Import Bentley i-model (.icm.dgn) Files

Import Bentley i-model (.icm.dgn) files to bring Infrastructure Concensus Models (ICMs) containing civil engineering objects into your project. For more information, see Understanding Bentley i-model (.icm.dgn) Files.

Note: This topic refers only to the newer i-model (.icm.dgn) versions of i-models, not the older i-model (.i.dgn) file format.
Note: Although i-model (.icm.dgn) files are intended to contain civil objects, it is possible to have a combination of civil and non-civil objects in the file. Importing non-civil objects is not recommended.

Importable Data

Here is a simplified view of how the civil objects (and their names) are mapped when they are imported from a Bentley civil product into Trimble Business Center via an i-model:

Note: The i-model (.icm.dgn)'s contents and the import settings you choose determine which objects are imported, and whether alignments that that are not associated with a corridor are imported as linework.

Bentley Civil Object		i-model (.icm.dgn) Object		Trimble Business Center	Notes
Point	>	Point	>	Point	Points can be either 2D or 3D.
3D Linear Element	>	Linear Entity 3D	>	2D or 3D polyline, arc, circle, spline, linestring (depending on what the original object was)	The original line styles are preserved, but all line style scales are set to 1.0. Note: Vertical arcs in 3D linear elements are not supported in linestrings; a straight line is drawn between the start and end points of the vertical arcs. Tilting of the plane of horizontal arcs is handled consistently. Note: For line styles, the Project Setting > View > Display Options > Show all lines as solid is honored. Other settings that affect line styles include Project Settings > View >Plan View > Plot Scale, and Properties> Linetype Scale.
Complex Element	>	Alignment	>	Alignment	Line, Bspline, spiral (clothoid), and arc segments in the horizontal plane are supported. Alignments with profiles that are not associated with a corridor can be imported as either alignments and profiles or as linestrings. See Import Options below.
Profiles	>	Profiles	>	Profiles	Line, arc, and parabola (symmetrical) segments are supported. Profiles are expected to be associated with an alignment, and an alignment can have multiple associated profiles.
Active Profile	>	Active Profile	>	Active Profile	Only one profile can be set as the active profile for the alignment.
3D Linear Element (chorded)	>	3D Linear Element (chorded)	>	These are not supported.
Station Equations	>	Station Equations	>	Station Equations
Mesh	>	Mesh	>
Mesh Surface (no depth)	>	Surface Mesh	>	Surface (TIN)	Surface meshes do not generate linework and corridor surface meshes are not imported if corridor linework is present.
Mesh Component (with a volume)	>	Component Mesh	>	Linestrings	The importer creates a closed linework polygon to represent each face of a component mesh. These faces are then shaded according to the imported mesh color.
Mesh with a corridor relationship	>	Corridor Mesh	>
Mesh Surface (no depth)	>	Surface Mesh	>	Surface (TIN)
Mesh Component (with a volume)	>	Component Mesh	>	Linestrings	The importer creates a closed linework polygon to represent each face of a component mesh. These faces are then shaded according to the imported mesh color.
Terrain Model	>	Terrain Model	>	Surface (TIN)
Hull/Boundary	>	Boundaries	>	Boundaries
Breaklines	>	Breaklines	>	Breaklines	Hole, island, and void boundaries are imported as breaklines. Contours and soft breaklines are imported with soft surface sharpness. Other breaklines import with sharp or sharp and texture boundary surface sharpness. The distinction between a void and a hole is only apparent when surfaces are merged. A void persists after the surfaces are merged, even if the other surface contains triangles over that area. A hole is filled in if the surface being merged contains triangles for that area.
Corridor	>	Corridor	>	Corridor
Corridor Baseline	>	Corridor Baseline	>	Alignment (main)
Hull/Boundary	>	Hull/Boundary	>	Boundary
ID of Top/Bottom Elements	>	ID of Top/Bottom Elements	>	Breaklines to form top and bottom surface

Non-importable Data

Data that is not included in the i-model (icm.dgn) format includes:

Non-civil objects (e.g., native MicroStation objects)
Corridor templates
Superelevation tables
Coordinate systems

To import an i-model (.icm.dgn) file:

Follow the instructions in Import Data.


Import Options
Data to import	All - Select this to import every object in the file, whether it is a civil or non-civil object. Civil Objects Only - Select this to import every object that is not a component mesh. Component Meshes Only - Select this to import only the component meshes in the file.
Import non-corridor alignments as	Alignments - Select this to import all alignments that are not associated with a corridor as alignments. Note: Alignments cannot be converted to linestrings after import,but alignments can be created from linestrings; either can be included in a surface. Linework - Select this to import all alignments that are not associated with a corridor as linestrings. Note: See the Imported Non-corridor Alignments table (next) for the effect of these options on different types of linear objects.
Create the corridor’s top surface from	Linestrings - Select this to use corridor sub-alignment strings (alignments and/or linear entities 3D) to form the corridor surface. Forming a corridor from linestrings has these advantages: The corridor is suitable for densification (although this is not turned on by default) The corridor includes the sub-alignment names And this disadvantage: When two corridors intersect each other, the linestrings get clipped but the boundary does not. The surface formation in this area can be poor. Tip: Both surface representations benefit from frequent template drop settings to increase the fidelity of the data written to the i-model (.icm.dgn) and consequently Trimble Business Center. See the Design Stage topic for more information. Surface Mesh – Select this to use the top surface mesh, if present in the file, to form the corridor surface. Forming a corridor from surface meshes has this advantage: The clipping problem between corridors is resolved And these disadvantages: They are not suitable for densification Sub-alignment names are not available Surface formation over non-planar mesh faces may not be what was intended Note: See the Corridor's Top Surface Formation table (two below) for the effect of these options on top surface formation. Tip: You may find that the Surface Mesh option creates a better corridor top surface for GCS900, but that the Linestrings option creates a better surface for SCS900. Depending on the road geometry, the triangulation you get from linestrings may be unexpected, although not necessarily incorrect. Experiment with the options to see which provides the optimal results for your needs.

Imported Non-corridor Alignments

Here are the effects of the Import non-corridor alignments as option on linear objects:


Object in the File	Resulting Object in Trimble Business Center
	When Imported as an Alignment	When Imported as a Linestring
Alignment
Line	Line	Line
Arc	Arc	Arc
Spiral	Spiral	Chords*
BSpline	Chords	Chords**
Profile
Line	Line	Line
Arc	Arc	Line
Parabola	Parabola	Parabola

*The chording setting (determining the amount of deviation) is specified in Project Settings > Computations > Surface > Breakline Approximation Parameters > Horizontal Tolerance.

**When imported as linestrings, alignment Bsplines are chorded because the importer first interprets the alignment as a Trimble Business Center alignment and then converts the alignment into a linestring.

Top Surface Formation for a Corridor

An i-model (.icm.dgn) may contain multiple representations of a corridor, including:

Sub-alignments of the corridor stored as alignments (with profiles) and/or linestrings (Linear Entity 3D)
A surface mesh of the entire top surface (Surface Mesh)
Individual meshes that can be merged to form the corridor surface; these may or may not correspond to the top surface

Two factors determine how the corridor’s surface is formed:

The contents of the i-model (.icm.dgn) file
The Create the corridor’s top surface from import setting:
- Linestrings - The importer’s default is to use the Linestrings option if alignments and/or linear entities 3D are present in the file. If the preferred surface formation data is not present, the importer will revert to the next best option and trigger import report warnings to advise you that your preferred option was not available (see Import Report Warnings below). If the corridor’s top surface is created from linestrings, the top surface mesh and individual corridor meshes for that corridor (if present) are not imported. If the top and/or bottom corridor surfaces have been constructed from linestrings, the names will be: <Corridor name> - Top Surface and <Corridor name> - Bottom Surface for the top surface and bottom surface, respectively.
- Surface Mesh - If the corridor’s top surface is created from the top surface mesh, then the individual corridor meshes for that corridor (if present) are not imported. If the corridor’s top surface is created from a top mesh, the name will be <corridor name>.default or <corridor name>.Top Mesh, dependingon the Top Mesh Feature Definition defined in the selected design stage.

