WGS84 to OSGB36

A transformation between WGS84 and OSBB36 requires a datum transformation. A 7 parameter transformation should give results accurate to about +/- 5m…

ArcGIS supports a number of 3 para, and 7 para transforms. CS2CS using Proj may also be used as follows :

cs2cs +proj=latlong +ellps=WGS84 +towgs84=0,0,0 +nodefs +to +proj=tmerc
+lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy
+towgs84=446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894 +units=m
+no_defs

(thanks to Paul from http://osgeo-org.1803224.n2.nabble.com/OSGB36-td2062542.html for settings)

However for a centimetre accurate transformation an NTv2 grid is required. The OS provide an online coordinate transformation service here (a site for bulk conversions is also available)

http://gps.ordnancesurvey.co.uk/etrs89geo_natgrid.asp

It’s also possible to download the grid in NTv2 format from here:

http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/furtherinfo/ostn02_ntv2.html

or directly from this link http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/OSTN02_NTv2_DataFiles.zip

Test for a point in Edinburgh (Scotland)

WGS84 –3.185, 55.95
OS Online NTv2 Conversion > 326097.07,673630.932 [taken to be correct conversion]
ESRI (3 par option_1) > 326097.86,673625.357
ESRI (3 par option_5) > 326095.042,673619.452
ESRI(petroleum option)> 326096.974,673630.405
CS2CS (not using NTv2)> 326096.98,673630.4

So it would appear that ESRI’s petroleum option and CS2CS (as above without NTv2) and the OS Online conversion tool all agree to within 1 metre, while the 3 parameter version are out by at least 5m. The options relate to which parts of the country the re-projection has been set up for… this test would suggest option_1 works better for Scotland.