[GIS] Correct CRS for the Ordnance Survey Open Data products

coordinate systemdataordnance-survey

What's the correct CRS for the Ordnance Survey Open Data Products?
The spatialreference website shows a number of projections identified as British National Grid:

SR-ORG:6812: BritishNatGrid
SR-ORG:6832: UK National Grid
SR-ORG:6971: Great Britain National Grid with additional Helmert Corrections
SR-ORG:7132: uk nat grid
SR-ORG:7237: test
SR-ORG:7340: test
EPSG:7405: OSGB36 / British National Grid + ODN height
SR-ORG:7415: ordinance survey boundry-line
SR-ORG:7460: osgb:BNG
EPSG:27291: NZGD49 / North Island Grid
EPSG:27292: NZGD49 / South Island Grid
EPSG:27700: OSGB 1936 / British National Grid

Best Answer

The EPSG code for OS products is epsg:27700. If you need further formats (though that should be enough for any good GIS) you can visit http://epsg.io/27700 which will allow you to download a specific format such as proj.4 directly.

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[GIS] What are the differences between 81989 and 27700 CRS for the British National Grid

The two coordinate systems are essentially equivalent: 81989 is the older "legacy" definition that used to ship with Oracle before 10gR2. 27700 is the EPSG definition that ships since 10gR2. Transformations give slightly different results due to the difference in datum shifts.

Onto your specific question: your Mastermap data is 27700, your point data is 81989. Will you be able to use your 81989 points to search the 27700 Mastermap data ? The answer is yes. Queries will automatically transform your points from 81989 to 27700 to perform the searches. That transformation will either retain the same coordinates, or slightly shift them. The main issue will be wasted performance due to this essentially unnecessary transformation.

A better solution will be to change the SRID in your points. For that, you need to drop the spatial index, update the SRID inside your points, change the SRID in your metadata and rebuild the index. To replace the SRIDs in the geometries use this approach:

update my_points p set p.geom.sdo_srid=27700;

i.e. remember to use an alias.

Generally speaking it is better to use the EPSG codes instead of the Oracle "legacy" SRIDs. EPSG codes are generally a bit more accurate, but more important they are more "standard" and make it easier to exchange data with others.

[GIS] CRS transform from EPSG 4326 (WGS 84) ? to EPSG 27700 (OSGB 1936 / British National Grid)

Solved, thanks.

Needed to being ST_Transform instead of ST_SetSRID (see last comment / link)

ALTER TABLE public.roads_sco ALTER COLUMN geom TYPE geometry(MULTILINESTRING, 27700) USING ST_Transform(ST_SetSRID(geom,4326),27700)

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Related Solutions

[GIS] What are the differences between 81989 and 27700 CRS for the British National Grid

[GIS] CRS transform from EPSG 4326 (WGS 84) ? to EPSG 27700 (OSGB 1936 / British National Grid)

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