This is a topic that has – historically - caused confusion across all levels of Revit users, myself included. I seem to have to re-learn the ins and outs of shared coordinates and the myriad array of associated tools every time I have to intervene on a project where multiple linked models are employed. This week has been one of those weeks, but this time I am thoroughly documenting this topic in an effort to shorten the learning curve next time this comes up once again!
For starters, HERE is a very good summary blog post by Steve Stafford with links to other of his posts on this very topic.
I will write more in detail on this as time permits, but in the meantime, I will offer the single most important strategy to avoid confusion: if you don’t need Shared Coordinates, don’t use them when linking and positioning models! Sounds simplistic, but it is valuable advice and you should ensure to touch on this in your BIM kick-off meeting. If you have separate discipline models for a particular building, they should link into each other with Auto – Origin to Origin from day one. This way you reserve Shared Coordinates for other purposes. Don’t use this system to fix sloppy and incorrect linking because you lose the flexibility to use it for other desirable purposes, such as reporting true site coordinates. For example if you want views showing Level 1 at 0’-0” in some sections/elevations, but want the ability to report the exact height above sea level or to report site building coordinates in a site plan, you won’t be able to do it simultaneously in the same project if you have consumed Shared Coordinates to fix improperly positioned models at project startup. EDIT: I should have worded this better. What I mean is that if you use Shared Coordinates as a linking fix with no regard to properly locating the shared coordinate origin and orientation based on a true survey mark, you would have to repeat the process later if you need to report coordinates relative to this point (especially the elevation above sea level), which can be a painful exercise later in the project.
The other problem I’m finding with the use of Shared Coordinates is that if a model with shared positioning is moved and this in turn causes Revit to save the location back to this linked model (sometimes even when nothing has actually moved), the Project Standards workset gets checked out and not properly relinquished. This results in a team member of one discipline (ex: Mechanical) owning the Project Standards workset in the Architectural model, which can lead to quite a bit of finger pointing! I’ve witnessed this several times when Architectural wants to edit the project address and they are locked out because Project Standards is owned by someone that has no business being in their file.
So are Shared Coordinates pure evil? I won’t dare go that far as their use might be absolutely necessary when exporting data to other applications, such as when needing IFCs for use in Solibri Model Checker. They are a great tool when used with care and caution, but they are undoubtedly one tough nut to crack.