How to define a custom projection?

Keith,

What would you consider "not spatially large" ?

Our site is over 300 square miles.

 Jim

 

Keith,

What would you consider "not spatially large" ?

Our site is over 300 square miles.

 Jim

 

I don't know if I could put a size on it but that sounds 'spatially large' to me!  I presume you are using a local CS with something like 0,0 at the lower left (or similar) of the site.  Perhaps you could define the appropriate state plane GCS for your DGN files and then define an ACS such that users can key in their coordinates from this known benchmark as they usually do.