Difference between NAD 1983 and NAD 1983(91) within ArcGIS

[BenjaminVang-Johnson]

Hello.  I often see reference to datum NAD 1983(91).  However, I don't see NAD 1983(91) listed as the datum in any of ArcGIS 10.4 coordinate systems.  Is NAD 1983(91) the same as NAD 1983, which is used in many of ArcGIS's coordinate systems?  If not the same, what is the difference between the two datums?  And does ArcGIS have any coordinate systems that use NAD 1983(91)?

Thank you.

[DanPatterson_Retired]

An old thread  https://community.esri.com/thread/171738

MKennedy-esristaff‌ might have more updated information, but there are some links and background there that may be useful in the interim

[MelitaKennedy]

Hi, 

We don't have any NAD83(91) definitions. A dated NAD83 like usually means a HARN re-adjustment. Those were done state-by-state or a few states at a time over about 10 years. I usually group them into NAD 1983 (HARN) which then means you can use the existing HARN transformations that we have.

Melita

[BenjaminVang-Johnson]

Very useful, thanks for your help!