Does Collector does spatial joins with underlying layers? In my searches I haven't been able to confirm or deny. Thanks.
Hi Bob -- Could you clarify a bit what you are looking to do? For example, you can use the Join Features tool in ArcGIS Online or ArcGIS Enterprise, save the result as a new hosted layer view, and then use that view in maps in Collector. However, these layers will be read-only and can't be taken offline.
(Source: Add editable layers to a map, step 7 - in Make a map—Collector for ArcGIS | ArcGIS)
I’m wondering if Collector can do a Spatial Join, where attributes of interest from an underlying layer are added to table of the newly-created feature.
No, this isn't something Collector supports. I'd recommend using other ArcGIS tools to do this once back in the office.
Relationships classes work great for this. Link the two tables instead of doubling up the attributes.
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