OSGB conversion error

Question

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!

Answer 1

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.