Assigning Coordinate system to CAD

[Craig_Eissler_Iceman]

Note there is a related Question also posted with regard to Changing/Assigning/Defining CAD coordinate systems terminology and two different approaches.

So, to tack on to that, or this can still be treated separately I think:

I read and interpreted (correctly or not) in an ArcGIS Pro Help Doc -- that if the CAD Data has NO geospatial coordinate system, but the Units are known to be in feet or meters, that I can Assign/Define the coordinate system to be "anything" I want. but to stick with a projected system in those same units. OK, fine.

But, what if the CAD units are in "inches or millimeters"? ...Do I still stick with their respective parent units of feet and meters, respectively? Or, do I need to do something different?

[RTPL_AU]

The simple answer is to ask the source to provide it in standard units - but you won't be here if it was that easy 🙂

If the CAD data is geolocated then assign a coordsys in Catalog before adding to a map.

If the data is not geolocated/in strange units then follow the georeferencing process and provide the coordsys at that point.

With Geolocated I mean the CAD geometry numbers match those of a coordinate system, not that it has a coordsys embedded (as you get with AutoCAD Map, etc.)

If you have ArcMap open a blank map, add a dataset with the same coordsys as the CAD data, and then clear the coordinate system from the map. Add the CAD data (without assigning it a coordsys) and if it is in the right units/location it will show in the right spot. 
Remove it and then assign the coordsys in Catalog.

If it doesn't show up in the right spot you'll have to follow the georeferencing process.

[Craig_Eissler_Iceman]

Thanks, but all the Help docs I've read seem to indicate that a PRJ coordinate system/projection file MUST be assigned/defined before anything else.

So, my questions are:

1a. IF the CAD data is NOT in a geospatial coordinate system, and I need to assign/define it, it sounds like I can choose whatever coordinate system I want to use?...

1b. ...but with one requirement; that the coordinate system Units I assign/define MUST match the CAD data. But, if the CAD Units are in, let's say "inches", what do I do? ...Of course, "feet" would be in the same Unit family, so does that suffice?

[RTPL_AU]

1a. Yes - if the CAD geometry is correct then you are off to the races.

1b. Follow the georeferencing process to locate & scale the CAD data and give it the required coordsys. 
The catch is often with the large values you find when a UTM aligned file is in mm instead of m.  Inches & feet are closer in real extent so should not make a mess (but we don't see those old units much anymore so your mileage may vary...)

If the CAD is nasty try to bring  it into Pro without a coodsys (use Unknown in the map). Apply a def query and export just the bits you need to shp (yes - without a coordsys).
Apply a coordsys to the shp and then edit the geometry to offset/scale/rotate as needed. Easy if you know the transform. Messy but sometimes easier to do than to wrangle a complex CAD file where you only need a single layer from a master design.

If this is a frequent occurrence with complex data I'd suggest working with the CAD team to have them run a script to scale/convert units to something useful - often much easier to do in CAD than GIS. If your project is cost bound find the lowest cost cost-centre to perform the most data wrangling.

[Craig_Eissler_Iceman]

Thanks, but all the Docs say to assign a PRJ first, so why do they say that?