In order to ensure that all information is authored in the correct geographical location but also within the accurate modelling area allowed by Revit (the limitation to draw accurately in Revit is a 10mile/16km radius from the Origin Point), the coordinate system has to be setup in advance of the project team starting to develop the BIM data for collaboration purposes.
This is done by creating a Master Coordinates model, used to manage the coordinates for the project. The Master Coordinates model is setup by specifying the Real World Coordinates, including elevation, for a specific point (e.g. a grid point) called a Survey Point (triangle). The Base Point (circle) has a function to identify coordinates different to the Survey Point, i.e. to the project grid.
The project model can then be linked in to the Master Coordinates model and have the coordinates published to it via a common internal site. If the coordinates later change or if a coordinate error occurs the coordinates can easily be managed and re published from the Master Coordinates model.
Setting up the master coordinates model
- Create a new “Master Coordinates” model using the relevant template.
- Save the new model to the project folder location.
To define the coordinates:
- Link in a survey or OS map where an origin point with known coordinates (using Northings and Eastings) can be identified.
Typically this would be linked Centre to Centre to ensure the model is within Revit’s accurate modelling area.
NOTE: If you are working to a linked CAD grid file, you may need to draw a location line before assigning the coordinates. The Specify Coordinates at Point tool doesn’t work with intersections. It’s also a good idea (although not critical) to place a Spot Coordinate at your “project origin” for information.
- Click on the Manage Tab > Coordinates > Specify Coordinates at a Point.
Click on the model line at the known origin point. The Specify Shared Coordinate dialog will open. Enter the coordinate values, the correct datum (Elevation) and the angle to True North. This will allow you to model orthogonally in Revit but still have reference to the correct coordination orientation.
The Master Coordinates model is now set up to use defined Shared Coordinates.
Next, set up a project site:
- Click on the Manage tab > Project Location > Location.
- Open the Site tab of the Location Weather and Site dialog box.
- Click Duplicate… to create a new site for the project.
Name the new site, make it current and delete the original.
- If the project model doesn’t yet exist, create it using the relevant template. Create an identical site in the project model.
Publishing the coordinates from the Master Coordinates model
- Link in the project central model into the Master Coordinates model by Centre to Centre. Move and rotate the project model to position it correctly against the survey and at the correct level above sea.
- Click on the Manage tab > Coordinates > Publish Coordinates.
Click on the project central model link to identify the file to which the coordinates will be published.
Check the defined project site is selected.
- Click Save to save the coordinates back to the project central model.
- Open the project model and check the coordinates are correct by switching between Project North and True North.
This process can be repeated on any models requiring the correct coordinates published to them. (Ensure the same site has been defined in all models that require coordinates published to them).
editing the position of the project model
If the building has to be moved in plan or elevation, the modifications should be carried out and managed through the Master Coordinates Model:
- Make sure the project central model is not open.
Open the Master Coordinates model.
- Position the linked project central model as required in plan and elevation.
This will prompt a Revit warning that the shared sites in the linked model have been modified but not saved back to the linked file (the project central model).
Click Save Now to update the project model with the new positioning.