Georeferencing is the process of spatially adjusting a CAD or Revit drawing without changing the original source data. In ArcGIS Pro, this is performed by assigning the proper projection file or using the control link points in the drawing to known geographic coordinates. Once you georeference your dataset or model, it is transformed on the fly in memory, while the source data remains unchanged in the native Autodesk file.
Limitations of Revit and CAD datasets
Revit, DWG, and DGN formats do not natively support the ArcGIS georeferencing system. To get around this limitation, ArcGIS Pro extends the properties of a CAD dataset or Revit model to include georeferencing control points as ancillary information. This information is defined as from-to coordinates and is stored in an auxiliary world (.wld3) file.
Georeferencing a CAD dataset is limited to a two-point transformation using the similarity transformation method. This adjustment method maintains the aspect ratio of the CAD drawing and prevents skewing to the x-, y-, and z-axes. The purpose of georeferencing a CAD dataset or Revit model is to accurately overlay the CAD drawing with existing spatial data without skewing its geometry. If a more complex transformation is needed, you can load the data into a geodatabase and use the Modify Features pane.
A projection (.prj) file is a text file containing information about the coordinate system and map projection. A projection file can be used to extend the properties of a CAD drawing to include a spatial reference. The information is stored in the well-known text (WKT) format.
A geographic coordinate system (GCS) includes information about the datum, spheroid, prime meridian, and angular unit of measure, usually in degrees. In addition to the GCS information, a projected coordinate system (PCS) defines additional parameters such as false easting, false northing, central meridian, scale factor, latitude of origin, and linear units, usually in meters or feet.
Revit and CAD drawings are often drawn in the correct geographic location, but when the file is read in ArcGIS Pro, the file does not have this information attached. In many cases, assigning the correct projection file allows the Revit or CAD drawing to be in the correct geospatial location.
Revit survey points
The project base point defines the origin (0,0,0) of the project coordinate system. It is also used to position the structure on the site and to locate the design elements of a building during the construction phase. The spot coordinates and spot elevations in the Revit file that reference the project coordinate system are displayed relative to this point. The survey point represents the known point in geographic space and is used to correctly orient the building geometry in another coordinate system, such as that used in ArcGIS Pro.
Since every Revit project contains a base point and a survey point, these points can be useful to help position the file for use in ArcGIS Pro. It is recommended that your users set this information and know which is the appropriate projection file. If this is used, then you just have to assign the correct Projection file for the revit file in Pro and it will be in the correct geographic location.
If you assign the correct projection file and the Revit file still does not appear in the correct location, you can use the control points to move it to the expected location. Control points define the displacement paths known as links and form the basis of georeferencing. A maximum of two links are possible with CAD datasets and Revit models. A one-point transformation comprises one link and moves the files. A two-point transformation comprises two links and moves, rotates, and scales the file uniformly.
Control points can be created manually with the pointer by clicking in a from-to direction, loaded from a world file, or by adding their coordinate information to the Control Point Table.
3D world files
3D world files are saved with the file extension .wld3. Using this 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 Revit file, the following criteria must be met:
- The world file and the CAD or Revit drawing must have the same name (prefix), for example, MyDrawing.RVT and MyDrawing.wld3.
- The 3D world file must exist in the same folder as the appropriate CAD or Revit drawing.
Universal 3D world files
A universal 3D world file defines control points for all CAD datasets or Revit models that are stored in the same folder and have not been georeferenced.
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 Revit model.