Ordnance Survey - Cartography

Cartography

The original maps were made by triangulation. For the second survey, in 1934, this process was used again, and resulted in the building of many triangulation pillars (trig points): short (approx. 4 feet/1.2 m high), usually square, concrete or stone pillars at prominent locations such as hill tops. Their precise locations were determined by triangulation, and the details in between were then filled in with less precise methods.

Modern Ordnance Survey maps are largely based on aerial photographs, but large numbers of the pillars remain, many of them adopted by private land owners. Ordnance Survey still has a team of surveyors across Great Britain who visit in person and survey areas that cannot be surveyed using photogrammetric methods (such as land obscured by vegetation) and there is an aim of ensuring that any major feature (such as a new motorway or large housing development) is surveyed within six months of its construction. While original survey methods were largely manual, the current surveying task is simplified by the use of GPS technology, allowing the most precise surveying standards yet. Ordnance Survey is responsible for a UK-wide network of GPS stations, known as OS Net. These are used for surveying but other organisations can purchase the right to utilise the network for their own uses.

Ordnance Survey still maintains a set of master geodetic reference points to tie the Ordnance Survey geographic datums to modern measurement systems including GPS. Ordnance Survey maps of Great Britain do not use latitude and longitude to indicate position but a special grid. The grid is technically known as OSGB36 (Ordnance Survey Great Britain 1936), and was introduced after the retriangulation of 1936–53.