October 22, 2010

Understanding Shared Positioning in Revit

James Vandezande, AIA
Firm-wide BIM Leader, HOK
Co-author of “Mastering Autodesk Revit Architecture 2011”

An increased importance has been placed on shared positioning in Revit in recent months. Even though this has existed in the software for quite some time, a general shift from “lonely BIM” to “social BIM” can be seen in the steady flow of new case studies and BIM awards. When I co-authored Mastering Autodesk Revit Architecture 2011 (Wiley, 2010) with Eddy Krygiel and Phil Read, I decided to focus some energy on a more complete understanding of this often misunderstood functionality. The following text is an excerpt from “Chapter 7: Working with Consultants” of the book.

In the collaborative process of sharing information via linked models, the coordinated positioning of each model is of paramount importance. Agreement on a common coordinate system and origin must be included in every project’s BIM execution plan to ensure accuracy. This section will help you develop a fundamental understanding of the coordinate systems within Revit, so you can configure and manage them in your projects. For a more complete history of coordinate systems and various examples of using them within Revit, we recommend the following class material from Autodesk University 2009, available at au.autodesk.com:

  • AB118-3: Finding Your Way Around Shared Coordinates, by Teresa Martin, Ideate Inc.
  • AB9114-1: Autodesk Revit Collaboration: Shared Coordinates for Projects Big and Small, by Steve Stafford, AEC Advantage, Inc.

There are two coordinate systems in a Revit project: project internal and shared. Each system has essential features and limitations.

Project Internal Coordinate System

Every Revit project has an internal coordinate system referred to in several places as Project. You can find this reference in the Type Properties of datum measuring objects such as levels and spot coordinates as well as in the settings for exporting CAD files. The project coordinate system cannot be changed and your model should be constructed within a one mile radius of the project origin. The true origin in Revit is referred to as the Project Start Up Point; the Project Base Point can be reset to this point by setting it to Unclipped, right-clicking on the icon, and selecting Move To Start Up Location.

Tip: Modeled elements should be constructed as close to and as orthogonal as possible to the internal origin and coordinate system.

A complementary component of the project internal coordinate system is the view orientation of Project North. This setting is the default and can be found in the View Properties of any plan. We strongly recommend that your model be created in an orthogonal relationship to the project north or as you expect the plans to be oriented on a typical sheet. Your project’s actual relation to True North will be established via shared coordinates.

Shared Coordinates

According to Ideate’s Teresa Martin, “shared coordinates are simply a way for the project team to utilize the same definitive work point.” In other words, the shared coordinate system consists of a single origin and true north orientation which can be synchronized between models and even AutoCAD drawings. In the diagram shown below, you will see an architectural model and structural model linked together. Each model was created using a different Project Base Point (PBP)—which is not the recommended method—but their shared coordinates were synchronized.

Although you can use either Project Internal or Shared as the setting for Coordinate System Basis when exporting CAD formats, there are some limitations. If you are exporting sheet views, the plan data will always use the Project Internal coordinate system. Using the XRef Views On Sheets option during export does not change this limitation. We recommend using views—not sheets—for issuing 2D CAD backgrounds to project participants not using Revit.

Although shared coordinates have not been supported for IFC export in previous versions of Revit, Autodesk has recently posted a notification that this functionality is enabled in the 2011 version. If you are planning to utilize IFC exported data in tools such as Solibri Model Checker, BIMServer, or even in collaboration with other designers using ArchiCAD; you must ensure the appropriate coordinate system is set to the current project location before exporting to IFC by activating the Manage tab on the ribbon and then clicking Location. If the appropriate shared location isn’t set to (current), select it from the list and click the Make Current button.

If you are using Autodesk Navisworks for 4D simulation or clash detection, you can also take advantage of your shared coordinates system. When you launch the Navisworks 2011 exporter within Revit, click on the Navisworks Settings button to open the Navisworks Options Editor. From here, you can specify either Project Internal or Shared in the Coordinates option drop-down as shown below.

Acquiring or Publishing Coordinates

When you attempt to synchronize shared coordinates between linked projects, there are two tools to achieve this: Acquire Coordinates and Publish Coordinates. A simple way to understand the difference between these tools is to think of them in terms of pulling versus pushing:

  • Acquire = Pull
  • Publish = Push

It is important to understand the situations where you would pull or push coordinates between linked files. A typical workflow for establishing a synchronized shared coordinate system on a single building project would be as follows:

  • A site model is generated in which the survey point in Revit is coordinated with geodetic survey markers or station lines. The site model is linked into the architectural Revit model. This file can be placed manually, then moved and rotated into a position relative to the building. Do not move or rotate your building to the linked site model!
  • From the Manage tab, select Coordinates, then Acquire Coordinates. Pick the linked file and the origin of the shared coordinate system and angle of true north in your Revit model will be synchronized with those in the linked file.
  • For all engineers or consultants using Revit, they should obtain a copy of the site model and repeat steps 1 and 2. When linking other project models that have already been synchronized with the site model, they can be placed using the Auto – By Shared Coordinates option.

Acquiring Coordinates from a CAD File

A common scenario in a project workflow begins with the architect commencing a design model and receiving a 2D survey file in DWG format from a civil engineer. The survey is drawn in coordinates that are geospatially correct but may not be orthogonal to true north. The architect should create the model close to the internal project origin; however, the architect will need to ensure the building and survey coordinates are synchronized for properly oriented CAD exports, and for coordination with additional linked Revit models from engineers in later phases of the project.

