Geo-referencing Revit models
This tutorial explains how to geo-reference your Revit model based on survey or Geographical Information System (GIS) data sets. Since geometry in Revit needs to be within 10 miles or 16 kilometres of the model’s internal origin, specific workarounds are required to maintain accuracy. By following the steps outlined below, it is possible to eliminate any graphical inaccuracies while maintaining the coordinate system. The tutorial uses Sydney as a case study. If you are unfamiliar with coordinates systems used within Australia, please refer to this tutorial.
Project Local Coordinate System
Many frequently used CAD/BIM software have inferior GIS capabilities. This can make geo-referencing models difficult. Revit, for example, has a maximum distance limit from its internal origin of 10 miles or 16 kilometres. Beyond this limit, the graphical representation of elements becomes less reliable and less accurate. This limitation applies to geometry created in Revit as well as incoming geometry from an import or a link.
Since surveys, city models and planning overlays are authored in GIS-enabled software, such as ArcGIS or QGIS, they are often geo-referenced and are located very away from the origin. Directly importing this information into Revit will cause an error, “Geometry in the file has extents greater than 20 miles (33km)”.
Coordinate system diagram
To avoid this issue, it is recommended to set up a ‘Project Local’ coordinate system. This is merely repositioning the model closer to the origin point and falsifying its coordinate values.
Step 1: Prepare the survey
Firstly, open the MGA56 positioned data set in AutoCAD and draw a circle at coordinate X=335,000m, y=6,250,000m (or some other known point if not in Sydney). Ensure all layers are turned on and unlocked. Select all elements and move the entire drawing from the centre of the circle to 0,0,0. The drawing will now be at the ‘Project Local’ coordinate system. The repositioned geometry will now be much closer to the Revit’s internal origin. This process should avoid any geometric error messages when linked into Revit.
To make sure others are aware that the drawing has been repositioned, add some text at 0,0,0: “This point has been repositioned from x=335,000m, y=6,250,000m. This corresponds to Project Local.”
Next, as per best practice, clean up the CAD drawing by running Purge and Audit. To do a super purge, you can run ‘Wblock’. You may also want to explode blocks as sometimes the insertion point, which isn’t always visible, is far from the main geometry and can cause errors. Check that the insertion scale is set correctly to meters.
Step 2: Set-up Revit site model
In a new Revit model, ensure that the length units are set to metres. This is important as most surveys will be in metres while Revit models in Australia tend to be in millimetres. While it is possible to link files of different units, this may cause confusion in Step 3 and is best avoided.
Next, ensure you are in a view orientated to True North and then go Insert > Link > Link CAD. Select the repositioned project local file from Step 1 and use ‘Auto – Origin to Internal Origin’ as the positioning. Import units should be ‘meter’, and colours as ‘black and white’. Deselect ‘Orient to view’ and ‘Correct lines that are slightly off axis’.
The survey will be linked in at ‘Project Local’ coordinates and will be pinned by default.
Note that Revit only has two orientations – Project North and True North. Technically our survey data is orientated to MGA56 not True North, but for this example, we’ll consider them the same.
Step 3: Set Revit site model coordinates
The next step is to set the coordinates so that it displays MGA zone 56 coordinates without moving the Revit model or the linked *dwg file. To do this, first turn on the Survey Point by going Visibility Graphics > Site > Survey Point. It should be located at 0,0,0 if Steps 1 and 2 were done correctly.
Go Manage > Project Location > Coordinates > Select Coordinates at a Point, and select the Survey Point and enter in the MGA56 coordinates that this point relates to. It is important to note that in CAD programs such as AutoCAD, the convention is X,Y,Z. However, in Revit it’s N/S, E/W, Elevation so you’ll need to flip the order of the values accordingly.
This will move the position of the Survey Point without moving anything else, including the Revit Project Base Point and Internal Origin which will be kept as-is. The Project Base Point will display the new coordinates, but it will still be coincident with the internal origin (0,0).
Step 4: Verify Revit coordinates
Without some visual reference, it can be challenging to understand a coordinate system. The simplest and easiest way to achieve this is to add a grid, typically at 500m intervals, with Spot Coordinates. By creating two Spot Coordinate types, ‘Northing_Horizontal’ and ‘Easting_Vertical’ you’ll easily be able to see if someone has accidentally moved the model. This is particularly important when using shared coordinates (refer Step 6).
Step 5: Set-up Revit’s building model (optional)
If you plan of linking in a Revit building model and sharing coordinates, first prepare the building model. Ensure the Project Base Point is coincident with the start-up location. To do this, select the project Base Point and unclip it (paper clip with slash). Next, right-click and select move to start up location.
You can model as much or as little information as you like, just ensure there are at least the two grids in there. Save and close the file. If the file isn’t closed, it may cause errors in the next step.
Step 6: Set-up shared coordinates (optional)
Next, we need to geo-reference our building relative to the site model. This can be done by either acquiring coordinates or pushing coordinates. This tutorial will describe the push method as I find it more intuitive.
Open the site model and link in the building model by going Insert > Link > Link Revit and insert the building model using ‘Manual – Origin’ as the position. Move and rotate your building as required so that it is in the desired position. Select the building model link and in the Properties pallet, click Shared Site and select the first option, “Publish the current shared coordinates…” and press Reconcile.
When you next save the file, you will be prompted if you want to save the position of this link. Select ‘Save’. You have now enabled shared coordinates.
Due to a lack of GIS capabilities, getting coordinates correct in Revit can be difficult. While it may be intuitive to keep all models and links geo-referenced for ease, this will invariably cause graphical representation issues which are less reliable and less accurate. By following the steps outlined above, it is possible to eliminate this issue while maintaining accurate coordinates.