LOCATING A PROJECT TO AN EXISTING GRID IN REVIT

Setting Shared Coordinates in Revit is straightforward. But what happens if you have been issued with an existing building grid or OS map and need to make sure that your model data is always coincident? If you have been given the coordinates for a known point and the project rotation, you can enter the Shared Coordinates manually, but that might lead to input error. Instead, follow the procedure below to use the coordinate system of the grid file directly.

Note: If you are working to a grid in DGN format, you will need to convert it to DWG first as Revit cannot interrogate the global coordinate system of a DGN file.

1. Create a new Revit project. Switch to a “Floor Plans” view, typically “Site”.
2. Set the View Orientation to True North.
   
3. In the Insert tab, pick the Link CAD tool.
   
4. Browse to the grid file and select it. Make sure the settings are:
   Positioning = Auto – Center to Center
   Place at = Level 0
   Orient to View = Ticked
   Click Open.
5. The grid will be linked around the Base Point and rotated as it was originally drawn. It would be usual to assume that the y-axis of the DWG is North.
   
6. In Revit, you would want to have your model orthogonal to your Elevation Tags, so that you can orientate to Project North or True North when required. Rotate the linked CAD file to suit.
   
7. Go to the Manage tab and pick the Coordinates > Acquire Coordinates tool.
   
   Click on the linked grid file.
   Revit will read the World Coordinate System from the file and reset its Shared Coordinates to match. The Survey Point (visible in the Site view) will be shifted to allow for the coordinate change, and True North will be assigned to match the WCS’s y-axis. As your view is set to True North, the view will re-orientate itself to show the true orientation of the grid.
   
8. If you now orientate your view to Project North, the grid will be orthogonal.
9. To check a known coordinate point, use the Manage tab Coordinate > Report Shared Coordinate It might be easier to draw a line or other object in order to be able to snap to a point of the linked file.
   
   
10. For further Linked files, you can use:
    Positioning = Auto – Shared Coordinates
    Place at = Level 0
    Orient to View = UntickedWARNING:   When saving the Revit model, you will be warned that the position of the CAD file has been changed. This is normal and is used to align Revit models using differing coordinate systems. However, you must pick “Disable shared positioning” to avoid Revit trying to write the Shared Coordinates back to the DWG file. If that happens, you’ll end up with all kinds of issues.