The architect will link the 2D CAD file into the Revit model but will first manually place it—moving and rotating the CAD file to be in proper alignment with the building model. Once the link is in place and constrained (or locked), the architect will acquire the coordinates from the DWG survey by switching to the Manage tab, selecting Coordinates > Acquire Coordinates from the Project Location panel, and then clicking the DWG link. This will not affect any views that are oriented to Project North—only those set to true north will display the orientation established by the coordinates acquired from the survey file.

For a campus-style project in which you might be creating multiple instances of a linked building model, you would most likely use Publish Coordinates to push information from a site model into the linked building model. Here’s how that would work in a hypothetical scenario:

  • Assuming a site model and building model were created in Revit, you would begin by opening the site model and linking the building model into the site.
  • Adjust the position of the first instance of the linked building model to location A.
  • From the Manage tab in the ribbon, select Coordinates, then Publish Coordinates and pick the linked model.
  • The Location, Weather and Site dialog box will open and you will create a duplicate location named Location A as shown below.

  • Copy the linked building model as required for each subsequent location. Repeat steps 2-4 for each copy.
  • When you close the site model and open the building model, you can link the site model to the building using any of the named location references you pushed into the building model.

Using Project Base Point and Survey Point

In prior versions of Revit, locating the project origin or shared coordinate point was only accomplished by using an imported AutoCAD file or by using spot coordinates and moving elements as required. Revit now provides two objects to identify these points: the project base point and the survey point. In the default templates, these points are visible in the floor plan named Site; however, they can also be displayed in any other plan view by opening the Visibility/Graphics Overrides dialog box, selecting the Model Categories tab, and expanding the Site category as shown below.

The Project Base Point (PBP) defines the 0,0,0 point of the project. Notice that we are not calling it the origin. Using that term, you might confuse the PBP with Revit’s internal project origin. The unclipped PBP can be moved in relation to the internal origin, thus creating a secondary reference point for spot coordinates, spot elevations and levels—as long as the measuring control is set to Project in the respective type properties. Moving the clipped PBP icon is the equivalent of using the Relocate Project tool, moving the project relative to the shared coordinates system.

Unless your project requires the use of a secondary point of reference other than the survey point, we recommend you do not adjust the PBP and make sure your building model lies within a close reference of this point such as the corner of a property line or intersection of column grids A and 1.

The Survey Point is the equivalent of a station pin or geodetic survey marker in a civil engineering drawing. This is the point that will be coordinated to real geospatial coordinates. For coordination with Autodesk Civil 3D, the survey point is used when a Revit project is exported to the ADSK file format.

Note that specifying a particular location for the survey point based on civil engineering data is not a requirement. For smaller projects, the survey point and shared coordinates may never be used at all; however, these are critical in the use of analytical tools for daylighting and solar analysis.

To further expand your understanding of these points and what happens when they are modified, you can use this sample file for your reference: c07-Shared-Points.rvt from the companion web page of “Mastering Autodesk Revit Architecture 2011” available at www.sybex.com/go/masteringrevit2011. In this file, you will find three copies of the floor plan Level 1. Each is configured to display the project coordinates, the shared coordinates, and a combination of the two. There are also two types of spot coordinates—one indicating project coordinates in which the values are prefixed with the letter p, and the other indicating shared coordinates with the prefix of s. You can open these three floor plans and tile the windows (View tab, Window panel, Tile or type the keyboard shortcut WT) to get a better sense of how these points affect each other.

Within this sample file, you can explore the effects of moving the project base point and survey point on your model’s coordinates. When selected, the project base point and survey point have paperclip icons that determine the behavior of the points when you move them. Clicking the paperclip icon changes the state from clipped to unclipped and back to clipped.

Modifying the Project Base Point and Survey Point

Following is a list of the possible point modifications and explanations of how they affect the project. Note that in most cases, you shouldn’t have to move the survey point or project base point if you are using a linked civil file (2D or 3D) and acquiring the coordinates from the linked file.

Project Base Point (PBP): Clipped

  • Move the PBP
  • PBP values change
  • Project-based spot coordinates don’t change
  • Model elements “move” relative to shared coordinates

Moving a clipped PBP is the same as using Relocate Project. That is, the model elements maintain their relationship to the PBP, but the relationship of the PBP to the survey point is changed.

Project Base Point (PBP): Unclipped

  • Move the PBP
  • PBP values change
  • Project-based spot coordinates change
  • Model doesn’t move

Unclipping the PBP essentially detaches it from the internal project origin. Moving the unclipped PBP is really only used to affect the values reported in spot coordinates set to the Project origin base. It does not have any effect on exported files.

Survey Point (SP): Clipped

  • Move the SP
  • SP values don’t change
  • Shared spot coordinates change
  • Model doesn’t move

The clipped survey point represents the origin of the shared coordinate system. Moving it is the equivalent of setting a new origin point. Use caution if you must move the shared coordinates origin, especially if linked models already exist in which the shared coordinates have been synchronized. In such a case, each linked model must be opened and manually reconciled with the model in which the origin has changed.

Survey Point (SP): Unclipped

  • Move the SP
  • SP values change
  • Shared spot coordinates don’t change
  • Model doesn’t move

Moving an unclipped survey point essentially doesn’t do anything. It doesn’t affect spot coordinates and it doesn’t affect the origin of exported files.

Use Pinning to Protect Coordinate Origins

An excellent way to prevent accidental modification of your project’s coordinate systems is to pin them. To do this, you must first make sure the survey point and project base point are visible in a view (as we discussed earlier in this article). Next, select each point and click the Pin button from the Modify panel when the Modify > Project Base Point or Modify > Survey Point ribbons appear.


Submitted by Steve

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