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I'm returning to Esri products after a decade away. I first learned on ArcGIS 9 but switched to FOSS alternatives and have never touched ArcGIS Pro or ArcGIS Online (AGOL) until the last few weeks.

How do I check my map's projection in ArcGIS Online's Map Viewer (classic or new)?

If I understand correctly, the map viewer's map projection is determined by the projection of the basemap, and any layers in another system will be projected on-the-fly for a visual match. But I've seen several sources saying "don't keep layers in the web Mercator projection if you want to do spatial analysis" - because of the Mercator distortion away from the equator.

I do not know what my basemap's projection is.

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  • Most of the standard basemaps (Google, Microsoft, Esri,...) all use Web Mercator, so odds are, your basemap is Web Mercator, but there are basemap providers that use other projections. If you have a local GIS client, it would let you inspect the CRS of the service. When using AGOL, you'd need to find the properties of your basemap in the UI. While in the abstract, you don't want to use Web Mercator for anything, as a practical matter, you'll get the best performance if your data layers are in the same projection as the basemap.
    – Vince
    Feb 5, 2022 at 5:10

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You can check the Spatial Reference of a hosted service through its REST end.

For example:

enter image description here

Scroll down to view the REST End of this service:

enter image description here

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