How it was done
In 1824, Parliament ordered Colby and most of his staff to Ireland, to produce a detailed six inch to the mile valuation survey. Colby and Lt Thomas Drummond, a leading mathematician and inventor, designed specialist measuring equipment, systematically collected place names, and reorganised the map-making process.
The maps were based on a framework of triangulated points. The first leg of the first triangle, known as the baseline, was measured along the flat eastern shore of Lough Foyle. The baseline, at almost eight miles, was the longest of its kind and was measured to the highest standards of accuracy available at the time. In 1960 it was re-measured using modern electronic equipment and the old measurement was only out by one inch.
The tools especially developed for the project, were an iron and brass bimetallic strip compensation bar, a strong limelight and a heliostat reflector for daylight observations.
These first maps provided the basis of an accurate survey of properties originally for taxation purposes.
By 1830 the Survey began to recruit staff to research geographical names and approve the forms of the names which would appear on maps – townlands, districts, hamlets and hill features. These names have been published on successive editions of maps in their original form. All other names on Ordnance maps are updated for each new edition and new, changed and obsolete names are carefully recorded.
When it was decided to put height points on maps, local ‘datums’, where the height is fixed at zero, were chosen at locations around the country. The datum for County Dublin was fixed at the low water mark of the spring tide on 8 April 1837 at Poolbeg lighthouse. This datum was later used across Ireland until new datums were adopted in the mid 20th century: mean sea level at Malin Head, County Donegal in 1958 and mean sea level at Clarendon Dock Belfast between 1951 and 1956.
Metric height information on OSNI maps
All large-scale mapping undertaken by OSNI is published with metric height information referred to mean sea level at Belfast. However, to make sure compatibility throughout the 1:50,000 and 1:250,000 map series which cover both Northern Ireland and the Republic of Ireland, the heights on all OSNI small-scale maps refer to mean sea level at Malin. Malin OD is about 0.037m above MSL Belfast.