Better Site Coordination and Topography: An AutoCAD Civil 3D and Revit Workflow
Does this conversation sound familiar?
Civil Engineer: Why doesn’t Revit have a project base point?
Architect: Revit has a project base point.
Civil Engineer: Then why doesn’t it reference the “Real-World” base point from our survey?
Architect: Well, why does your DWG come in way out of left field when I insert your file into my project?
Civil Engineer: Because you’re using the wrong 0,0 point.
Architect: Why can’t you just relocate the corner of my building to the 0,0 point?
Civil Engineer: Why can’t you move your project base point to reflect where the building actually sits in the real world?
Coordinating the Autodesk® Revit® building model with the AutoCAD® Civil 3D® site plan has been a thorn in the side of many project teams. Everyone wants to get along and create a well-coordinated site plan. However, the strengths and limitations of both Revit and Civil 3D cause miscommunication and misinterpretation that can result in the above conversation.
Some Limitations of Autodesk Revit
Autodesk Revit is… well… Revit. It is crucial to understand that while Revit can link and import DWG files into the project environment, and can extrapolate topography based on connecting a series of points derived from a CAD file, Revit is not able to incorporate certain types of data that Civil 3D takes for granted.
These data types include applying rules in how the data is connected, no control lines, feature elements, breaklines, etc. When Revit interpolates the elevation points acquired from a DWG, it undergoes a triangulation process with the two nearest adjacent points and extrapolates the topography.
Assigning “real-world” coordinates to a Revit project requires some input and an understanding of how Revit “sees” its location. In other words, to use a common phrase among Revit users: You have to know “What Revit wants” before it can do what you want. Let’s start with some basic Revit terminology.
Origin or Startup Location
The Startup Location is the original position of the Project Base Point of a new project. This is analogous to the 0,0 point in AutoCAD®. However, because civil engineers use State Plane Coordinates (or something similar) to establish the project location, linking a Civil 3D plan in Revit by the 0,0 point is less than helpful. Spot Coordinates and Spot Elevations set to Relative reference this location.
Revit Project Base Point
The Project Base Point (PBP) defines the origin of the project coordinate system, which may (or may not be) the same as the Startup Location (see Figure 1).
The PBP is often used to define the location of the building on the site. Spot Coordinates and Spot Elevations that reference the project coordinate systems are displayed relative to this point.
The PBP also establishes the “Project North” for laying out the building plans on sheets.
Revit Survey Point
The Survey Point represents a known point in the real world. Although the Survey Point can represent something like a geodetic survey marker, it can also represent any known physical location on or adjacent to the physical site.
The Survey Point (when used correctly) orients the building geometry in another coordinate system, such as Civil 3D and established “True North” of a Revit project (see Figure 2).
What Revit and Civil 3D Need from Each Other
To best coordinate the Architectural Building Model with the Civil Site Plan, Revit and Civil 3D need to share some information with each other. This information exchange requires some back-and-forth in data sharing that should start with the architect.
Ideally, the architect should create a Site Model separate from the Building Model, especially if the intent is to build accurate topography for rendering purposes.
If, however, the architect is not going to create a realistic topography of the site, then the following steps may be carried out in the Building Model.
Establishing Shared Coordinates in Site Model (Just one Method)
1. Create a floor plan view (I am going to assume the architect knows how to do this part) and hide all categories not essential to representing the building footprint.
2. Export the new Floor Plan (let’s call it Plan-Export) to a DWG format (again, I am going to assume the architect knows how to do this) and share this file with the civil engineer.
a.NOTE: The architect can also export the entire Building Model as a 3D DWG or an ADSK, allowing the civil engineer to locate a 3D building on the site.
The Civil Engineer
1. Insert this file (DWG or ADSK) as an External Reference and locate the building on the site (moving, rotating, etc.).
2. Once the building has been located on the site plan, the civil engineer (in all his magnanimous generosity) will create a Reference Point (RP) on the Site Plan for the architect (see Figure 3).
3. This RP will show the Northing, Easting, and Elevation of some point on the site that will become the Site Reference Point between the Site Plan and Building Model.
1. Link the Civil Site Plan into the Building Model (or Site Model if you are going that route), making sure you are using the Positioning: Origin to Origin option.
2. Locate the Reference Point on the Civil Site Plan and take note of the values. (Record on a sheet of paper if your memory is anything like mine.)
3. Access the “Visibility/Graphics Overrides” for the Site Plan within the Revit Project.
a.Open the “Site” category (Figure 4).
i.Turn on the “Survey Point” subcategory.
4. In the Site Plan view, select the Survey Point.
a.Click on the paperclip icon to “unclip” the Survey Point (Figure 5).
5. Copy or type them into the appropriate fields of the Survey Point.
a.NOTE: Do not copy/paste the values from the reference point directly into the Properties palette.
