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Round-Trip Design with Autodesk Civil 3D and Revit

Bennett EarleJuly 8th, 2024

Working on multi-disciplinary projects, amongst diverse teams means keeping a keen eye on data, coordination, and collaboration. Without a dedicated project manager or data coordinator, this role can be sorely missed. Today, we’ll explore how engineers and architects can tackle this role, and facilitate data sharing throughout the project life cycle. Doing so will have many benefits throughout the design process; it will enable your organization to go beyond 2D plans and capture project designs in high detail and provide insights can have a large impact on a project’s success no matter the phase.

As we’ll explore in this article, we will geolocate and coordinate an Autodesk Civil 3D project file with our Autodesk Revit BIM model. Once this coordination is completed, we can easily round-trip the design information between both software tools!

Working with Civil 3D Coordinate Systems

Graduating from a drafting workflow such as Autodesk AutoCAD to intelligent civil information models (CIM) design with Autodesk Civil 3D comes with many benefits specific to the tasks that a civil designer, surveyor, planner, or mapper might require. The focus of today’s discussion surrounds geolocation and coordinate systems and how our civil project files can play nicely with Autodesk Revit (and beyond).

Civil 3D has an extensive database of coordinate systems that can be assigned, and this database is common to and shared across multiple Autodesk products. The first step in assigning the coordinate system is to select the category that your coordinate system resides in. The categories are based on geographic location. Since Civil 3D is used worldwide, its database contains most of the standard coordinate systems. Once the category is selected, the available coordinate systems within that category will be listed in the Available Coordinate Systems.

It is important to keep in mind that Autodesk AutoCAD and Civil 3D are very different pieces of software when compared to Revit in the way that they handle precision and units. Additionally, as civil engineers we add a couple of additional curveballs to the mix – such as the fact that our project sites are typically very, very far from the model origin, and we have a wacky concepts such as coordinate systems, transformations, and US States Planes and we do not respect fractions. This makes the marriage between Civil 3D files and Revit files a bit tricky, as Revit does not immediately or natively understand the internal origin or coordinate systems that Autodesk Civil 3D supports.

During project kickoff, and through consideration and discussions with the project team (specifically the surveyors) a coordinate system should be specified for the project. All Civil 3D and Map 3D files should be set up accordingly – optionally, for larger projects, a project-specific template may be recommended to standardize any project requirements across all model files. Moving forward this coordinate system will be used consistently throughout the project, facilitating collaboration across Civil 3D project files, Autodesk SDF layers, Esri ArcGIS databases, geolocated LiDAR scans and so much more. On the civil infrastructure side of the project, we can easily coordinate this variety of project data. Next, we will explore ways to export, share, coordinate and round-trip our civil design information with the architectural team.  

Preparing the Civil 3D Model File for Revit

Ownership of the coordination of the project files and the creation of a federated model can fall on any project member or team, however due to the uniqueness of civil infrastructure files (coordinate systems, large/weird terrain objects, and geometry far from the origin) it will typically fall on the plate of the civil engineer or site designer. Let us examine the steps and best practices when exporting a Civil 3D project file for use in the Revit BIM model.

When leaving one design software for another, and potentially converting file types it is imperative to review the collaboration standards for the project and to consider the impact of the conversion on data loss. A Civil 3D model will contain many intelligent design objects and understanding the best method to export, to share, and to preserve that data enhances collaboration across the project and provides stakeholders with better project insight.

When exporting from the Civil 3D environment it is important to consider what information will be useful on the Revit side of the equation. Will our teammates need 2D linework? 3D elevations? Contours? Points? Triangles? Breaklines? Solids? Text and annotations? When we simplify a Civil 3D object through this process, the intelligent Civil 3D style is crushed down to its most basic AutoCAD entities, it is important to consider your style choice wisely as only those components will be exported during this design coordination process.

If the architect is looking to build a Revit Toposoild (to emulate terrain in the BIM model) what is the best DWG export to provide? Contours will tell one side of the story. 3D Triangles a different side. Points will show another angle. Revit does not perform any triangulation for Toposolid objects, so as an engineer or surveyor providing the correct detail is more important than more detail. Think about how much context is lost when exporting 2’ contours when a 6” curb exists on the site! This is one place when you may unintentionally experience data loss. Exporting the proposed Civil 3D TIN may be the easiest option, however the same surface definition could be recreated in Revit by leveraging existing 3D geometry (points, feature lines, 3D polylines, etc.).

Once the Civil 3D model has been cleaned up, cropped down, and the correct styles have been applied the Export to AutoCAD command is recommended. Intelligent Civil 3D objects only exist in its universe so simplifying to the AutoCAD drawing format is preferred when coordinating with Revit (or other CAD/BIM software) at this step. There are many additional formats for exporting a Civil 3D model to such as IFC, LandXML, DGN, SDF, and more. Selecting the correct format will be determined by the BIM/CIM execution plan and coordinated with the project stakeholders to populate the federated project model.

Once an AutoCAD drawing file (DWG) is created, it is important to again reduce file size and complexity without losing data. Purging blocks, layers, and content from the DWG is a must. Additionally, it may be a good idea at this step to define an outer project boundary for coordination. This boundary can be shared between project drawings to coordinate and place any additional drawing exports from the Civil 3D environment. Save this file, and we are now ready to coordinate this project with Autodesk Revit.

Revit Coordinate Systems

In Autodesk AutoCAD or Civil 3D, the concept of an origin is pretty simple. There is coordinate location (0,0,0) in our model, all the project geometry refers to that location and it can be used to locate one file to another. In Revit there are several origins (or basepoints) that we need to keep an eye out for – the Survey Point, the Project Basepoint, and the Internal Origin.

The Survey Point is a common reference point that the project can relate too. Typically, identifies a real-world location nearest the project or campus. (Building corners, Property Line intersection, Iron Pin, LiDAR coordination target). For larger scale – campus-wide planning with multiple Revit and Civil 3D projects – the survey point can be used to relate all of these models together. Since Revit does not have the same understanding of precision and coordinate systems, we can use the Survey Point to trick Revit into thinking the project is somewhere it's not. The survey coordinate system is synonymous with the following terms:

  • Global Coordinates
  • GIS Coordinates
  • Grid Coordinates
  • Surveyor Coordinates
  • State Plane
  • Shared Coordinates

The Project Base Point identifies the origin of the project coordinate system and is typically used to locate measurements across the site. It is typically located at the intersection between 2 grid lines (these architects have is so easy!) The project coordinate system is synonymous with the following terms:

  • Local Coordinates
  • World Coordinate systems (WCS)
  • World coordinates (in a DWG context)
  • User Coordinate System (UCS)
  • CAD coordinates

The internal coordinate system provides the bases for positioning all elements in the model. It used to establish the Survey Coordinate System and the Project Coordinate System. This system is at the same angle as the project coordinate system.

And finally, the Internal Origin identifies the origin of the system and cannot move. All modeled geometry must be positioned within a 10-mile radius from the internal origin. Exceeding this distance may reduce reliability and result in undesirable graphic behavior. Understand Shared Coordinates

In the next discussion, we will examine the ways to coordinate the Autodesk Civil 3D project drawing with our Autodesk Revit model.  

Link the CAD file and Acquiring Coordinates Revit

Once the Civil 3D file has been prepared and exported (with care), the drawing can now be readily used in Revit and used to set up the architectural project base point and survey points. Fm the Insert tab use the Link CAD to link in the DWG. The Positioning location and Import units are vital to dial in correctly. For an Imperial Civil 3D project file, feet should be selected as the units (boo fractional inches).

Once the unit scale is determined, the positioning can be selected. To easily center the Civil 3D or AutoCAD drawing with the Revit model, center to center is recommended. However, there are additional link CAD positioning options including: • Auto - Center to Center: Places the center of the linked model at the center of the host model • Auto - Internal Origin to Internal Origin: Places the internal origin of the linked model at the internal origin of the host model • Auto - By Shared Coordinates: Uses the Survey Coordinate System to locate the model. This only works if both models share the same Survey Coordinate System. • Auto - Project Base Point to Project Base Point: Places the Project Base Point of the linked model at the Project Base Point of the host model. • Manual - Internal Origin: User manually places the linked model based off the linked models Internal Origin. • Manual - Base Point: User manually places the linked model based off the linked models Project Base Point. • Manual - Center: User manually places the linked model based off the linked models center.

Linked CAD content can be later translated in the Revit environment so this step may only have to occur once. However, if there are multiple DWG files that are linked throughout the project, consistency in the extents of the DWG model geometry and completing this step identically each time will be important. You will not be able to mix and match the positioning types when linking CAD and if the overall model extents change then the positioning will change as well when the link occurs.

RevitPlacement.png

Once the AutoCAD drawing content is linked and placed in the Revit site model, the drawing can be used to set up the shared coordinate system for the project. The Acquire Coordinate command (from the Manage tab) will examine the coordinate system utilized by the DWG and acquire its geolocation information to establish shared coordinates for accurate positioning of the model on the site and to coordinate other Revit models and files. The host Revit file acquires the True North from the linked model and the origin of the linked model's shared coordinates are set as the origin of the host Revit model's shared coordinate system. In this way a single Civil 3D file could be used to locate a series of Revit model files.

RevitAquire.png

Once setup the Project Basepoint will now reflect the location based on the acquired Northing and Easting values. Architects and designers can relocate this Project Basepoint relative to the BIM design.

RevitCoords.png

Setting Up the Vertical Datum in Revit

Thinking about the intelligent data linked from the civil infrastructure model, elevations may be one of the most important components. Whether we are using a 3D polyline to set a FFE (finished floor elevation) or creating a Toposolid from contours, coordinating not only the coordinate system but the vertical datum is critical.

Once the CAD Link is positioned, review the Project Base Point location along with the CAD geometry in an elevation or section view. The Project Base Point should be located at the preferred FFE elevation, and the CAD should sit about that point. This step will match the building entry elevation to the propsed building pad elevation from the civil site model. To achieve this coordination, the Project Base Point (current Revit project datum of 0 ft/m) will first be moved to the proposed project FFE with a vertical move command. Notice the linked CAD will react to this relocation of the Project Base Point and will perform the identical move. To correct for this, simply move the CAD Link inversely back down to the correct elevation. At this point the vertical datum and the project coordinates have been setup for the project.

Real-World Survey Point & Project North Coordination

To capture the real-world location and direction of our project site Revit allows us to relocate the Survey Point as well as define a new Project North. To match up the northing and easting coordinates in Revit we must first relocate the Survey Point to a known location within the civil model. With the Survey Point is selected you may see a paper clip, indicating that the Survey Point is clipped (or unclipped). During this relocation step, unclip the Survey Point to avoid relocating the entire site. Move the Survey Point to a common, known project location shared between the Revit models and other project files.

RevitNorth.png

The Revit Rotate Project North command can be used to adjust to any rotation from True North that the Civil 3D / AutoCAD file might indicate for a real-world rotation. The Rotate Project command allows users to specify a line – for example from our CAD file – to define the new project north. Utilize the 90 degree rotation to make any cardinal adjustments once a bearing line is selected.

RevitPlane.png

Round-Trip and Wrap-Up

Once the BIM modeling is underway in Autodesk Revit, it may be a good opportunity to coordinate the Revit model with the Civil 3D model and round trip the data back into Civil 3D. Revit fully supports exporting its intelligent BIM model to a simple CAD format such as DWG, a 3D model and more, and it will preserve such properties such as coordinate systems and project origins.

Coordinating the BIM file back to Civil 3D users is as easy as: creating a view / floor plan for the export, updating the Visibility/Graphics settings as required to display the most important content, setting the rendering style, and finally exporting to DWG format.

RevitExport.png

Just as we had concerns about preserving data when exporting from the Civil 3D model, we want to make sure that we can preserve as much information from the Revit model when exporting to CAD. Review the export setup for Layer Mapping, Units & Shared Coordinates. If set up correctly the DWG will drop in perfectly to existing Civil 3D project files. 2D CAD drawings and 3D models are supported through the AutoCAD DWG format.

RevitExport2.png

Hopefully, this overview can get you started with collaborating between the civil infrastructure CIM models and the architectural BIM models!

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