Acquiring Coordinates between Trades

Revit's shared coordinate features routinely confuse people. I believe I understand them well but I find the various ways people manage to use them can be quite confusing. I've written about this feature quite a bit, enough that I've created a summary of related posts. I was reading a thread at RFO where a user was describing an error message about Shared Sites.

That message appears when a shared coordinate relationship exists between two files and the linked file involved is moved. The thread at RFO hasn't shared a resolution because the person who wrote the original post hasn't replied to my last round of questions. Their claim is that the situation I've described isn't involved. Without more evidence it's hard to say what is causing their situation.

The point of this post is that they also described how they used Shared Coordinates to adjust the position of their work compared to the other team's project file. I've found with some experimentation of my own that it is effective until (IF) the source file alters its own shared coordinate relationship. If that occurs then its necessary to start over, first using the reset technique listed later in this post and then repeating these steps.

Keep in mind, this approach works if you begin modeling your work before you receive a model to use as a reference. Ideally this wouldn't happen but apparently it does. When you do receive another model it's very likely that it won't align with your work. Take these steps to reconcile the misalignment:

• Insert ribbon > Link Revit > select a Revit Model
• Positioning: Auto - Origin to Origin
• Move their model into alignment with ours
• Manage ribbon > Coordinates > Acquire Coordinates - select their linked model
• Delete their linked model (yes, it's counter-intuitive)
• Insert ribbon > Link Revit > select their model again (this time it IS sharing coordinates)
• Positioning: by Shared Coordinates
• Save your work

Our model should now be in sync with theirs until (IF) they alter their shared coordinates. We are using the Shared Coordinates feature to share a common understanding of respective project's shared coordinate origin.

For some background information, Revit projects have a file (reference) origin that does not change, in a way it's like AutoCAD's World Coordinate System (WCS). The shared coordinate concept we are using is similar to using the User Coordinate System (UCS) in AutoCAD to provide an alternate way of seeing the model.

If the team whose model we used to acquire coordinates from revise their shared coordinates in the future we'll have to repeat the process. To do that we'll have to reset the shared coordinates first. We'll also be able to avoid generating the Shared Sites error message above. Revit stores information in our project file about a linked file that it uses to know if a project is sharing coordinates with it or not and we have to break that relationship to reset shared coordinates. This technique uses a separate temporary project file to acquire coordinates from instead. A file can only acquire and share coordinates based on one source file at a time.

We can follow these steps to reset shared coordinates:

• Create a new blank project file
• Put a pair of crossing grids in plan so we can see it when it is linked later.
• Save the project
• Close the project
• Open our model
• Delete the existing linked model (click Remove Link in the warning that appears)
• Insert ribbon > Link Revit > select our temporary new blank project
• Positioning: Auto - Origin to Origin
• Manage ribbon > Coordinates > Acquire Coordinates > select the blank project we imported
• Save our project - it now thinks it is in sync with this new project instead.
• Delete the linked project (Remove Link)
• Save our project - Coordinates are still reset, we can link the real model and acquire coordinates again

Ideally when we link models together we should start with Auto - Origin to Origin because that's all they have as a common reference. Projects usually start with the architect and they establish the coordinate relationship with the site and survey data. Using Auto - Origin to Origin works if at the outset we agree that somebody establishes the site survey relationship and the rest of us all use their model to define our relationship to theirs.

The technique I've describe is using Shared Coordinates to align models instead of using it to align it to the site/survey data. This means that our models will align but exporting to DWG using Shared Coordinates won't necessarily align with the survey data. If this relationship is established after we've aligned our models with this method it is necessary to reset and repeat the process described above to allow our model to acquire the new coordinate relationship.

It's been my observation that Revit's Acquire Coordinates tool does not work well on projects that have VERY large coordinate values (based on survey data). When I write "does not work well" I mean Revit fails to acquire the correct coordinate values. When this occurs I've found that the coordinate values I expected to see were off roughly (less than they should be) by a factor of 10 or 100. In contrast, Revit's Specify Coordinates at Point (SCaP) tool always works when Acquire Coordinates doesn't. It does not define a shared coordinate relationship however. Using SCaP defines what the shared coordinates are at a specific location in the model but it does not store a "relationship" to another file like using Acquire Coordinates does.

I've described one approach to dealing with multi-model multi-discipline sharing of coordinate data HERE. The process described by that post is far more comprehensive and is meant to deal with the entire team. This post is meant to help resolve the relationship between one trade and another. As soon as everyone else gets involved it is important to have one team establish the single source coordinate system to use. A Revit project file can only share a coordinate relationship with one other source file.