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.
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There are two different geolocation types in AutoCAD-based products;
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:
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.
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:
Image 2 - Example of Drawing Settings dialog in Civil 3D
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:
DWG is a unitless file format. A product can handle and manage units differently from another. The same applies to DWG based connectors.
Drawing units shown above are used in iTwin through Civil3d connector.
AutoCAD and AutoPLANT Connectors
If a DWG file has a GCS that is supported by a DWG connector (refer to above “Types of Coordinate Systems in DWG”), and the GCS has a linear units defined, its units are used by AutoCAD connector.
If a file does not have a GCS, or it has a GCS but the GCS does not have linear units defined, the connector determines file units in the following order:
When system variable INSUNITS is set to real-world units, the units will be used for iTwin.
When INSUNITS is set to Unitless, but system variable is set to Architectural or Engineering, International Inches will be used.
When none of above information is available, Meters will be assumed.
Setting correct units in a source DWG file will ensure you get the correct units in iTwin. This includes externally attached reference files (xReference). Particularly, if an xReference is a Civil3d model connected by a Civil3d connector, iTwin Civil tools, and other apps depend on the correct units to work correctly.
The following sections describe how to best configure DWG models to achieve perfect geospatial alignment.
MicroStation can work with several built-in geographical coordinate systems (GCS). Using a DGN as a geolocation file for DWG models can be useful when:
Connecting model files
Regardless of the synchronization method chosen for connecting model files, always ensure that:
In this setup, it may be a good idea to check if a DWG file matches the DGN file before adding the DWG file to the iModel connection.
A recommended check may be done in MicroStation:
When a DWG file has a valid GCS that uses a geographical coordinate system, as opposed to a local coordinate system, it may be used for the geolocation of an iModel. Other files added to the iModel connection will use the first DWG file's GCS. If an added file is a DGN or another DWG file, checking the model alignment before starting a new connection is a good idea.
Validating model alignment
If a to-be-added file is DWG, it can be checked in either Civil 3D or MAP 3D:
If a to-be-added file is DGN, the file can be checked in MicroStation: