Tis the season

Tis the season to give. To give change notices and site instructions. It’s also a season of sharing. Sharing CAD files with consultants so everyone is on the same page. In this weeks post I’m going to look at sharing information with non-Revit users and how to make it all line up. Yup in this post I’ll be explaining the Revit coordinate system and which tools are available to get it to line up with the AutoCAD coordinate system.

This is a topic filled with mystery and confusion. Partially because Revit doesn’t let you see the coordinate system like AutoCAD does (and most Revit users are ex-AutoCAD users) but I think the main reasons are poor documentation from Autodesk and really bad tutorials or explanations on the web. When I sat down and really looked into it I found it simple and easy to use. It’s just a matter of explaining it in a way that lifts the veil of mystery and lets every see how the machine underneath works.

AutoCAD coordinates

AutoCAD use a Cartesian grid called the “World Coordinate System” (WCS) to know where to place all objects in the file. This grid cannot be changed by the users. AutoCAD also has a “User Coordinate System” (UCS). This is another Cartesian grid that the user CAN change. The user can move and rotate the UCS anywhere on the WCS.

Revit coordinates

Revit has 3 grids systems. The Project grid which acts like the WCS and also the Project Internal and Shared coordinates which act like the UCS. Until version 2010 the only way you could see where 0,0,0 was in Revit was to import a cad file using origin-to-origin. With 2010, came 2 new features: Project Base Point (which shows the location of the Project grid) and Survey Point (which shows the location of the Shared Coordinates grid). Both of these features are hidden by default. To make them visible in a view go to the “Visibility\Graphics Overrides” setting for a model view and look under the “Site” category. Their default position is on the origin of the Project grid.

Project Base Point shown selected.

Project Base Point (PBP)

The PBP is just a blue circle with and ‘X’ in the middle. When you select the PBP additional information is shown including it’s location on the Shared Coordinate grid. Also displayed isĀ  a paper clip to the upper left.

Moving the PBP when clipped will relocate all the geometry and model views the same as the “Relocate project” command does. This however does not relocate 0,0,0. As you can see in the image to the left the PBP has been moved.

Moving the PBP when unclipped will change the reference for levels, spot elevations and spot coordinates only. The annotations read there position based on the PBP not the Project Internal Grid.

Moving the PBP whether clipped or unclipped will not move the SP.

Survey Point shown selected.

Survey Point (SP)

The SP is a blue triangle with a cross in the middle. When selected the SP also shows additional information and has a paper clip. The SP is meant to show an actual physical location in the real world, like a surveyors pin. Something that doesn’t move and the exact location can be determined on site. This point can be set to a linked file by using the acquire coordinates command and it is what is used to make sure exported CAD files will line up. Only one file in a project can be used to acquire coordinates so all consultants should use the same file. I’ll run through a scenario later on in this post.

Moving when SP when clipped will move the origin of the Shared Coordinate system and the linked file as well.

Moving the SP when unclipped doesn’t do anything really except change the coordinates of the SP marker. I don’t know why anyone would want to do this.

Moving the SP whether clipped or unclipped will not move the PBP. However, the PBP reports it’s position based on the location of the SP. So whenever the SP is moved the coordinates of the PBP change. This part is confusing I know but you can still get your CAD files to align without understanding this.

Note: clipped is not the same as pinned. Pinning the PBP and SP will prevent you from moving them completely.

One final crack at clarity

If your still confused I will offer this one final example using real world items.

Your sitting at a desk and on the desk is marked a Cartesian grid with 0,0 in the middle of the desk. Sitting on top of the desk is a clear glass plate it also has a Cartesian grid on it. Paper clipped to the middle of this plate is a picture of a triangle with a cross through it. On top of that plate, there is a second glass plate with yet another Cartesian grid on it. Paper clipped to this glass plate is a 3D model of a building. Moving either one of the glass plates does not affect the other glass plate.

Does that help to make it more clear. The desk is the Project grid you can’t move it at all. The first glass plate is the Survey Point and Shared Coordinate grid. If you remove the paper clip from the SP you can move it around on the glass plate without affecting the grid location. If you leave it clipped you move both. The second glass plate is the Project Base Point and Project Internal grid. If you remove the paper clip on this plate you can slide the plate under the 3D model. If you leave it clipped you move both together.

A scenario in Revit

Alright let’s do a step by step using the Big Box Blog Store as a scenario. However, I’m not going to be starting with the civil drawing like most other tutorials do. The firm I work for has a very large client with many stores. It is important that the CAD files all line up to the clients fixture CAD file. Also the building has usually been modeled in Revit before the client sends the finalized fixture layout. Which means I can’t link the fixture DWG using the origin-to-origin option.

Step #1 – Link the fixture layout into the Revit project using the centre-to-centre option. Next move and rotate the fixture layout to align with you building and pin the linked file to the location. This has been done in the image below. The text note is pointing out the origin of the CAD file, not Revit.

Floor plan with linked fixtures and SP selected

Step #2 – Activate the “Acquire Coordinates” command (Manage tab -> Coordinates panel) and select the linked file. The Survey Point should now appear when the linked file’s origin point is and it’s coordinates should read 0,0,0. In the image below, you’ll also notice the Project Base Point hasn’t changed location.

Floor plan showing relocated SP

Step #3 – When exporting to DWG make sure you set the “Coordinate system basis” on the “DWG Properties” to “Shared”. To make sure it all works open the linked DWG file in AutoCAD and insert your exported floor plan to the 0,0,0 origin. The plans should line up perfectly and so should the consultants if they all use the fixture DWG to define their origin points too.

Well that was a long post for a simple 3 step process. If you still have any questions send me an email or leave a comment on the ReVVed Facebook page.

PS – with the coming onslaught of the Christmas season I’ll be taking a break from blogging to do some serious nogging. See you next year.

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Revit and Building Information Modeling