Every height on every Ordnance Survey map of Wales is measured from a single brass bolt at Newlyn in Cornwall, where the sea was recorded continuously for six years between 1915 and 1921 to fix Ordnance Datum Newlyn, the zero point of the national height system. This interactive map shows, for the first time, how that datum was carried into Wales and what remains of the network that carried it.

This is the master map, the lead of a county-by-county series recording the complete height network of Wales: this national map of the special marks, followed by a companion map for each of the twenty-two local authorities of Wales presenting its ordinary marks in full. The county maps will be published here in sequence as each is completed, beginning with Carmarthenshire, and together the series will place all 46,522 recorded bench marks of Wales onto interactive maps for the first time.

The master map presents the 2,916 special marks of the network in three layers: the nineteen Fundamental Benchmarks, granite pillars anchored to bedrock from Holyhead to Penhow, still used by Ordnance Survey today; the 1,472 numbered flush brackets that carried the precise levelling lines between the pillars, many set into the walls of chapels, churches and bridges; and the 1,425 bolts, pivots and brass rods that completed the system, from a tollgate cottage two metres above the tide to a mountain pillar at 642 metres. Each mark is shown at its recorded position with its height above Ordnance Datum Newlyn to the millimetre, its levelling history and the surveyor's original description of where it stands. The zero point at Newlyn is included as the origin to which the whole network aligns.

The ordinary cut marks and rivets, more than 43,000 across Wales, have been unmaintained since the 1980s and are quietly disappearing with the buildings and walls that carry them. This series records the system while it can still be found. Compiled from the Ordnance Survey Complete Benchmark Archive, published under the Open Government Licence. Contains Ordnance Survey data © Crown copyright and database right 2026.

Interactive map: https://tinyurl.com/How-Wales-Was-Measured

Flip Book: https://tinyurl.com/How-Wales-Was-Measured-FB

Graham T Emmanuel, Kidwelly, 2026