Project Coordinates, Shared Coordinates, and the new Survey Point

[dbaldacchino]

Did I ever mention I hate coordinate terminology and behavior in Revit?

Ok let me clarify. If you try to link a dwg into Revit, the location of the dwg geometry with respect to the Global Coordinate System origin is somewhat irrelevant as long as it all lies in a 1 mile radius. Revit will allow you to link it, but if it's that far from the Global Coordinate System Origin, Revit will not link it with the Origin To Origin option and will instead default to Center to Center. The dwg's origin is really irrelevant because now you set your Survey Point to a known value and move the dwg so that this point co-incides with a known datum.

However Revit will not let you move the Project Base Point (unclipped) outside the 1 mile radius from the startup location (which defines Revit's Internal Fixed Origin). So even though you can move your linked site far from the startup location, you won't be able to move the project basepoint outside that radius, which could limit you.

If you try and link a dwg whose geometry lies outside of the 1 mile radius circle, Revit won't even let you link it now, and you'll be presented with a warning dialog. This won't impact a large user base I think, but there will be projects that will need to employ workarounds to circumvent this limitation.

Jeff, I look forward to those Skill Builders! I'm currently working on some graphical examples to help explain the terms and clear up confusion. I'll blog about it in the coming weeks and post a link here.

[Wes Macaulay]

Is there a reason you'd want to move the Project Base point that far away from the startup location? Couldn't you just relocate Shared Coordinates to handle the DWG's origin? (We are coordinate geeks, aren't we)

[dbaldacchino]

Yes we are! I guess it's the challenge of truly understanding this darn thing is what gets me hooked, so I don't have to think it through another time. And hopefully I'll have enough documentation to explain it to anyone in a year's time when all this will be fuzzy in my head

There really isn't a reason you'd want the project to be so far away from the fixed origin. I was just pointing out that you could, but if you tried moving the unclipped Project Base Point icon outside that boundary, you'd get a warning dialog and Revit won't let you do it. I wasn't aware of that warning until I tried tonight!

[David Conant]

To put things in real world terms.

Think of the Survey Point as a stake placed by the surveyor that has a known location in the surveyor's world. It will have a shared coordinate value equal to its distance from the surveyors origin. By moving it in a clipped manner, you are sliding the site around under the model. (assuming you have linked the site in using shared) Moving it in an unclipped manner is the equivalent of pulling up the stake and putting it in a different spot on the site. Changing the shared coordinate value of the Survey Point controls the location of the Shared Coordinate System's origin

Think of the Project Base Point as a convenient spot on the project from which to measure and place things. It might be the intersection of grid lines or the corner of some geometry. You will have carefully measured the distance and direction to that spot from the surveyor's stake. By moving it clipped, you drag the whole project around over the site (if it is linked to shared). By moving it unclipped, you are saying that you will be measuring from a different point on the building. The PBP always sets the 0,0 point of the Project Coordinate System. It's orientation also indicates Project North.

Both of these systems exist as overlays on the internal coordinate system, and in a new project share the same origin point. This point can be recovered at any time by unclipping the Project Base Point. When the unclipped PBP is selected, there is a command on the right click menu "Move to Startup Location".

The magic 1 mile radius around the internal origin is still enforced to prevent bad system behavior. The base points can't be moved out of the circle because they too have to be drawn and will thus trigger the same issues as project geometry.

[dbaldacchino]

Thanks for taking the time to explain David. After spending a day yesterday fiddling around, I feel confindent I finally understand the system and can relate to your explanation with diagrams. I'm attaching a draft of what I have so far.

[jeffh]

One more clarification: exporting to the ADSK file format -- this is always done with respect to the location of the Survey Point, whether it's clipped (at the origin of Shared Coordinates) or unclipped?

The ADSK file export is for sharing a file with Civil 3D. The Civil engineer and Architect agree to a position where the "survey point" should be placed and the Civil Engineer then supplys the Architect what the coordinates are at this position with respect to their Civil work. Now when the Civil Engineer imports an ADSK file created by Revit the project will appear in the right position.

[patricks]

I'm having an issue where I bring in a CAD file with the origin very clearly at the property corner when I view it in AutoCAD. When I link the file into Revit, rotate it, and acquire coordinates, the survey base point shows near that property corner, but it's at least 50 feet away from the actual corner of the property line. I checked the CAD file and it was set to the World coordinate system, with the origin being at the property corner.

What could be causing this?

[dbaldacchino]

When you acquire coordinates, if the dwg file doesn't have published coordinates from Revit in it, the Revit Shared Coordinate system will match up with the WCS of the dwg. So the 0,0,0 location will be the 0,0,0 of your shared coordinate system in Revit.

Question: The Origin in your DWG represents a known benchmark or is it the Project Basepoint origin?

[patricks]

When you acquire coordinates, if the dwg file doesn't have published coordinates from Revit in it, the Revit Shared Coordinate system will match up with the WCS of the dwg. So the 0,0,0 location will be the 0,0,0 of your shared coordinate system in Revit.

Question: The Origin in your DWG represents a known benchmark or is it the Project Basepoint origin?

The origin in the DWG is a property corner, where an iron pin is located on the site.

I finally got the Revit survey origin to match up with the property corner after several operations of specifying coordinates at points, moving the CAD file, relocating project up or down (project and survey base points ended up 500 ft. above my model at one point).

So now all appears to be correct.

[cphubb]

Guys,

I think you are specifying the shared/project coordinates backward. Correct me if I misunderstood. We are using other downstream applications that do not see Revit shared coordinates and will have issues if you use the Survey point as the TRUE origin of the entire project. We set the project coordinates to a known point x,y,z and then relocate the project to a more convenient location that coorisponds to campus plans or survey locations etc. We have found that exporting to other downstream applications including Navisworks up to June 2009 the shared coordinates messed up the ifc and dwg export and required manual adjustment in the other application to get linked models to corrispond in origin. So our procedure is to set the model x,y,x relative to the origin, adjust the origin Lat/Lon and set the true elevation of a floor or level. We then relocate the project to work with survey campus etc using the relocate project tool. (We are still not using 2010 yet so no clip unclip) but will probably adjust to that method when we switch to the new interface (maybe in 2020 when the fix the problems)