Thursday, April 03, 2008

CAD Surveys and your Revit Project

This text was posted at AUGI by David Conant (Autodesk Revit Product Designer) in February this year. It is very important information.

This method should ALWAYS be followed. It is critical to follow when the survey coordinates of your building site have large values (> 2 miles/3km from site 0,0):

ALWAYS locate sites underneath buildings. Revit buildings like to stay near home and be oriented to project north. Sites can wander about and talk to the buildings later. DON'T try to pan your view 40 km northeast of 0,0 and try to build your building model there because your site coordinates have a remote origin. This will work in AutoCAD but in Revit you will be very unhappy.

NEVER import or link a site with large coordinate values Origin to Origin! This may seem correct based on AutoCAD experience, but here too you will be very unhappy in Revit. You will get the correct origin later in the process.
  • Build your revit model at or near the position of the elevation marks in the default template with the building oriented orthagonal to your desired printing frame. i.e. use a Project North not True North for your working environment.

For one building on a site:

  • Link in a site model or site dwg.
  • Move and Rotate the SITE so that it is located correctly relative to the building.
  • Acquire the site's coordinates. Now, the site's origin will be the origin of your shared coordinates. The site's Y axis will be True North. The site coordinate values can be arbitrarily large without disturbing Revit's internal calculations.
  • You can later open the site model and link in the building using shared location and it will land in the exact position.

For multiple buildings on a site:

  • Create a Site project, link in a site dwg, placing the center of the building site near the center of the default view.
  • Acquire coordinates from the site.
  • Build any site elements, topos, etc.
  • Link in building rvts. Move and rotate them so that they are correctly located on the site.
  • Publish coordinates to the rvts. Now all models will have the same Shared origin and True North orientations.
  • You can now link the site into any of the buildings (the other buildings can be linked in as well) using shared location and it will be in the exact location.

8 comments:

Anonymous said...

Steve,

We meet again!
"ALWAYS locate sites underneath buildings. Revit buildings like to stay near home and be oriented to project north. Sites can wander about and talk to the buildings later. DON'T try to pan your view 40 km northeast of 0,0 and try to build your building model there because your site coordinates have a remote origin. This will work in AutoCAD but in Revit you will be very unhappy.

NEVER import or link a site with large coordinate values Origin to Origin! This may seem correct based on AutoCAD experience, but here too you will be very unhappy in Revit. You will get the correct origin later in the process."

What makes you say this? Why would I be unhappy?

Thanks again!
JB

Steve said...

David's choice of words. If you do what he says you won't be unhappy. If you do what he says not to do you run the RISK of being unhappy but it may not make you unhappy.

We accept no responsibility for your definition of unhappy. The names have been changed to protect the innocent. Any similarlity to people, places or things is merely coincidental. Please do not bend, fold or mutilate. Objects in mirror are closer than they appear. Thank you for playing...

Anonymous said...

Are you going Rain man on me?

I only ask the question because David's choice of words come across as though he had a bad experience by doing so. I was asking because I wanted to know the RISK before I jump! If they're not your words then... do you know why I wouldn't want to do as David described?

Thanks again Steve!

Steve said...

Just which "JB" are you? Apparently not the one I thought you are...

Full disclosure, I am not a mathematician. Yesterday I used a formula that said that one half plus one third is two thirds...and apparently it isn't!! But its close! So beware the math, check your work!

Revit's mathematical design for calculations is based on a relatively small footprint (2 mile dia. David mentions), the footprint of a building, not a city or even a small town, a building.

Exceeding this footprint means that the farther away from this footprint you get the "less" accurate it becomes.

I have seen files and projects that begin to distort their linework, when you zoom in really close you can see this in a normal file too, and appear to snap to points that aren't on the the element. I have also seen this in AutoCAD files within AutoCAD under similar circumstances but more extreme distances involved.

As I understand it there are two mathematical models involved, one for generating display of the model and one for actually creating the model elements.

To sum up, if David says not to, its good enough for me. He's the is most "senior" ranking remaining Revit team member as the other two Revit people with more years with Revit have both retired (the founders) if we don't include lawyers and accountants naturally.

And David, if you read this, I don't mean senior as in old, I mean it as in the most years with Revit (which could also mean you are old, this isn't getting me out of trouble is it?)

Anonymous said...

Ah that answer works for me. Thanks Steve (and David)!!!
JB

Malleristic-Revitation said...

Steve-

Perhaps i can further articulate *why* he will be unhappy. I have a Revit model here that was built... lets call it a *good distance* from the Revit origin.

When working in such a file, you dont seem to notice that there are any problems with it, until you begin to zoom in close to things. now, i dont mean obsessively close, just close enough to drop a metal stud detail component. You suddenly notice you cant lock it to the line of the wall, but rather it is locking to a "phantom" or "ghost line" NEXT to the wall stud. trying to use the aling tool, it snaps to the ghost. And when you export or plot to pdf, it is (to no suprise) not where it should be.

Plus, the ghosting is not consistant, because of the two different math engines Steve described. meaning, the ghost is "slightly below and to the right" of the model, until i roll my zoom scroll one click, then suddenly it is up and to the left. But all of the previous items inserted are now down and to the right.

And alas, trying to xoom in closer to circumvent begets you a myriad of "an error has occured while drawing the contents of this window. this window shall now be closed" issues.

This is just the first of major problems. DWG imports come in completely annihiliated, and its just a mess.

Im a firm believer in that text Mr. Conant and Steve have posted, not by faith, but by brutal experience of having gone the wrong way, LOL...

Charles Vincent said...

I'd add to the NEVER IMPORT ORIGIN TO ORIGIN:

Always import Origin to Origin, provided your design will be at or close to the Origin...
Provide this in the imported files, prior to importing, of course.

Michael Cervantes said...

Steve,

Was searching for some updated information on how to place a Revit model in relation to state plane coordinates, and wanted to confirm if your suggestions are valid for the current release of Revit given the new radius boundaries?

Also, if you already have actual coordinates of a monument on a site that you would like to establish as a common work point for your bulding, is it better to just type in those coordinates as the origin of your model, as opposed to using shared coordinates, and then distributing that as the project template for your team?

I say this, because, as I understand it, it seems shared coordinated can reset if multiple files are linked and shared coorindates are used. Whereas, if the Revit origin is relocated within the project, and then pinned, and then distributed as a template, every one is assured a common work point. This also essentially moves the origin of the site, as opposed to moving the building itself, making it easier to underlay AutoCAD Site plans

We have experimented with this here at LA Community College District, and it appears to work fine. However, since we are implementing the BIM Process and not actually doing the design work, we wanted to know if anyone out there is actually working this way, and are getting positive results.

Thanks, and keep up the great work on the blog. It is a wealth of information.

Regards,

Michael Cervantes