Coordinate System differences - NAD 27 vs. NAD 83

[DavidGray1]

Hi Community; I have a question regarding a step by step process for getting certain data from a working coordinate system into a different coordinate system.

The issue seems to be beyond the usual issue of having a feature layer adjust to the coordinate system of a project that the layer is added to.
The factors:

-I have a dataset of point data where the raw x,y cords come from an outside table source. They are in NAD 27 UTM Zone 11.
-I wish to import and portray them in NAD 83 (2011) UTM Zone 11. I understand that the two NAD systems are quite different being based upon significantly different spheroids with significantly different centroids. But how significant would the location difference be when each coord system is portrayed in UTM Zone 11's(US)?
-I assume that if one were to enter the x,y NAD 27 coords into an attribute table in NAD 83, then PRO would treat the x,y location data as being numbers in NAD 83.
- However I wish to portray the NAD 27 locations in the NAD 83 map to see and compare differences. Would the difference be a few feet, hundreds of feet, a few inches?
-If one were to enter the data in a table in a NAD 27 project - where the original map frame is the NAD 27 coord system, can one bring that over into the NAD 83 project and have it located by the NAD 27 coord values? There must be a better "workflow".

I almost always get useful help with these kinds of inquiries in this community. Hope so this time around, too. Thanks; Dave

[DavidSolari]

By default Pro maps will reproject all data into the coordinate system of the map, which includes any necessary datum transformations. Geoprocessing tools will also try to reproject and transform input data into the target format, but you either have to set these values on the "Environments" tab of the tool if it creates a new feature class, or hope it uses the correct transformation when you add data to an existing feature class.

When in doubt, use the Project tool to get all your input data into the correct coordinate system (or the correct datum at the very least) before processing to ensure errors are minimized. Datum transformations aren't always lossless -- and I'm pretty sure NAD27 to NAD83 is a bit lossy -- but the error is often well within the tolerance of your task.

[DanPatterson]

off by potentially 100's of feet depending on latitude etc

Make XY Event Layer (Data Management)—ArcGIS Pro | Documentation

and define your NAD27 data with the appropriate utm zone and datum within that tool

It will "project on the fly" to line up with map data set to a NAD83

If you want to compare differences between point values between the 2 datums, use

Calculate Geometry Attributes (Data Management)—ArcGIS Pro | Documentation

twice, once using the NAD27 and once using the NAD83 datum, within the coordinate system parameter

Amongst the volumes of references on coordinate systems, projections, datums and the like,

Coordinate systems, map projections, and transformations—ArcGIS Pro | Documentation

is a good place to start