By Dennis Collin
When working in Revit and need to share models with other consultants it’s important to understand the 3 origin references that come with Revit. These origins are referred to when placing Coordinate and Elevation markers and can also be referenced when generating exported CAD files, IFC files and when linking in other Revit projects.
They are.
- The Internal Origin. – Used for Revit’s own purposes only. It cannot be changed or altered. From Revit 2020 onwards the internal origin can be displayed in plan views. As a guide it should be situated somewhere within the extents of the model. Although provided as an option for linking Revit models, it is rarely used in coordination purposes. It will also warn users when importing CAD data that contain elements located more than 20 miles from this origin location.
- The Project Base Point. – A local user definable origin usually situated close to a building or structure. Think of it as a local ‘Setting Out’ survey station defining a grid situated from the Project North. The grid orientation mimics the orientation of the grid of the Internal Origin, except that its position can be changed.
- The Survey Point. The ‘Important’ One! This point can provide a real-world context to the model and provides a grid translation from local or project-based coordinates to a true or real-world context like an Ordnance Survey grid.
When defining coordinate labels, it is important to ensure they refer to the correct coordinate system. Which one should be used?
When working in a federated model and making use of the Acquire/Publish coordinate tool. Use the Survey Point to represent the position of the Shared Coordinate location, along with the deviation between Project (Sheet) and True (Actual) norths.
Comparing two consultant models alongside, they may be positioned at different offsets from Internal or Project Base Point origin locations. But they would have the same offset from the Survey Point.
Therefore, when linking in coordinated Revit or CAD files the default origin to select when linking files should be SHARED COORDINATES. The exception is when setting up files for the first time and the process of defining the Shared Coordinate System Origin and orientation needs to be performed.
In this case either Centre to Centre or Origin to Origin can be used. It doesn’t matter as generally the next step is to define a shared coordinate relationship between the two files. Which will be the preferred option moving forward.
I have written a lengthy white paper to positioning Revit Models in the Real World which can be accessed here.