Regardless of how the corridor’s top surface is formed, a corridor’s linestrings are imported. Corridor surface meshes are not imported if the corridor linework is present.

In the absence of top surface information, it is better to have a corridor surface than no surface, so an Individual Corridor Surface Meshes option (fourth column) is used as the last choice used in forming the corridor surface. While the surface that the individual surfaces represent is valid, there is no indication as to which corridor surface they reflect. To prevent many small surfaces being created in your project, the importer collates the individual corridor surface meshes into one surface on import.

Here are the possible effects of the Create the corridor’s top surface from option on the imported top surface (with the defaults in green):


Import Option for Top Corridor Surface Formation	Contents of the i-model (.icm.dgn)			Warning in the Import Report
Import Option for Top Corridor Surface Formation	Top Surface from Linestrings	Top Surface from Surface Mesh	Top Surface from Individual Corridor Surface Meshes	Warning in the Import Report
Linestrings	—	—	—	None
Linestrings		—	—	1
Linestrings			—	1,3
Linestrings				1,3
Surface Mesh	—	—	—	None
Surface Mesh	—		—	2
Surface Mesh			—	2,3
Surface Mesh				2,3

Import Report warnings:

Corridor '{0}' doesn't have top surface linework
Corridor '{0}' doesn't have a top mesh surface
Corridor '{0}' doesn't have a top surface definition

Bottom Corridor Surface Representation

An i-model (.icm.dgn) can also contain a mesh representing the entire bottom corridor surface. This mesh encompasses the extents of the corridor, which generally means that it joins the top corridor surface. However, the routine that forms the bottom surface selects the next offset out from the edge bottom subgrade point to connect to; this may or may not reflect the intention of the original design. For example, the subgrade may connect to the top surface with a vertical, or even over-vertical, face. When this occurs, the importer takes a conservative approach and forms an alignment surface from the bottom only sub-alignment strings; no connection to the top surface is made.

Layer Creation and Entity Values

The i-model (.icm.dgn) importer creates layers in Trimble Business Center that correspond to the Bentley levels used by the objects in the i-model. When importing terrain models, the importer creates two layers for each surface:

<surface name>_Breaklines - This contains the associated breaklines, contours, and soft breaklines for the surface. These are automatically associated with and used in the surface formation.
<surface name>_Boundaries - This contains the boundary, holes, islands, and voids associated with the surface. These objects are automatically added to the surface as closed breaklines, as they are in Bentley products.

Holes, Islands, and Voids in Surfaces

Both Bentley civil products and Trimble Business Center handle holes and islands in the same way. An island must be enclosed by a hole. Holes and islands cannot overlap. Void boundaries in Bentley products are treated as holes in Trimble Business Center.

Component Mesh Handling

Component meshes are not natively supported in Trimble Business Center. Component meshes can, however, be imported using a Data to import option:

Separately, by selecting Component Meshes Only
Along with the rest of the civil data, by selecting All.

Component meshes are not imported when the Civil Objects Only is selected.The i-model (.icm.dgn) importer creates a closed linework polygon to represent each face of the component mesh. These faces are then shaded according to the imported mesh's color.

Scenarios:

If you import the same i-model (.icm.dgn) file more than once, the first imported model does not get updated by changes in the second; you simply have both models in your project. All the data is imported from i-model (.icm.dgn) model into the Trimble Business Center project. The i-model (.icm.dgn) importer does not provide options to update or overwrite existing data in the project; so you may end up with duplicates of data in the project, depending on the import settings when you reimport data from an i-model (.icm.dgn) into the same project. Each import, however, does create a separate selection set (and thereby view filter) which enables you to hide either set of data.

Dependencies:

None