Geographic Information Systems Asked on December 21, 2020
In ArcMap, I regularly use the Append Geoprocessing tool to append data from a dataset in one projection into another dataset that is in a different projection. When you append between projections, as expected, there are slight shifts in the data. I’m fine with this as the shifts have always seemed to be consistent, meaning it appears to use a consistent transformation in this process.
However, I’m now in a situation, due to some additional datasets in other projections getting involved, that I need to know what transformation is being applied by the append tool.
I know I can use project on the data to a temporary dataset, defining the transformation in the project tool, and then append from that temp dataset. However, that doesn’t change the fact that if I do that, without knowing what transformation has already been being used by the append tool, then my data will no longer match where boundaries have historically been according to the append tool workflow (which creates edge-matching issues with other data-sets).
So, does anyone know how the append tool transforms data when it is appending between projections?
There are a few things that could be happening in the Append tool depending on how it's implemented. I would try using the Project Tool, and take the transformation/path that's listed first.
Compare those results with the existing Append workflow. If they match, you have your answer. If they don't match, try reprojecting the input data in ArcMap by setting the data frame to the target coordinate sytem. Do NOT set a transformation. If you look at the geographic transformation dialog (data frame properties, coordinate system tab, transformations button), it should say None. Compare results.
If those results don't match either, you'll have to do some more digging. In the pedata folder of your ArcGIS Desktop installation folder is a file called gtdefaults.json. That's a fall-back file for certain workflows when no other transformation has been set. However, this file uses the WKID (well-known ID) codes for the geographic coordinate systems.
The reason why Project Tool (or Transformation dialog in ArcMap/ArcGIS Pro) may differ from what gtdefaults.json is because the latter is very generic and assumes you're using the full extent of the intersection of the input/output geographic coordinate reference systems while the former take into account data extents.
You can comment/edit your question with the info and I'll amend my answer.
Disclosure: I work for Esri on this stuff and I am a member of the subcommittee that manages the EPSG registry.
Correct answer by mkennedy on December 21, 2020
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