Correctly geolocated iModels can provide additional benefits to a model coordination workflow. Not only can the background map provide more context to the model, measuring coordinates and sharing this information can be valuable for model review tasks.
To take advantage of an iModel’s capability to work in a multidisciplinary environment and with various other source files and formats, it is important to correctly align DWG models.

A DWG file may be connected to an iModel using one of the following iTwin Connectors: AutoCAD, AutoPlant, and Civil 3D. 

Types of Coordinate Systems in DWG

There are two different geolocation types in AutoCAD-based products;

• Use of local coordinate system
• Use of a defined Geographical Coordinate System (GCS)

To properly position DWG files, they need to have the GCS defined. Currently, a GCS created using local coordinate system is not supported, and there could be cases where the results might be spatially misaligned after successfully connecting such a DWG file to an iModel. 

Differences in design applications:

• In AutoCAD, a GCS can only be created using a local coordinate system. In this workflow, a GCS is placed at a given point in the model coordinate system. This process may be compared to attaching a raster file as a background image.
• In AutoCAD MAP 3D or AutoCAD Civil 3D, a GCS can be created using a geographical coordinate system. In this workflow, a GCS is assigned to a model, effectively making the model in the same geographical coordinate system.

Thus, Civil 3D and MAP 3D, but not AutoCAD, are recommended for GCS creation in a DWG file, which will be used to geolocate an iModel.

Geolocating DGW models

Assign a GCS in AutoCAD Civil 3D or AutoCAD MAP 3D

Civil 3D or MAP 3D could be used to create a GCS in a DWG file. They both have a command MAPCSASSIGN used to assign required GCS to the DWG file. Keep in mind that the library's appearance or content may change from one product version to another, but the command shall hopefully remain the same.

Image 1 - Example of GCS library invoked by MAPCSASSIGN

There is a different way of setting a GCS in Civil 3D. If you are a C3D user and prefer using its TOOLSPACE, you have this alternative:

1. Select the "Settings" tab in TOOLSPACE
2. Right mouse click on the file name
3. Select "Edit Drawing Settings"
4. Set options as appropriate in Drawing Settings

Image 2 - Example of Drawing Settings dialog in Civil 3D

Replace existing Geographical Coordinate System

A GCS created in plain AutoCAD does not work for iTwin as the model may not be in the geographical coordinate system.  In this, the GCS may be replaced in Civil 3D:

1. Open the source DWG file in Civil 3D
2. Invoke command GEOREMOVE
3. Accept the prompt to delete existing GCS
4. Create a new GCS by following the steps outlined in the above section Assign a GCS.
5. Invoke command GEOMAP to check if the new GCS is against the model.
6. If the model is not aligned with the new GCS, re-align it.
7. Save the file.