6. Clip the Survey Point (making sure the red slash is no longer visible.
7. Move the Survey Point to align with the Reference Point on the linked Civil Site Plan.
Shared Coordinate System
If the architect is using a Site Model to coordinate one or more Building Models on the site, then the architect will not be able to utilize the “Publish Coordinate System” to the Building Model(s).
1. Link Building Model(s) into the Site Model.
a.Ensure the Positioning: Origin to Origin option is selected.
2. Locate the Building Model(s) on the site to their final resting place(s).
a.Ensure the correct elevation of each building from an elevation or section view.
3. Ensure that all other users have Synchronized to the Central file and, preferably, have exited the project until the following steps have concluded.
4. Publish Coordinates to Building Model(s).
a.Manage Tab > Coordinates Panel > Publish Coordinates Tool.
b.Select the Building Model(s).
i.Use the “Internal” Coordinates for each Model.
ii.Create a Duplicate Coordinate system for each Model that is repeated in the Site Model.
5. Save the Site Model.
a.A dialog box will appear with the message that the “Location Position Changed” for each model to which the new coordinates were published.
b.Select “Save” for each occurrence of this message.
6. Close the Site Model.
7. Open the Building Model(s).
a.Repeat the following steps for each Building Model.
8. Link the Site Model
a.Insert Tab > Link Revit > Select Site Model.
b.Positioning: Auto – By Shared Coordinates.
9. Verity that the Site Model is in the correct position.
10. Pin the Site Model in place (if desired).
Rock the Topo!
Coming full circle to the limitations of Revit Topography, there are some things civil engineers can do to maximize the usability of their 3D surfaces in Revit.
Typical workflows for creating surfaces are perfect for the run-of-the-mill topographies in Revit. However, there are times (like when the architect wants to dazzle the client with a higher quality rendering) the standard workflows lack the necessary level of detail.
So, what can you (as an amazingly awesome civil engineer) do to provide that greater degree of detail as well as expand your list of added services? The following steps will help you facilitate that extra dazzle the architect is seeking (Figure 6).
The Civil Engineer
1. Extract the 3D faces from Surface (combining the EG/FG surface if necessary).
a.Put triangles on a different layer in Surface Styles so triangles will populate a specific layer during extraction.
2. Create COGO points along all TIN (Triangulated Irregular Network) edges due to significant grade changes i.e. curbs, retaining walls, etc. with the applicable method(s) below.
a.Survey Figures, Feature Lines, and Grading Object Daylight Line, Alignments/Profiles.
i.Point Creation Tools > Command Settings
1.Prompt for Point Names: None
2.Prompt for Point Names: None
3.Prompt for Descriptions: Automatic
4.Next Point Number: 10000 or similar
ii.Create Points > Miscellaneous: Measure Objects (Figures, Feature/Daylight lines).
iii.Create Points > Alignment: Measure Alignment (Alignment/Profiles).
iv.Create enough points so that triangulation in Revit (based solely on points) will be in the correct direction. Some experimentation is required. However, spacing the points to match the height or width of a transition will usually yield the desired result.
i.Increase frequency along TIN edges of significant grade change.
ii.Select Corridor > Contextual Ribbon > Launch Pad Panel drop-down > Points from Corridor.
iii.Select Point Codes corresponding to applicable TIN edges. Typical examples for curb include Top_Curb, Flowline_Gutter.
3. Add new “Revit” points to new “Revit” surface.
a.Extract 3D faces from this surface.
4. Provide call-out of precise 3D location on site using a GOGO Point with Northing, Easting, and Elevation labeled.
a.Explode COGO Point to turn it into a block.
5. WBLOCK 3D faces and label new file for use in Revit.
a.Ensure nothing other than the 3D faces have been selected.
6. If using Autodesk Shared Reference Tool (not discussed in this article), run that command now.
a.When prompted for the first point, pick 3D location mentioned in Step 4.
b.Pick second point (or turn on Ortho) in direction precisely north of 3D base point. Distance is not important.
c.Confirm other default settings: origin, etc.
d.Save as XML file and provide this to Revit user along with accompanying DWG file.
There is still no “Easy Button” (as much as I’ve been begging Autodesk to create one) to coordinate between Autodesk Revit and Autodesk Civil 3D. After all, Civil 3D is AutoCAD based. And Revit is... well… Revit! Revit and AutoCAD still have a language barrier inherent to their base programming, so until a Universal Translator comes along (come on, you Trekkies out there) we’re going to have to just learn how to communicate better with each other.
After all, the project is bringing us together. But the software doesn’t have to keep us apart.
Adam Nicholes is a BIM modeling expert based out of Utah. Adam has been a part of the design community for roughly 11 years in all aspects of design, construction, and software implementation. He has implemented and trained companies across the country on Revit and other design tools and their best practice workflows. Adam can be reached for comments or questions at firstname.lastname@example.org.