Layer has no spatial reference

[JuliaGoldsworth]

I imported a PDF of an old map and made a layer of polygons for land use.  This new layer, though, has no spatial reference.  I cannot project the layer because there is no coordinate system to project it from.

How do I give this layer of polygons a spatial reference?

Thank you!

Julie

[ModyBuchbinder]

In ArcMap you can find Define Projection GP tool that will do it.

You need to know what is the correct spatial reference - no tool can find this for you.

Have fun

[DanPatterson_Retired]

Importing an old pdf of a map will just give you a map referenced in pixel coordinate space.  You have to ... georeference ... the file in order to provide it with real world coordinates.  Just defining its coordinate system will make a bad situation worse.

[JuliaGoldsworth]

The pdf of the old map was georeferenced and it sits nicely in place above the basemap.

That should not affect my new polygons layer.  I did Define Projection to the same projection as the other layers in my map, but that has not helped.  

[DanPatterson_Retired]

you set the projection of the new polygons to that of the old map then, not your other files.  If the new polygons are given a wrong coordinate system, they won't align with your old data.  If you want the new file to be in a different coordinate system, you need to define the projection correctly first, then you use the Project tool to produce a new file with the desired coordinate system.

I wrote a visual guide as to what can happen if you mix and match the order of things... define projection vs project