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Interpreting 'GIS' in the geographical information science sense, I have a historical question about the 1936 OSGB datum following the 1936-62 retriangulation of Great Britain - hopefully not too off-topic..

Does anyone know of the reason for the 49°N 2°W "true origin" of the datum? Ordnance Survey website's closest comment on this appears to be:

The True origin latitude and longitude coordinates of the National Grid are 49 degrees north: 2 degrees west. The False origin latitude and longitude or coordinates are 49 degrees 45 minutes and 58 seconds north : 7 degrees 33 minutes 23 seconds west.

The False origin which lies slightly southwest of the Isles of Scilly was devised to ensure that all National Grid coordinates were positive

So the "false origin" at 0 Easting 0 Northing (approx 7.56°W 49.77°N) is chosen so the Scilly Isles and western point of Northern Ireland would remain in positive coordinates - an admirably egalitarian nationalist justification. But 2°W seems a very England-centric longitude, approximately bisecting the 1.7°E and 5.75°W extremes of the nation. And the only redeeming feature of 49°N seems to be that the 49°N 2°W "true origin" is marginally closer to a UK dependency (Jersey) than the French mainland. So why this parallel/meridian intersection?

  • Possibly because Ireland (inc the North) use a different grid? – Ian Turton May 22 '14 at 13:54
  • Ordnance Survey Zero Meridian is slightly west of prime meridian en.wikipedia.org/wiki/… – Mapperz May 22 '14 at 16:16
  • Yes, for the relationship between OSGB36 and global longitude (now WGS84). – geotheory May 22 '14 at 16:54
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    You're not asking about the datum origin but the grid or projected coordinate reference system origin. The origin of the 'datum' was originally "fundamental point: Herstmonceux, Latitude: 50°51'55.271"N, longitude: 0°20'45.882"E (of Greenwich)." (from EPSG Registry) – mkennedy May 22 '14 at 18:25
  • Good point thanks, I wasn't aware of Herstmonceux. Will read into it. – geotheory May 22 '14 at 22:06
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For the meridian, the answer is simple: It lies almost in the middle of England, this meridian will be parallel to the local transverse mercator grid, while the others have minimal bendings. This is how the OSGB grid looks on a lat/lon map:

enter image description here

For the parallel, it does not make a difference for the projection which one you take for as center. German tmerc Gauss-Krueger projections use the aequator.

Maybe they intended to use Jersey as origin, but then decided to cut of the first 100km.


The 100km grid is actually a subsystem of a 500km grid which is indicated by the first grid letter, as explained here: Eastings/Northings OSGB36 Grid origin

  • Historical is London where Ordnance Survey was based until a fire in Tower of London (1841) - Meridian was in walking distance to start triangulation of GB hrp.org.uk/TowerOfLondon/stories/buildinghistory/… – Mapperz May 22 '14 at 16:22
  • Thanks Andre, sounds right. If they wanted an n*100km vertical gridline on the true origin so it pointed due North, 400kms makes sense for its proximity to the population centroid. Which would also mean the true origin's exact position was determined by the western point of N.I. I'll leave the question open for a bit in case anyone else can contribute, but thanks. – geotheory May 22 '14 at 16:38
  • Interesting link thanks @Mapperz, although surprised it makes no mention of Major General William Roy and the original baseline he surveyed in 1780s. – geotheory May 22 '14 at 16:43
  • No, the other way round: true origin is where meridian and projected grid align, and should always be in the middle. 2°W was just a good and even value. The false origin was defined n*100km westwards where all islands were included. – AndreJ May 22 '14 at 16:50
  • Ah of course. I was tripping over the coincidence that the false origin happens to be almost exactly on the same longitude as the Western tip of N.I.: 54.464754°N, 8.177431°W converts to -66 meters easting (which I'd guessed was due to error). – geotheory May 22 '14 at 21:23

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