This series consists of books and diagrams recording the exact positions of stations of the Old Triangulation, the calculated distances between them, and the calculations necessary to construct the framework of one-inch and six-inch maps. The types of records in the series are:
- County guard books of the Old Tertiary Triangulation. These consist of station description books, containing locating details for each triangulation station, and trigonometrical and meridional distance books, recording (a) the co-ordinates of each station and the names of the stations used to produce those co-ordinates, and (b) the calculated distances between adjacent triangulation stations which provided the scale control for the detail survey of the large scale plans.
- One-inch diagrams referring to the Old Tertiary Triangulation of Great Britain. These consist of Ordnance Survey maps annotated to show plotted positions of stations as recorded in the guard books. Some maps also indicate intervisibility between adjacent stations. The dates given are those of the edition of the map.
- Descriptions of restored primary stations and restored secondary stations. These contain locating details for Old Triangulation stations whose positions have been verified on the ground.
- Meridional distances of transferred points. These record local co-ordinates of Old Triangulation stations which were transferred from one county origin to another county origin.
- Town sheet books. These record the old Cassini co-ordinates, based on an origin located in each town, of trigonometrical stations needed for the survey of each town.
- County sheet books. These record Cassini co-ordinates for six-inch sheets.
- Projection books. These contain calculations of latitude and longitude of sheet corners of one-inch and six-inch maps.
- District co-ordinate books. These record Cassini co-ordinates for district stations in each county.
- Secondary co-ordinate books. These record Cassini co-ordinates for the primary and secondary stations in each county.
- Piling book. One volume recording positions of ground stations.