In 1935 the Ordnance Survey - the UK mapping agency - began the huge task of re mapping the British Isles. This involved building and surveying over 6500 triangulation or 'trig' pillars - the familiar concrete obelisks that frequent the British landscape, countryside and mountains. The Re-triangulation took many years and was finally completed in 1962. Between 1936-37 the survey was at work in Wales. This is the tale of how the Welsh Primary pillars came to be, the reconnaissances, permissions, construction and the observations taken from them. It is the tale of the men who did the work, toiling in all weathers, shifting and transporting tons of material and equipment and the thousands of hours of effort. The result of their work was the creation the OS maps we all know and use. Today the trig pillars are redundant, overtaken by new digital mapping technologies, satellites and GPS, they remain only as analogue memorials to a hidden human endeavour. This book aims to record their labours.