Crossing the River Lossie


The maps collection held by the National Library of Scotland is one of the finest in the world, and the foundation of the collection are the Ordnance Survey maps first produced in the mid-19th century. These maps, and those that followed, were made possible by a process called triangulation.

Triangulation is a means of determining the location of a fixed point by measuring angles to it from other fixed points. The original triangulation of Great Britain, called Principal Triangulation, was carried out between 1784 and 1853, and provided the foundation for the Ordnance Survey mapping of the country. In 1935 the Director-General of the Ordnance Survey, Major-General Malcolm MacLeod, set out to modernise the country’s triangulation, and in a military style operation, called Retriangulation, running from 1936 to 1962, hundreds of triangulation stations, or ‘trig points’, were carried up hills and mountains in order to make the mapping of the country more accurate. Several thousand secondary points were placed across the country.

Crossing the River Lossie
Come hell or high water! Crossing the River Lossie, 1938. Copyright: Ordnance Survey

Each ‘trig point’ was visited by an Ordnance Survey team, who used theodolites to measure angles to adjacent trig points, building up a series of interconnecting triangles across the country. The process was accurate to 20 metres over the scale of the entire country.

Sgurr Na Ciche
A primary pillar, Sgurr Na Ciche, Inverness-shire. Copyright: Ordnance Survey


An extract from a map showing the retriangulation of Great Britain. Copyright: Ordnance Survey

There are a number of books in the collection which outline the history of triangulation. Two of the best are Ordnance Survey: Map Makers to Britain since 1791 and The History of the Retriangulation of Great Britain 1935-1962. Alternately, if you want to see the tangible outcome of the triangulation process, why not visit the National Library of Scotland’s maps website.