CAD and BIM data positioning
CAD and BIM data is a common source of GIS content, but before it can be used within ArcGIS Pro, the data must have a defined geospatial location. CAD and BIM files are required to have a valid Esri coordinate system (PRJ file) and may require an optional coordinate transformation information (WLD file) to identify how the coordinates in the CAD or BIM data should be positioned on the Earth's surface. You will be alerted by ArcGIS Pro when attempting to add CAD or BIM data that does not have a proper coordinate system. Notifications will guide you to choose the proper coordinate system.
Properly positioning CAD and BIM data in ArcGIS Pro may involve two distinct parts. First, you must identify the Esri coordinate system reference that defines how ArcGIS will interpret the coordinates stored in the file. If the coordinates within the CAD or BIM file do not correspond to the identified coordinate system, you must define how the coordinates should be transformed to move, scale, and rotate the data into the proper position within the chosen coordinate system. You have the option to reposition data using the 2D georeferencing or 3D georeferencing tools for CAD and BIM data.
You can author CAD or BIM files using coordinates that correlate to a known world coordinate system, and then add Esri coordinate system reference information from an projection file (PRJ), or include the coordinate system definition in an AutoCAD drawing created with ArcGIS for AutoCAD. Such data would not need a spatial offset world file (WLD3).
Validate CAD and BIM file positioning information
You can analyze the spatial positioning information of a CAD or BIM file from the file properties dialog box of CAD and BIM files in Catalog, or from the Manage ribbon tab of CAD or BIM layers in ArcGIS Pro. You can validate the positioning information for any CAD or BIM file by inspecting the file properties in Catalog and fixing warnings and errors with positioning information. Once added to a map or scene, you can modify the position of CAD or BIM data with various positioning tools found on the Position tab that appears with the Manage main ribbon for CAD and BIM data. Tools on the position ribbon reposition your CAD or BIM data without changing the original source data. Positional modifications made using these tools update a world file (WLD3) that is stored in the file folder of the CAD or BIM file.
Required coordinate system definition (PRJ)
An Esri .prj file is a text file containing information that defines a coordinate system, data, and map projection. A projection file with the same name as the BIM or CAD file is used to define that file's coordinate system (<filename>.prj). 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. AutoCAD drawings used with ArcGIS for AutoCAD will have a coordinate system included and recognized in the DWG file and do not need an ancillary projection file (PRJ).
CAD and BIM data can be drawn in the correct geographic location, but when the file is read in ArcGIS Pro, the file may not have an Esri coordinate system included. In many cases, assigning the correct Esri coordinate system allows the CAD or BIM design files to be positioned in the correct geospatial location without the need to define a coordinate transformation file (WLD3) using ArcGIS Pro positioning tools. To accurately place CAD or BIM data in ArcGIS Pro, determine the system of linear units of the CAD or BIM files. If the CAD or BIM file is drawn in feet, choose an Esri projection file that is also in feet. If your CAD or BIM data is in metric units, select a coordinate system reference in meters. The elevation units of the layer should match too; otherwise, the BIM model may appear taller or shorter than it should. In most cases, CAD and BIM models are unlikely to need scaling. An exception to this is the ground-to-grid workflow for CAD, which is based primarily on a scaling factor.
If a .prj file is not specified, a suggested UTM zone .prj file is provided, which will allow you to add the data to ArcGIS Pro. This may or may not be correct for the data. For proper positioning, you must specify an appropriate .prj file where the linear units and range of coordinate values in the CAD or BIM file are contained within the range of coordinates allowed by the chosen coordinate system. Accepting the default UTM spatial reference without knowledge of the actual coordinates of the data is not recommended but can be useful to inspect the data, or when it doesn't matter because the coordinates of the file have no inherent spatial location, such as a typical structure or model home that could be placed anywhere.
Universal projection file (esri_CAD.prj)
A universal projection file defines the coordinate system for all CAD or BIM files that are stored in the same file folder and that do not have their own .prj file that matches the CAD or BIM file name. The universal projection file is a convenient method of defining a coordinate system for a folder of CAD or BIM files whose coordinates use the same coordinate system definition.
The universal projection file will be used when the following criteria is 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.
- The CAD or BIM data does not have its own Esri coordinate system definition.
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. By also including a matching Esri .prj file, Revit models will be properly positioned in ArcGIS Pro without modification.
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. The display units of the model must 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 must define a spatial offset to properly position your BIM data. You also must define a coordinate system 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, you do not need to define a spatial offset using a .wld3 file and the georeferencing tools in ArcGIS Pro.
World files (WLD3)
3D world files are saved with the file extension .wld3. Using a 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 world file, but it contains a coordinate so the file can be placed in the appropriate x,y,z location.
To associate a 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 world file must exist in the same file folder as the matching CAD or BIM design file.
Universal world file (ESRI_CAD.wld3)
A universal 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.