drummond made use of a brighter light called a limelight to facilitate this task. However, Colby was not happy with the technology available then for measuring his initial baseline distances. Drummond set about developing a compensation bar made from two different metals, brass and iron, which compensated for temperature fluctuations allowing distance calculations to become more accurate.

Colby's chose Lough Foyle as his baseline location. The flat eastern side would enable the most accurate recordings ever taken where a distance of eight miles would be measured. Measurements were recorded over 60 days between 1827
and 1828.

Colby's measurements were so accurate using his compensation bar that when his baseline was remeasured using a tellurometer in 1960, his eight miles were accurate to within an inch.

Once a baseline had been taken, the rest of the country could be measured using similar trig points…

Trigpointing Ireland - About (Various short articles)