Georeferencing BIM data from Revit or IFC files using a feature layer of a BIM file workspace in an ArcGIS Pro scene is the way to accurately position BIM data without changing the original source data. Georeferencing is performed by assigning the proper required Esri coordinate system definition file (PRJ), and if needed, an optional offset definition stored in a 3D world file (WLD3). Once the spatial reference (PRJ) is included and any optional offsets are defined (WLD3), the BIM coordinates in the design file are transformed on the fly in memory, while the source data remains unchanged.
BIM data may have been authored using projected coordinates that correlate to a known world coordinate system and may include a matching Esri coordinate system definition file (PRJ) and would therefore not need the optional spatial offset world file (WLD3).
Required spatial reference (PRJ)
An Esri PRJ file is a text file containing information that defines a coordinate system and map projection, which is also known as its spatial reference. A projection file with the same name as the BIM or CAD file is used to define that file's spatial reference. Optionally, any CAD or BIM file that does not have a matching PRJ file can default to a specially named coordinate system definition file, ESRI_CAD.prj, in the same file directory. The ESRI_CAD.prj file is sometimes called the universal projection file.
BIM data can be drawn in the correct geographic location, but when the file is read in ArcGIS Pro, the file may not have a .prj file included. In many cases, assigning the correct projection file allows the BIM or CAD design files to be positioned in the correct geospatial location without the need to define a .wld3 file using ArcGIS Pro georeferencing tools. To accurately place your BIM data in ArcGIS Pro, first determine the system of linear units of your BIM files. Then choose a projection for your scene that matches those units and the geographic location of the data. If your BIM file is in feet, choose an Esri projection file that is also in feet. The elevation units of the layer should match too; otherwise, the BIM model may appear taller or shorter than it should. Avoid using control points to georeference a BIM model in 2D, which may unnecessarily scale a BIM model. BIM models are unlikely to need scaling.
If a PRJ file is not specified, a default PRJ file is temporarily used when BIM data is added to set the linear units for the purpose of inspecting BIM data only. This may or may not be correct for the data. IFC files default to metric units and Revit files default to imperial units. For proper positioning, you must specify an appropriate PRJ file.
Universal projection file (ESRI_CAD.prj)
A universal projection file defines the spatial reference for all CAD or BIM files that are stored in the same folder and that do not have a .prj file that matches the CAD or BIM file name. The universal projection file is a convenient method of defining a spatial reference for a folder of CAD or BIM files that are all in the same geographic location.
To define a universal projection file, the following criteria must be met:
- The projection file must be named ESRI_CAD.prj.
- The projection file must exist in the same folder as the CAD drawing or BIM model.
Revit project coordinates
In Autodesk Revit software, you can define the project coordinates of BIM data according to a well-defined geospatial coordinate system. Using Revit tools, such as the Specify Coordinate At Point command, you can build your models in a real-world coordinate space. Including a matching Esri .prj file effectually creates Revit models that are properly positioned in ArcGIS Pro without modification or further georeferencing.
Revit geographic location
In Autodesk Revit software, you can set a geographic location. This information is used by Revit software for various lighting and modeling processes. The geographic location information is not sufficient to define an accurate geospatial position and is ignored by ArcGIS Pro.
Revit display units
In Autodesk Revit software, the author of the Revit model can choose the display units of the coordinates and scalar measurements. It is important that the display units of the model match the linear units of the Esri .prj file. ArcGIS Pro automatically scales correctly to Revit display units of inches or millimeters. The important distinction is that the Esri .prj file must be either feet or meters to match the display units of the Revit model.
Optional spatial transformation offset (world files)
It is common for BIM design files to use a relative or local coordinate system rather than a geospatial coordinate system. In such cases, you need to define a spatial offset to properly position your BIM data. You also need to define a spatial reference by including a .prj file. Spatial offset information defines the offset from a known geospatial coordinate system defined in an existing .prj file and the coordinates of your BIM data. A .wld3 file needs a matching .prj file. The spatial offset information created by ArcGIS Pro georeferencing tools is stored in a .wld3 file with the same name as the BIM file. If the coordinates of the BIM model are based on the coordinates of an included .prj file, there is no need to define a spatial offset using a .wld3 file and the georeferencing tools in ArcGIS Pro.
3D world files (WLD3)
3D world files are saved with the file extension .wld3. Using a 3D world file to store control points is the best practice for sharing and reusing links in other projects and map documents. ArcGIS Pro uses the file name and its location to link the world file to a particular CAD dataset or Revit model. This file is similar to the 3D world file, but it contains a coordinate so the file can be placed in the appropriate x,y,z location.
To associate a 3D world file with a CAD dataset or BIM file, the following criteria must be met:
- The world file and the CAD or BIM design file must have the same name (prefix), for example, MyDrawing.RVT and MyDrawing.wld3.
- The 3D world file must exist in the same file folder as the matching CAD or BIM design file.
Universal 3D world file (ESRI_CAD.wld3)
A universal 3D world file defines offset control points for all CAD or BIM files that are stored in the same folder and that do not have a .wld3 file that matches the CAD or BIM file name. The universal world file is a method for defining a single spatial transformation offset for a folder of CAD or BIM files that have the correct positioning relative to one another but need to be repositioned.
To define a universal world file, the following criteria must be met:
- The world file must be named ESRI_CAD.wld3.
- The world file must exist in the same folder as the CAD drawing or BIM model.