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Updated guide to coordinate systems link

Q: If I load my ARSF data with a GPS dataset in a GIS, they don't match. Why not?

This is (probably) because the ARSF dataset has been provided in British National Grid (BNG) projection and many GIS programs (such as ENVI) do not convert correctly between BNG and projections using the WGS84 datum. Note if you don't understand the difference between an ellipsoid, a datum and a projection, you're going to have trouble here - see this FAQ.

In a nutshell, BNG projection used to be defined against the OSGB36 datum, generated by traditional surveying methods. The UK Ordnance Survey (OS) wanted to move their mapping datum to the satellite-based datum ETRS89 (this is mostly the same as WGS84, it's just moving eastwards by 2.5cm per year or so relative to WGS84 to account for continental drift), but they wanted all their old maps (using the OSGB36 datum) to remain correct for most of their users. The solution was something called the OSTN02 transformation, which provides a non-linear transformation from eastings-northings points defined in ETRS89 to eastings-northings points defined in BNG against the OSGB36 datum. All this is is a grid of points every 1km throughout the UK at which the eastings-northings co-ordinates are defined for British National Grid projection both against the ETRS89 datum and on the old maps (essentially against the OSGB36 datum). To obtain the coordinates of a point between these postings you have to interpolate.

Confusingly, to go from Lat/Lon (unprojected) data defined against the ETRS89 datum to eastings-northings (projected) data defined in the BNG projection, you first have to project the points using the BNG projection information (using ETRS89). You then use OSTN02 to transform them into BNG co-ordinates that can be matched to an OS map.

There are two solutions:

  1. Tell your GIS that your ARSF data are defined against the OSGB36 datum (not ETRS89). This will introduce errors of the order of 10cm or so, but will otherwise match your other data.
  2. If this accuracy is not sufficient, the data will have to be reprocessed by ARSF-DAN using WGS84 throughout. Please contact arsf-processing@… to discuss your requirements, bearing in mind that we cannot guarantee to improve accuracy beyond this level in any case.

Relevant links:

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