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,