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My question is specifically referencing the file-based grid transformation methods that are now available in ArcGIS. I thought this grid-file method was exactly the same method that NGS's NCAT used when moving from NAD 83 to NAD 83 HARN, however I am not getting the same outputs. They are very close and seem to always been in the correct direction, but ArcMap's outputs fall short when compared to my outputs from NCAT. Either way, these state-specific methods (at least in my state) appear to be far more accurate than any others that are listed in the transformation drop-down menu.

Since this doesn't require concatenating transformations at all, why would Arc's outputs be different than NCAT's?

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NCAT contains several countrywide transformation files between NAD83 (1986) and HARN because it's using NADCON5 files. The files in ArcGIS are the older NADCON files which were built for individual states or a small group of states.

NCAT/NADCON5 also differentiates between different HARN re-adjustments that were done in the various states. The older NADCON solution only has files for the first HARN re-adjustment in each state.

National Geodetic Survey (NGS) did not try to replicate the results of the previous HARN transformation files.

We do plan to add the NADCON5 files to ArcGIS but I don't have a particular version or date yet (as of May 2019).

Disclosure: I work for Esri as a Sr. Product Engineer specializing coordinate reference systems and transformations.

  • Thanks mkennedy, makes sense. I was hoping you would see this question. – vce500 Jun 4 at 19:33

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