Why does the geographic transformation involving the 1st layer get listed?

[Craig_Eissler_Iceman]

Hello,

Question: Why does the geographic transformation involving the 1st layer get listed in the Map Properties> Transformations section? (see below for example):  I'm simply adding a layer in NAD 83 to a brand-new ArcGIS Map set to WGS 84 (Web Mecator, auxiliary sphere, WGS 1984)

By the way, I understand what a geographic transformation is, and the concept of projection on-the-fly. 

But I may not be fully understanding this specific ArcGIS behavior, because my knowledge is that:

• The 1st layer added does NOT need to be projected on-the-fly because it simply resets the Map Display coordinate system -- so that all other layers added "THEREAFTER" get projected (on-the-fly) to this coordinate system
• In other words, I would expect to see Geographic Transformations to be listed only "AFTER" that.

...What am I missing?

The only thing I can think of that could be the reason the 1st geographic transformation gets listed is, perhaps, because the Basemaps already in Contents (on the map) get re-projected on-the-fly?

...Is that the reason?

[DavidPike · Accepted solution]

Yes, it's due to the default World Basemap/Hillshade in Web Mercator Auxilliary Sphere.

[Craig_Eissler_Iceman]

Thanks for confirming, David.

[CalvinHarmin]

For what it's worth, if you are an admin of the Portal/AGOL organization that you are signed into Pro with, you could potentially change the org's 'default' basemap to a projection that you all primarily use. In my case it's NC State Plane. I created a custom vector tile basemap using that projection and set it as my Portal default basemap.