An interactive ten-layer map plotting 4,608 Ordnance Survey benchmarks across Carmarthenshire, drawn from the national benchmark archive and presented with a full companion document. 

For more than a century, surveyors chiselled cut marks and set rivets into the walls, bridges, churches, chapels, farms, milestones and mountain rock of the county, each one recording a height above sea level measured to the millimetre. Maintenance of the network ceased in the 1980s and the marks have been quietly vanishing ever since.

Three county layers carry the complete record for the west, centre and east of Carmarthenshire, while seven thematic layers gather the marks by what bears them: churches, chapels and cemeteries; farms; bridges and railways; milk stands; milestones and wayside features; village landmarks; and upland rock. 

Read one way the map is the levelling network of a county; read the other it is an index in stone and brass to the built and working heritage of Carmarthenshire as the surveyors found it, from a coastal mark last verified in 1878 to a rivet near the summit of the Black Mountain pass. 

Every mark is presently recorded as unvisited, and the map is offered as the starting point for a county-wide effort by walkers, schools and local history societies to find and record what still survives.

• Interactive Map Link: https://tinyurl.com/Carms-OS-Benchmarks-Map

   

• Companion Master Project: https://tinyurl.com/How-Wales-Was-Measured-Master