Wednesday 23 March 2016: Albert Allen: Mapping Ireland
Having introduced himself as a land surveyor/cartographer with almost 50 yrs experience, Albert Allen talked about the mapping of Ireland between 1822 and 1836. This mapping was the first time a whole country was mapped at large scale. The man in charge was Lt.Col. Colby, an experienced surveyor who won the case for adopting the English standard yard as his base measure as opposed to the Irish perch of 7.0 yards related to the Irish mile. In the 19th century, surveyors used a system called triangulation. The method was to establish a baseline, and from this measure the angles in the network of triangles throughout Ireland. Some of these measurements were over 50 miles. A team of surveyors would shine a limelight which could be observed from other mountains. Angles were measured by purpose built theodolites. The baseline was a precisely measured line of almost 8 miles. It was re-measured in the 1960s using electronic distance measuring and found to be correct to less than 1”. Albert illustrated his talk with early maps of Omagh and was thanked for a fascinating lecture by Harman Scott