# Longitude Latitude to British National Grid XY PROBLEMS GALORE

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10-15-2012 04:18 AM
New Contributor III
Hi,

I am trying to convert some Lat/Long coordinates to the British National Grid to no avail.  I have read MANY posts here and so many people have the same problem and get no satisfactory answers OR incomplete answers.

Please could someone explain how to do this properly because I notice that some just stick to the 'ArcMap will project on the fly' method which has never worked for me and, if it does, then its not clearly explained from the beginning.   This is something that is so important and has been raised by so many people so many times that a solution or tool is required.

I have attached the file with the latitiude and longitude coordinates.  I did get them to display in ArcMap but again (like EVERYONE points out) they weren't projected properly and all the points lay to the south west of the UK in the sea!  At one point I managed to get the X coordinates using the Project tool but not the Y coordinates (which were all minuses).  It beggars belief that there is not a simple way of doing this.

I would be grateful for any help on this and if it works then I'll post an explanation of  how it worked.

Thanks

Scott
Tags (2)
5 Replies
Occasional Contributor III
In AcrCatalog, right click on your table, click create Feature Class, and use Geographic Coordinate System\World\WGS 1984 for your new shapefile.

Open a new map, and use the British National Grid as the coordinate system. Insert your new shapefile.

Open the attribute table, add new X and Y coordinate columns in your new shape file.

Right click on the new "X" coordinate field and click Calculate Geometry, and use the coordinate system of the data frame. Do the same for the new "Y" field.

Your new shape file now has the Lat Lon and BNG coordinates. Export the shapefile, using the same coordinate system as the data frame.
New Contributor III
Hi Michael

Thanks for the post you sent regarding my query - it was definately the best explanation yet and it worked!

The only downside is that the coordinates are all about 30 metres out from where they should be - although the route is correct!

I've attached a file to illustrate this as the points should really be positioned on the road and path network.

Do you know if there's a way to improve the accuracy of the points?

Thanks again though - much appreciated,

Scott
Occasional Contributor III
You'd have to find out what coordinate settings the GPS used in the first place to collect the data and create the shapefile based on that rather than WGS 84 I used. I'm not sure if this will shift them for you or not though.
New Contributor III
Thanks again Michael - I'll give that a go and post a reply
Esri Notable Contributor
If the lat/lon coordinates really are in WGS84, you need to set a geographic/datum transformation to convert between WGS84 and OSGB36. The best solution is to follow Jim Sibbald's instructions here which has you download a file from OS, and set up some files on your computer to properly use it. If you have 10.1, you only need to download the file, and place it in your ArcGIS installation, pedata\ntv2\uk folder.

A faster solution to see if the problem is datum transformation-related is to try to set OSGB_1936_To_WGS_1984_7PAR. The points may still be off a bit, but should be better than 30m.

Melita