I recently started a new project file at the office, and looking back over the archives, I figured it was a good time to revisit some of my workflows for file set up. At some point since I last posted about establishing coordinates, I discovered on Revit OpEd that you can actually acquire coordinates from a dwg file. Acquiring coordinates is by far the superior method for establishing coordinates. Not only is it more efficient but it is also more accurate and leaves less room for human error.
Steve’s post goes into some good detail about how to clean up your dwg file and also how you can link subsequent CAD files via Shared Coordinates once you establish coordinates. So I would definitely recommend reading his post, but if you are unfamiliar with either the Survey Point or Shared Coordinates, here is the step by step process.
When starting a new site file there are a few rules that I always abide by:
- Do not change Project North or True North- the site file should always be set relative to real world coordinates or WCS (in AutoCAD terms).
- Establish at least 2 benchmark points for reference.
These two rules will reduce the possibility of alignment issues in the future. And if file alignment issues do crop up, they will be easier to resolve if you have done both of these things in your file set up process.
So with that in mind, the first step in establishing coordinates is to link in a dwg file that is set to WCS. By default, this will usually be Origin to Origin, but depending on the dwg extents and coordinates system, where the file appears may vary.
This survey came in centered on the origin, but that doesn’t always happen.
As always, you’ll need to unpin the survey (I frequently forget this step). And then move the dwg so that one of the benchmarks is aligned with the Project Base Point. (Do not move the Project Base Point. The PBP should always be pinned in place and never moved).
When snapping to a dwg import, I would recommend snapping to the center of your benchmark (using SC) and potentially even verifying the accuracy of the benchmark block (sometimes the cross hairs and the center of the circle can be very slightly misaligned). Also, once you have moved the CAD link, I would recommend that you pin it (it shouldn’t be moved again).
Next, unclip the Survey Point. This will ensure that it will not move when acquiring coordinates. And if you want to move the Survey Point to the second benchmark, now is the time to do it.
Since the Survey Point has been unclipped prior to moving, it will now have coordinates relative to the Project Base Point.
Then, select the dwg import and click on <Not Shared> (under Shared Site) in the Properties palette. Toggle ‘Acquire…’ and then click Reconcile.
If everything works, both the Survey Point and the Project Base Point will now have coordinates that perfectly match the linked CAD file. You can see that by selecting the point or by placing a Spot Coordinate at the point.
After successfully acquiring coordinates, remember to pin the Survey Point so that it isn’t moved accidentally (both the CAD link and Project Base Point should already be pinned).
And, as I mentioned at the beginning, if you have multiple dwg links, you can now link them By Shared Coordinates (under Positioning in the Link CAD dialogue).
I have not done this very extensively yet, but so far I am impressed with this workflow, which is much quicker and more accurate than moving a dwg link manually.
8 thoughts on “Establish Coordinates: Acquire Coordinates from a CAD Link”
As a friendly reminder, make sure to export to your CAD consultants using the shared coordinate system, and not from the Revit coordinate system! Great article! this is my #1 pet-peeve with architects that don’t understand this process. 🙂
Also, here is a link to a similar workflow using Shared Coordinate Systems. https://knowledge.autodesk.com/support/revit-products/learn-explore/caas/screencast/Main/Details/05f943be-4d60-4ec2-889f-f8ec57f2b19b.html
Thanks for sharing this. Unfortunately, I do not have C3D in the office, but it’s good to know the interop is starting to improve between C3D and Revit.
This is the method we always use and I won’t allow any other method in the office. We buy an Ordnance Survey map as a DWG, and acquire coordinates from it.
Because of the extents they always need to be manually positioned to that the PBP/SP sit on a grid intersection.
When you attach additional DWG files, such as topo surveys, you can use Shared Coordinates to load them, but Revit will give you an error…which you can ignore…they always land in the right place. You MUST remember to select them and turn off Shared Coordinates in the Properties though, otherwise they are vulnerable to misbehaving.
I’m a bit confused about this Paragraph?
“As always, you’ll need to unpin the survey (I frequently forget this step). And then move the dwg so that one of the benchmarks is aligned with the Project Base Point. (Do not move the Project Base Point. The PBP should always be pinned in place and never moved).”
Should this Read “Unclipped” Instead of “Unpinned” ?
I am assuming that the CAD link was brought in Origin to Origin, in which case it will be pinned by default. To move it you will need to unpin it. Only the PBP and SP have the clip state.
All Good. I’m trying to update our standards & your blog is a must read for me at lunch. We sometimes Clip / Unclip the PBP & SP’s. Again – Thanks for your very very helpful blog
All Good. I’m trying to update our standards & your blog is a must read for me at lunch. We sometimes Clip / Unclip the PBP & SP’s. Again – Thanks for your very very helpful blog.