TM65 to WGS 84 geographic transformation issues

[SiobhanMcDermott]

Hi there,

I am trying to projected data from Irish National Grid TM65 to WGS 84 Auxiliary Sphere for upload to an ESRI StoryMap. The data was generated as part of a detailed diff-GPS survey and it needs to have an accuracy of 10 - 20cm. However when I use the project tool, and any or none of the geographic transformations, the data is off by about 8 - 12 m to the south east. 

I have tried a number of solutions beyond the project tool - opening data into fresh mxd, setting data frame projection, exporting data. I'm wondering if this issue may be similar to those encountered with BNG Why are my OGC® Web Services displaced by about 120 metres? — Esri UK 

Any help would be really appreciated. 

S. 

[JayantaPoddar]

Have you tried both the following transformation methods?

• TM65_To_WGS_1984
• TM65_To_WGS_1984_2

It would be better to move this post to https://community.esri.com/groups/coordinate-reference-systems?sr=search&searchId=86647257-4720-4783...‌

[SiobhanMcDermott]

Yes, all the available transformations. They are all out - shifting to the east or south-east by about 8 - 12 m depending on which transformation used. 

I think it may be similar to the BNG issues mentioned here Understanding which Transformation to choose in ArcGIS — Esri UK 

But I can't find any info on how to solve this. 

[MelitaKennedy]

How close is it if you unproject to TM65 and redefine it as WGS84 AKA no transformation? 

Alternatively, redefine the data as TM75 and use this transformation: TM75_To_WGS_1984_4_NTv2 which uses an NTv2 grid file. 

Who created / what's the source of the Web Mercator data that you're comparing against?

Melita

[SiobhanMcDermott]

Do you mean to use the define tool rather then project? I just took this literally and did both, just to make sure.

If I define WGS84 from TM65 then it is just off the west coast of Africa. I can't seem to preform the project without selecting one of the transformation options? I believe I did manage to do it when I was trouble shooting this a day or so ago and it was out as well. 

But could that be a clue to the problem? 

I carried out the TM65 to TM75 then TM75 to WGS 84 using the transformation you suggested and it is still out by about 9 meters. The grid files I have in my system are tm75_etrs89 (Ireland folder) and OSTN02_NTv2 (UK) folder. 

The area I'm looking at is in the North of Ireland. So it is on the same land mass and grid as the Irish Republic. 

I'm using the ESRI aerial imagery basemaps as my target comparison.

S. 

[MelitaKennedy]

Hi Siobhan, 

Do you have access to the data producer? Was the diff-GPS survey originally done in ITRF or ETRS and converted to TM65 Irish Grid? If that's the case and we can find out what transformation was used, we can try to 'undo' it. Could it really be on IRENET95 Irish Grid?

Melita