I publish hosted feature services from an arcmap document that are in the NAD_1983_StatePlane_South_Carolina_FIPS_3900_Feet_Intl coordinate system. When they come into the webmap that they are used in, they shift into WGS 1984. Periodically I export and download them as FGDB, and then use these data to bring back into our SDE, which is in the NAD 1983 State Plane Coordinate System. When i see the downloaded fgdb in arc catalog it says that it is in the NAD 1983 State Plane. How can i trust that this is true? Is there a way to verify that the conversion from WGS 1984 to NAD 1983 State Plane is successful?
When the data is added to a WebMap that use WGS1984 ArcGIS Server re-projects the data on the fly (meaning it does not actually change the source data) so your data stays in State Plane and you can be assured that when you download the data it is actually State Plane.
Thanks for the answer, Robert. I had the suspicion that such is the case. However, I need to back up this assumption with evidence. Can you let me know which transformation takes place on the fly? And if there is a way to verify accuracy? I do have some of the gnss-dedicated metadata fields in the layers (pdop, horizontal and vertical accuracy), but is there anything else that I can tell the people asking that is easily conveyed?
The on the fly re-projecting is basic builtin functionality of ArcGIS in general ArcMap does it and ArcGIS Server does it. Not sure you are going to find info on accuracy. Pdop and horizontal and vertical accuracy are terms associated with data collection. I think you are thinking to hard about this. This is just basic ArcGIS functionality.
For an indication of which transformation will be used, you might try to make use of the Find Transformations operation of a Geometry service resource. This will give an indication of what ArcGIS thinks is the most appropriate transformation from your input spatial reference (WKID 2273) to output (WKID 3857). You can find the WKIDs here: Projected coordinate systems—ArcGIS REST API: Services Directory | ArcGIS for Developers.
If you don't have ArcGIS Server running a Geometry service you can use a publicly-accessible one, like the one from the USGS National Map:
Put in your input SR, output SR, desired number of results (the parameter defaults to 1 if it is left blank), and click FindTransformations (GET). The output will rank the transformations from most appropriate to least appropriate. Your first result is likely the transformation that the software is using. Then I would add your state plane data to ArcMap, then add your hosted feature service to the same MXD and pick the transformation returned by FindTransformations. If things line up as they should you will have some indication that the software is doing the job right.
Note/Disclaimer: I do not work for Esri and cannot guarantee that this is how the transformation is selected! However, I do think this would show a level of dilligence that you tried to determine which transformation was used and validate that it was an appropriate choice.