Transformation warning: What does it mean and what should I do?

[HeatherSmith]

You just opened a project in ArcGIS Pro and a little message popped up, warning you that a datum transformation cannot be found and that the data may draw with an offset.

If that sounds scary or confusing, read this article on ArcGIS Blog to find out why you are seeing it and what you can do: https://www.esri.com/arcgis-blog/products/arcgis-pro/mapping/transformation-warning/ 

[BrittanyBurson]

Hi @HeatherSmith, thanks for the post. My understanding is the Esri basemaps always use the Web Mercator (auxiliary sphere) projection as a default, and while the map projection can be adjusted within Pro during the session, this setting cannot be saved beyond that. Our rest services and Portal/AGO layers are typically published in our local projection, CA State Plane VI, so I run into this alert each time we open a web map using an Esri basemap. Do you have recommended best practices for this scenario? Or is it just to use a locally hosted basemap? Thank you, and apologies if I missed something obvious.

[HeatherSmith]

Hi @BrittanyBurson 

Yes, almost all Esri basemaps use the Web Mercator projected coordinate system. You can't change the coordinate system (CS) of a basemap, although you can make it display in another CS by changing the CS of the map. (what we call projection-on-the-fly).
The recommended best practices for this scenario are outlined in the blog I linked to above - if you walk through those steps they should cover your scenario.

If you have a locally hosted basemap in your state plane CS, using it instead of Esri basemaps will prevert the warning, but it shouldn't be necessary. We have transformations available between state plane coordinate systems and web mercator, so you should be able to resolve the warning by following the steps in my blog.

[BrittanyBurson]

Thank you @HeatherSmith. I was mostly curious if the display change could be saved back to the web map somehow, instead of setting it each time you open the map in Pro. Sounds like no, which is OK. Appreciate the reply!

[HeatherSmith]

Good question! You are correct, but here's a bit more explanation:

Web maps (unlike maps in ArcGIS Pro) can't have a different coordinate system than their basemap. Because most basemaps are in Web Mercator, this means that most web maps also have to be in Web Mercator. In your case, yes, the web map will always be in Web Mercator, and when you view the web map in ArcGIS Pro, you can temporarily change its display.

This lesson shows you how to make web maps in other coordinate systems, by creating and publishing simple basemaps: https://learn.arcgis.com/en/projects/make-a-web-map-without-web-mercator/