Written between 1834 and 1841, the Ordnance Survey Letters comprise notes and correspondence from the fieldworkers to the headquarters of the Ordnance Survey in Dublin. They are available for 29 counties – unfortunately, there are no letters for the Ulster counties of Antrim and Tyrone (and neither is there one for Co. Cork). The collection was later transferred to the Royal Irish Academy. Arranged by county, PDFs of the manuscripts can be downloaded from the Ask About Ireland website.
In addition, there are typescript copies of the letters, prepared in 1927–30, in the RIA and some other libraries. In more recent times, Four Masters Press has published editions of the letters, under the editorship of Michael Herity, including for all seven counties in Ulster for which the Letters are available: Donegal (2000); Down (2001); and, in a single volume, Armagh, Cavan, Fermanagh, Londonderry and Monaghan (2012).
More information on the Ordnance Survey Letters is available from the Royal Irish Academy and the OS200 Ireland Mapped Project. Images of the manuscript and typescript letters are also available through the OS200 Digital Resource.