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I am currently working alongside city council datasets derived from Ordnance Survey; this is projected in their ArcSDE system as "British National Grid" or EPSG:27700. My problem is that I am trying to overlay data downloaded from OpenStreetMap - which I do using osm2pgsql to export into a new PostgreSQL db creating PostGIS layers with the projection system ESPG:27700.

However, when I try to overlay these, the two data sources are offset from each other by 50m. The city council state that the OSM data is wrong, siting that it does not overlay well onto their aerial photography; whereas if I add the datasets to something like MapBox Streets, it would appear that the Ordnance Survey data is the one that is offset.

Which data source is wrong? How does such an offset arise? What methods can be used to align & project them correctly?

Three questions that have been burning in my mind for the last few weeks; any advice or insight on the problem would be most appreciated.

Yours in geography,


share|improve this question
is… any help? – iant Jun 4 '13 at 12:12
It depends on if the OS data has been projected correctly in the first place, but it is hard to tell from a description why there is an offset, can you provide a sample area of both datasets? – Mapperz Jun 4 '13 at 13:34
Sounds like a datum shift problem. For starters, it might be worth importing the OSM data to QGIS in its native EPSG:4326 CRS and letting it transform to EPSG: 27700 on-the-fly. Then check how this aligns with the council data. N. – nhopton Jun 5 '13 at 8:36
Thinks. Another thing to try is to load some OS OpenData rasters (Streetview, Vectormap District Raster, &c.) into QGIS and try overlaying these with your OSM data. If this works properly it means that the problem is almost certainly with the council data. N. – nhopton Jun 5 '13 at 8:48

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