Projecting a basemap?

[JillClogston]

I am working in ArcGIS Pro v 3.2.0. 

It appears I have suddenly forgotten everything I have learned about map projections! I want to work in NAD 1983 UTM Zone 10N. When I add a new map to my blank project, it creates the new map in WGS 1984 Web Mercator (auxiliary sphere) with World Topographic Map as the base. From the map properties window I can change the projection to NAD 1983, but the World Topographic Map doesn't change to the new projection and I'm not seeing how to change it? Tried the Project (Data Management Tool) but that doesn't allow me to select the basemap. 

Pretty sure I am overthinking this so any help is greatly appreciated!

Jill

[MErikReedAugusta]

Before I got around to changing my defaults, when I started a new project in ArcGIS Pro, it'd always be in WGS 1984, just like you describe.  I'd then go into the properties of my Map and change the coordinate system to the one my organization uses (also a NAD1983 derivative, coincidentally, but a different one), and then make sure to select a transformation from the drop-down there.

Then, as soon as I hit okay on the Map Properties and my map refreshed, I could see it visibly distort relative to WGS 1984 as it applied that transformation.

My bet is that either you forgot to specify the transformation when you selected the projection, or the distortion is happening, and you happen to be in a part of North America (I'm not familiar with 10N offhand) where the difference is slight enough that you didn't notice it'd changed.

[ChristopherCounsell]

ArcGIS Pro by default loads an Esri basemap and temporarily sets the map properties to match its coordinate system - WGS84.

When you first add a layer to the map, ArcGIS Pro will update the coordinate system of the map to that layer. This change only occurs once - adding more layers won't continue to update the coordinate system.

Layers in your map will project - and transform - to the coordinate system in your map properties. Transformations are set in the Map properties and ArcGIS Pro will default to the most accurate one; you can change them.

https://pro.arcgis.com/en/pro-app/latest/help/mapping/properties/coordinate-systems-and-projections....

This includes Esri basemaps - ArcGIS Pro will attempt to project and transform this on the fly to align with your map coordinate system.

You'll need to export the basemap out to create a new projected dataset. Different basemaps support exporting. You won't be able to export the entire basemap. You can find the export-friendly versions e.g. here's the World Imagery basemap below. 

https://www.arcgis.com/home/item.html?id=226d23f076da478bba4589e7eae95952

You could try creating a map of just the basemap and exporting to .tiff, just make sure to check the 'write world file option'.

https://pro.arcgis.com/en/pro-app/latest/help/sharing/overview/export-a-map-or-layout.htm

[BarryNorthey]

Might be relevant to your question. You can set the spatial reference system for all new Maps to the one you use most or all of the time under Project | Options | Map and Scene | Chose spatial reference:

It will otherwise use the default (top radio button).

You can also control basemap loading here. I set mine to None and add a basemap as needed.

[JillClogston]

Thank you everyone! I just tried the suggestion of setting my default projection to NAD 1983. I then created a new blank project, then inserted a new map. The map indeed is in NAD 1983, but why would the basemap still be in WGS 1984?

[BarryNorthey · Accepted solution]

All layers in the Map are Projected on the fly by ArcGIS Pro to the coordinate system of the Map. This is to automatically align them with one another in the Map display. The coordinate systems of the layers in the Map do not change.

[Craig_Eissler_Iceman]

...Well, this is not totally true; though it may just be a matter of "word -phrasing". If a LAYER has the SAME coordinate system as the MAP DISPLAY, no Projection on-the-fly is necessary by ArcGIS. And, this is what you want; it's better to avoid Projection on-the-fly if you can, by re-Projecting your LAYERS "permanently" using the PROJECT tool. The main reason for this is more reliable spatial analysis and accurate editing. In other words, it's preferable to have all of your LAYERS and the MAP DISPLAY to be in the SAME coordinate system.

And, of course, we're referring to an appropriate 'Projected' coordinate system. You do NOT want to be conducting analysis or editing in Geographic coordnate systems like WGS 84 or NAD 83 ...as have been mentioned a few times in this thread.

Esri Basemaps are in the default WGS 84 Web Mercator (auxiliary sphere) projected coordinate system. Unfortunately, they cannot be (re) projected because they are Vector Tile layers which is something different than a Vector or Raster data type. ...Yes, it is being Projected on-the-fly if you change the Map Display coordinate system. So, for that reason, plus that you may not know how positionally accurate the Esri Basemap is, you may not want to use this as a reference for digitizing or editing features. Typically, adding your own Basemap in the form of a recent orthophoto (aerial image) is best for those purposes.

[PROBERT68]

One other thing may I suggest to you if you don't need the basemap.  You can remove them and then  make sure your projection is align with what you want . 

Unless the others suggest to you is what I agree with them.