@zwaarcontrast/ol-graticule-os

Historical Ordnance Survey grids for @zwaarcontrast/ol-graticule. The first is the yard grid of the 1930s, the Ordnance Survey's transverse Mercator grid before the metric National Grid.

Live demo: https://zwaarcontrast.nl/ol-graticule/ol-graticule-os/

The yard grid

| Parameter | Value | | ------------ | ------------------------------------------------------------ | | CRS | OS:YARD_GRID | | Projection | Transverse Mercator, true origin 49°N 2°W | | Ellipsoid | Airy | | Scale factor | 0.9996 (reduced by one part in 2500 on the central meridian) | | False origin | 1 000 000 yards west and 1 000 000 yards south of the origin | | Unit | British yard, 0.9143984 m | | Coverage | Great Britain with Orkney and Shetland; not Ireland |

It was printed on the One-inch Fifth Edition (1931, lines every 5 000 yards), the ten-mile maps (1932) and the Quarter-inch Fourth Edition (1934, lines every 10 000 yards), and dropped after the Davidson Committee recommended a metric grid in 1938. The maps never named it; instructions in the margin explain it as "the position of a point ... defined by its distance in yards East and North of a point lying to the S.W. of England". The German 1940 reprints of the quarter-inch sheets call it "das englische Gitternetz".

Sources: R. Oliver, "The evolution of the Ordnance Survey National Grid", Sheetlines 43 (1995) 25-46 (origin and scale factor, p. 36); the OS sheet instructions quoted by R. Hellyer, "The first National Grid map?", Sheetlines 55 (1999) 31-34 (false origin, and coverage "to the very north of the Shetland Islands" with Ireland omitted).

It is the National Grid projection (EPSG:27700) in yards with another false origin. EPSG gives the National Grid a scale factor of 0.9996012717 rather than 0.9996; the two differ by less than 2 yards anywhere in Great Britain.

The grid is clipped to the faces of the Quarter-inch Fourth Edition sheets, whose sheet lines it laid out (union of the 1934-39 coloured edition footprints in the National Library of Scotland catalogue). The worked examples printed on those sheets, Helensburgh (Upper) station at E 813,800 N 1,856,600 and Horse Isle at 804,400 1,812,200, are in the tests.

The shift to WGS84 is the OSGB36 one (EPSG:1314). The grid predates OSGB36, so positions on the original maps carry the older triangulation's offsets.

Usage

import { UniversalGraticule } from '@zwaarcontrast/ol-graticule';
import { createOSYardGridSystem } from '@zwaarcontrast/ol-graticule-os';

map.addLayer(new UniversalGraticule({ gridSystem: createOSYardGridSystem() }));

createOSYardGridSystem(options) takes the ProjectedGridSystem options except crs, proj4Def and extent. Labels are full figures in yards (813,800 yd) from YardFormatter, which also parses E 813,800 style input.

OS_GRIDS holds every grid keyed by CRS (crs, name, proj4, validityWgs84, createGridSystem()). The CRS code, proj4 string, validity rings (in yards and in WGS84) and YardFormatter are exported ol-free from @zwaarcontrast/ol-graticule-os/headless.