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I have a polygon layer of a grid of USGS Quads Index clipped to the Southwestern US (https://catalog.data.gov/dataset/united-states-7-5-usgs-quad-index). I also have a polygon layer of county boundaries (https://www.census.gov/geo/maps-data/data/cbf/cbf_counties.html). I would like to create a new field in the quad index file that lists all of the counties that each quad intersects. I know I can use Intersect and Union in QGIS and ArcGIS, but the trouble is that there are duplicate rows for each Quad. Whereas when I started there were 4080 records (1 for each quad), there are now something like 5852 records (multiple for each quad). Like this:

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I basically want the original 4080 records (1 for each quad) but with a list of all the intersecting counties. Something like this:

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What is the right process for doing this in ArcGIS Desktop?

I was looking at the "Make Query Table" option in ArcGIS.

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    Use spatial join. In the field options, right click the field you want to populate. Select 'join' for the merge rule and use a comma as the separator. – jbalk Oct 3 '18 at 19:41
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    jbalk is right, except that this only works for text fields that have enough characters to allow the entire list of every feature to fit in the field. So in addition, you have to change any non-text field type to Text in the merge rule and expand the number of characters allowed in the field to be sure it is long enough for the list (255 characters is the max in a shapefile/dbf file, but a gdb allows for more files characters if needed, so switch to that if you get an error about the list not fitting). – Richard Fairhurst Oct 3 '18 at 20:08
  • Thanks @jbalk and to you Richard for the swift replies! I came to the same conclusions as you both and so posted my answer as well below. – BenW Oct 3 '18 at 20:19
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I found this Q&A helpful:

Dissolving shapefile but retain attribute fields

It wasn't exactly the same, but you can use the spatial join tool from the ArcToolBox -> Analysis -> Overlay -> Spatial Join tool. I used the "CONTAINS" as it suggested, "JOIN" merge rule, elongated the field I needed (GEOID) to 50 characters, and used a comma delimiter. Results were exactly what I wanted:

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