Coordinate Systems in ArcGIS for AutoCAD; Is it morning already?
The Earth is round, so even though we may have an accurate measurement that same measurement has a geographic relevance for a particular place. The watches on both of our wrists were accurately measuring the time in our zone. Trouble was I was out of my zone.
For many CAD related efforts I may be entering coordinates accurately in my zone, be it state plane or UTM or the alike. However, without some means to identify which zone I am in, or more basically what coordinate system my coordinates are depicting, when I am outside that zone things don’t line up.
In ArcGIS for AutoCAD you can identify which coordinate system you are in so that map services from other coordinates systems will automatically line up in the right locations. Likewise when the drawing is added to ArcGIS software it will also line up. By default ArcGIS for AutoCAD will assume that if you are adding a map service within an AutoCAD file without a coordinate system defined you will want your drawing to have same coordinate system as the map service. This may not be true; therefore you will want to select your coordinate system from a list of some 4000 coordinates systems that ship with ArcGIS for AutoCAD first. If your AutoCAD file has a coordinate system definition in an external .PRJ file, ArcGIS for AutoCAD can import that .PRJ file and store it inside the CAD file according to the Mapping Specification for Drawings.
For Microstation files and AutoCAD files not used in ArcGIS for AutoCAD you would copy a .PRJ file of the appropriate coordinates system in the same directory old-school.