The Ordnance Survey of Ireland, carried out in the early-nineteenth century, was not just the process of mapping and collecting place names for translation as it is frequently depicted. The director of the Ordnance Survey, Sir Thomas Colby, decided to also use the Survey to carry out statistical, antiquarian, and geological surveys. The results of this trigonometrical survey include the so-called Ordnance Survey Memoirs and the Ordnance Survey Letters. Both sources provide valuable information about life in Ireland in the 1830s and early 1840s. Focusing in particular on the province of Connacht, this article argues that the Ordnance Survey Letters should be considered an important source of information about folklore and folk beliefs which were still extant or had been until shortly before the Survey visited the locality. This essay examines how, in a period of change and decline, the Ordnance Survey wrote local cultural heritage and identity onto the landscape.