I am trying to add a shapefile to a mxd file in =ArcGIS 10.6. When I do, I get the following error message:Unknown Spatial Reference

The polygon is always completely off the map. I have tried to use the define projection tool, but that does not put the polygon in the correct place. Is there another way to set the projection in the correct spot?

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    That message isn't an error, it's just a warning. Do you know what the correct spatial reference is for the shapefile? – Dan C Jan 17 '19 at 19:05
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    Ideally there should be a .prj file along with .shp file. Did you create the shapefile or get it from an external source? – anjama Jan 17 '19 at 19:06
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    This may or may not help: desktop.arcgis.com/en/arcmap/latest/map/projections/… – mkennedy Jan 18 '19 at 17:35
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    Also look at a sample of the YX coordinate and add it to your question, it would help to determine if you are looking for a projected coordinate system or not (and if you says where in the world it should be, somebody may recognize it...) – J.R Jan 23 '19 at 16:09

In order for the layer to show up in your map, you will need to know what the data source is for the layer with the "Unknown Spatial Reference". If you don't know it already, you need to find out. It is best if you can find out by referring to wherever the data came from. If you find out, skip to the next paragraph. However, if you are really stuck, you can use a little bit of "trial and error" if you know some of the common projections for data in your area/region.

Try changing the projection of the data frame to one that you think (or know) it could be. For example if you are in Indiana, you might try their state plan coordinate system and if that doesn't work maybe try the UTM zone that is appropriate for your location. Just right click on the Data Frame (typically named "Layers" in the table of contents) and choose the projection tab (see my screenshot).

Once you have found the one that makes the data appear correctly you may want to assign that projection permanently using the Define Projection Tool. That one is in "Data Management Tools"->"Projections and Transformations"->Define Projection.

Projection Dialog

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