Using Custom Transformations

[BerndtNording]

I am trying to use some custom datum transformations in Pro. These were created to work in conjunction with a standard transformation as a composite transformation. I have noticed that the standard transformations are bi-directional, i.e. the same transformation is used no matter which is the "from" datum and which is the "to" datum.  The custom transformations are also bi-directional, but they only show up for one direction in the transformations list box in Map properties. I've found some workarounds but neither is ideal:

If my custom transformation doesn't show up, I can change my map spatial reference to be the same as the layer that needs the transformation, then select the transformation that uses the custom transformation, and then switch the map spatial reference back again. (The selected transformation remains and works as it should)

The second workaround involved writing a .net AddIn to set the transformation which works great but is limited to the custom transformations it is written for.

The third workaround is to develop a second set of custom transformations that work in the reverse direction.

Is there some way to get Pro to consider custom transformations as bi-directional when it decides what transformations to show in the dropdown list?

[Robert_LeClair]

Tagging @MelitaKennedy for this question.  She's the best person I know in Esri to answer this question.

[MelitaKennedy]

Berndt, 

Are you finding that the custom transformation isn't showing up / cannot be chosen when it needs to be applied in the opposite direction? We do only define transformation in one direction, but the code knows to apply them in either direction. We didn't want to spend the time to flip the transformation name in the UI. 

Melita

[BerndtNording]

Thanks to @Robert_LeClair  for "Tagging" Melita, didn't know how to do that. I have spoken with Melita several times before concerning issues on another island in the vicinity!

Melita,

  Yes, that is exactly what I am finding, and likewise I didn't want flip the transformation either. You can try it yourself: I'm attaching a zipfile containing 3 .gtf files and 3 more that are "flipped". Also in the zipfile is a little file geodatabase with point features of USGS benchmarks using the published coordinates on each of three islands (Saipan. Tinian, Rota) in both UTM as well as the local coordinate system so you can see the result of the transformation.

Rota has a couple additional issues:  I have been unable to find any information on the details of the spatial reference used there, nor have I encountered anybody out there that knows. I know it uses the same Geo Datum as the other islands and also the same projection but with different parameters. I stumbled across one single .prj file and used it to create a custom transformation which works well, but the Geographic offsets therein are so large that I don't think the .prj is right.

These custom transformations were made to work together with the "Guam_1963_to_WGS84_Saipan" transformation which was fine in ArcMap, but Pro ignores it - I think because Rota is a little bit south of the bounds in the documentation for that transform.