Assigning Coordinate system to CAD

Question

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?

Craig_Eissler_Iceman · Answer

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

RTPL_AU · Answer

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 · Answer

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?