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So I am using ArcGIS 10 and trying to work in British National Grid. My shape file is in WGS84 UTM30N. By chance I passed the shape file across to a colleague and we both transformed into BNG.

When we compared the data there was a significant difference in our results. I have since cross checked with QGIS, Global mapper and a website or two and got a whole range of results but in general the most common is the result my colleague got.

My question I guess is three-fold:

  1. What have I done wrong?
  2. Why is there such a variation (~150m) in values? (1 or 2m error is probably ok for what I am looking at.)
  3. Which value is "right"?

Bit of a beginner-ish question I fear but help would be much appreciated!

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British National Grid which one did you pick? - the 'OSGB_1936_to_WGS_1984_Petroleum' is the most accurate for WGS84 re-projection geoxchange.esriuk.com/journal/tag/projections –  Mapperz Jan 15 '13 at 17:07
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1 Answer

I suspect that you and your colleague have chosen different datum transformations when converting from UTM to BNG.

This results in a datum shift (see http://en.wikipedia.org/wiki/Ordnance_Survey_National_Grid#Datum_shift_between_OSGB_36_and_WGS_84 ) - 150m would be expected.

I'm not sure of the best way to determine which shapefile is now "right" - I suspect that they are both still right, but different.

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