revit and accuracy vs AutoCAD

[ray salmon]

recently I had a conversation with an architect of a firm I do not work for who was curious about Revit.. but was surprized with the statement
we will stay with 2d AutoCAD because it is very accurate... Revit would be good for spec models etc... I don't find this statement to have any basis... but apparently is might be a common view... I did not know a good answer to this... except personally I findRevit greatly reduces error prone issues due to its linked data... but the response was.. well maybe it does but 2d is just very precise.

anyway just how would one respond to this without getting into some big contention.... architects as you know can get their thinking set in concrete too...

ray

[Scott D Davis]

Accuracy is not a product of the software but of the user that inputs the data. I can draw 2 lines in CAD that are 6 feet apart, dimension them, then override the dimension to read 6". Not very accurate.

[cliff collins]

In Revit, like other software, you can set dimension tolereances.

Set tolerance to 1/256", which is industry standard. Model all walls, etc. at this tolerance.
The key is to actually model at 1/256" tolerance--and make walls lenghts to even feet, inches, and reasonable fractions of inches.

Then exports to cadd from Revit will be at correct 1/256" tolerance.

If you model w/ tolerance at 1/8", exports to cadd will not be accurate--which I suspect
is why you got some negative feedback about Revit being "innacurrate".

Cheers......

[ray salmon]

ok, i did not know about about the tolerance but did notice it was set to that when I did my first Revit project...

however, it does puzzle me why would someone want to spend all this up front time on just a model which would amount to all the floor plans and elevations in Revit then not use that to finish the CD set. exporting out to a 2d format would really add unnessary time. now you got torture your brain with crossections, interior elevations etc which are already done for you in Revit...

I guess its just hard to get people to jump the fence...

r

[DaveP]

We did a project about a year ago that had one portion with orthogonal grids, and another section that was at a 30 degree angle to it. The project had gone through SD in AutoCAD, so we "just" Linked the grid DWG in and traced over it.

BIG MISTAKE!

It turned out that the AutoCAD grid lines varied by a few thousandths or a hundredths of a degree. AutoCAD was fine with that because you could cheat the dimension string. Revit would not let us dimension the grids because they weren't parallel. We had to go through the entire building and add angular dimensions to every grid to move them around so they were at the correct angle. A hundredth of a degree may not seem like much, but by the time you go out 400 or 500 feet, you can be several inches off.

I also used to believe that AutoCAD was super-accurate, but not after that experience!

[Mike Sealander]

There are some basic accuracy issues with Revit.
First, many of us have experienced object drift, which is an inexplicable tendency for Revit objects to occassionally drift. This may well be pilot error, but it happens nevertheless.
Another is Revit's strong desire to change nearly orthogonal objects to orthogonal objects. This can be frustrating.
Another issue is in trying to move something an eensy weensy bit. Revit sometimes responds with some gibberish about screen resolution.
I have no doubt that all 32-bit software is equally precise, but the dumbness of ACAD makes precision much easier to maintain.

[cdetore]

One funny thing about the accuracy.

Zoom in (way, way in) and draw a line 1/32" long. If you slowly drag the "pencil" up or down to 1/32" there are actually a number of different length line segments that can be drawn which will dimension at 1/32". You can split these lines into smaller segments, you just can't draw them that way. If they are scaled their size difference scales as well. Good to know if you scale information from DWG files in your model.

Typically you don't need to model to that kind of accuracy but it's interesting to know the issue exists.

CD

[sfaust]

I saw a demonstration about acad one time how you could zoom from galaxy level to the text on the lunar lander as a demonstration of the precision level. Great, but I really don't need that. Revit may not allow that, but it links my elevations and sections and callouts and everthing and does a hundred other things that make my job easier, faster, and better.

Would you rather measure an existing building with a tape measure marked by a laser in 1/512" increments, or with a laser scanner accurate to 1/8"?

Been using Revit for 6 years or so now, and I've never had an issue with acuracy or precision.

[dgreen.49364]

Been using Revit for 5 years. I've never seen anything "drift" that wasn't drifted by the user. Give me the accuracy of Revit over any 2d program out there.

[hand471037]

Everything (and I do mean everything) on our site was modeled in Revit and then cut out with a CNC machine directly from those Revit models. Sometimes the vectors get more added to them after export from Revit, but all the important joints and connections are 100% straight from Revit without any editing. We are now starting to scale up to do whole houses this way.

If there were terrible inaccuracies I certainly would have noticed them by now.