Wednesday, November 19, 2014

Importing CAD Files and By Shared Coordinates

When we link/import DWG files with survey data Revit often encounters file extents that are quite large. The developers have always encouraged us to use the Auto - Center to Center positioning option with those files. Revit will forcefully use that option when the extents of the file violate the current 20 mile tolerance it has.

I wish I could write that linking/importing survey files was very simple and error free. The reality couldn't be further from that. I recommend these steps to improve the odds for success:
  • Import multiple Survey files individually (don't nest them as xref's)
  • Purge everything you don't need, purge again
  • Use Wblock if you can't get Zoom Extents to focus on just the relevant portion of the site
  • Remove Named UCS (Revit only wants the World Coordinate System)
  • Set UCS (User Coordinate System) to WCS and Plan to WCS
  • If the survey isn't oriented to WCS, North is "up", have the civil engineer/surveyor change their file first
  • Identify a specific location within the relevant part of the survey, put a marker, identify its coordinates, better still make those coordinates easy to use, even clean numbers.
  • Make sure everything actually aligns correctly in AutoCAD first, no point setting it up in Revit if it doesn't work there
  • Once you get a working first survey file, pass it back to the surveyor so they know what you need in the future
Once we've used Acquire Coordinates on our first survey file and verified the resulting coordinates are correct we can import the rest of the site related files. If Acquire Coordinates didn't work then we need revisit the items above, especially Named UCS. I find that Revit will acquire very large coordinates accurately IF the file is pristine.

In contrast I also like to use Specify Coordinates at Point (SCaP), using the marker I created earlier. Keeping in mind that doing so doesn't establish a shared coordinate relationship between the CAD file and the project. By relationship I mean that Revit records the identity of the linked file in the project when/if Acquire Coordinates is used to align the project with the linked file's WCS origin. SCaP does not.

Now regarding the title of this post, importing the rest of the files. Once shared coordinates are defined based on our first file we can import other site files using the positioning option: by Shared Coordinates. This assumes that each of the site related files are already aligned with each other using the same WCS origin. When we link a file this way a warning will appear while it is loading.

Revit is being precise, warning us that this project doesn't have a shared coordinate relationship with the incoming file. That's true, it is just another CAD file as far as this project is concerned. The only shared coordinate relationship that is established is between the first file and this project.

The last part of the warning is the significant part, The link's World coordinates will be aligned with this project's Shared coordinates". That means it will line up correctly because our project is aligned with the same WCS and both cad files already use the same WCS origin between themselves. Clicking Close accepts the warning and the link should land in the correct location. We can repeat this as many times as we have site related files to use.

When we save our project we may receive this warning. In this circumstance choose the bottom option Disable shared positioning...

We really don't want to create a named UCS in the CAD file the dialog references. It's linked and lined up correctly, its WCS is already correct, there is nothing to be gained by letting Revit store a named UCS in the file. This is what happens if you click Save instead, we end up with the named UCS in the image below.

I don't recall Revit bringing up this dialog in the past, unless I moved the link later, and I don't think it should be doing it in this circumstance. Fwiw, it doesn't in Revit 2015, at least not with the files I've been experimenting with for this post. It may be related to having a Named UCS. In some further testing I was able to link and save without generating the dialog. The inconsistency seems to consistently fall back on the condition of the DWG file though.

If we are careful to link each file and Disable the shared positioning Revit seems to think needs to be established all our linked file should line up very nicely. When they don't I find it necessary to revisit the list above. Skipping over them is very likely to bring on heartache later.


Unknown said...

Hi Steve
I am such a big fan of yours.Great piece of explanation.After specifying coordinates I was trying to test on the Autocad survey if it falls in the right place? "Curious mind" It did only after I disabled shared positioning.While the other way (xref into Autocad survey)worked very well.

Revito said...

Hi Steve,

Thanks for the information. It works well however, i notice that it only works when Project North orientation is current. When you choose True North the link subsequent files it doesnt work. Am i doing something wrong.


Steve said...

I don't think so. In recent releases of Revit it seems to be insistent on Project North being assigned in the view that the DWG file is linked into. If you reload a link while True North is active it will cause the DWG to move out of alignment. If Project North is active then it works properly.