Revit and AutoCAD UCS

[kparks140020]

Using UCS in AutoCAD for many years, and now I am using Revit which does not have a viewable 0,0,0 origin. I have imported a DWG floor plan using the "origin to origin" setting but am I by faith letting Revit control the correct origin in the drawing environment? Occasionally the Story Level Tags are far out in space in my elevation views, but not sure if this is a related problem. How does Revit handle the "X, Y, Z" orientations and placement of imported DWG files and other objects.

Thanks.

[aaronrumple]

0,0 is there. Just not of much use in Revit. Shared coordinates are more important.

However, for AutoCAD users I put in a couple of ref. planes in my template at 0,0. To find 0,0, just draw a simple X at 0,0 in AutoCAD and import the file using origin-to-origin. There you are. (...and emember: Wherever you go, there you are.)

[kparks140020]

Thanks. I'll share this with my fellow employees. If 0,0 is not much use in Revit, then I will not be to concerned about it myself. I went, and here I am.

[JamesVan]

What we've found after discussing the issue at great length with the facotry is the following methodology. When you already have some DWG data before beginning in Revit:

1. Link in the DWG file using "Center-to-Center" option
2. Use the "Acquire Coordinates" tool and pick the DWG link to align Revit's shared coordinates to the DWG's World coordinates.
3. Other DWG's can now be linked using "By Shared Coordinates"

Think of how you would translate data in Autocad as compared to Revit. If you had a physical model (CAD data) on a sheet of paper (the coordinate system), in Autocad you would move the model around on the paper; whereas in Revit, you move the paper under the model.

By linking the DWG in first using origin to origin, your levels and standard elevation views will be way out in space.

[A3D]

What we've found after discussing the issue at great length with the facotry is the following methodology. When you already have some DWG data before beginning in Revit:

1. Link in the DWG file using "Center-to-Center" option
2. Use the "Acquire Coordinates" tool and pick the DWG link to align Revit's shared coordinates to the DWG's World coordinates.
3. Other DWG's can now be linked using "By Shared Coordinates"

Think of how you would translate data in Autocad as compared to Revit. If you had a physical model (CAD data) on a sheet of paper (the coordinate system), in Autocad you would move the model around on the paper; whereas in Revit, you move the paper under the model.

By linking the DWG in first using origin to origin, your levels and standard elevation views will be way out in space.

This should be made a sticky. Virtually every new user will come across this problem at some stage.
Unless the Factory fixed this issue in the latest build - Tatjana was saying something about fixing a DWG import.

BTW, thanks James.This workaround you previously posted helped me a lot when I started my current project.

[kparks140020]

This is really fascinating the internal workings of Revit and AutoCAD. I have come across some "out in space" situations, but have not delayed my work, but concerned about accuracy. This will be a issue for my office as we are converting from ADT to Revit.

[Duncan Lithgow]

I just want to add to this old thread that Revits internal coordinate system is not completely unimportant. It's a good idea to keep your model close to the internal 0,0,0 because as you get further away accuracy falls. Until recently anything more than 32km (20 miles) was simply dropped, now it's accepted but accuracy suffers. (don't ask me why this is true, it's something to do with the way Revit internally calcluates the positions of things relavtive to the 0 point)

[MikeJarosz]

My theory about keeping the model close to the origin involves some computer literacy. When studing computer programming, it comes as a suprise to many people that computer hardware has limited precision to represent numbers, sometimes severely limited. There is also a maximum and minimum value for numbers. Numbers are represented as integers, single precision floating point, double precision, sometimes decimal and maybe some other formats. These decisions are made by the hardware manufacturer, most of the time that means Intel. I won't go into the details, but the truly interested can google "floating point numbers".

When writing a program, the author must choose what level of precision is needed for a number. My theory is that the native code for Revit uses a relatively low precision. That would create a minimum/maximum constraint, just like we see with the 'distance from origin' limitation in Revit. In general, the lower the precision, the faster the code. I have brought up this idea of mine in several forums in the past, but no one has bothered to prove or disprove it. The topic is, after all, boring to a lot of people. But it is critical to the program author.

One good reason why Revit moves the tablecloth instead of the wineglass is the many view specific objects scattered around the model. Would you want to visit every view to make sure that everything stayed in alignment when you move the building? BTW, an old CAD system I used for many years had a snap to 0 0 0. It worked even if off screen. You never had to search for 0 0 0.

The Acad UCS is no picnic either. Once you dig in under the hood, it turns out that Autodesk implemented 3D using a higher level of math than most of us architects are comfortable with. They put the x and y axes in cartesian space, but they made the z axis a vector, throwing the math analysis into linear algebra. In order to manipulate the z coordinate, you need dot products and cross products. If you don't know what those are, you've proved my point. I once had to write a script to draw a line across the face of a 3D polygon in any possible orientation. (think 3D wire frame of a biomorphic building design). I took me a week. I had to buy a linear algebra handbook!

WOW! This thread is really old. It actually contains a post by James Van.

[Duncan Lithgow]

Thanks for that - one day I'll understand better what you're talking about, but I got a bit closer today with the help of google and wikipedia!

Any idea how I can publish the project coordinates of our revit model to the landscape designers DWG without messing up their coordinate system? I had them create a UCS named DNU but when I try to publish coordinates to their dwg I only get one coordinate system to choose. I was expecting to be able to choose which coordinate system to align with ours from Revit. A link to an explanation would be great! I've not much experience with manipulating coordinates and UCS in AutoCAD.