Revit Accuracy

[PaulB]

Can anyone tell me why I can't get an even distance of 1000 between levels R.L. 0.00 and Level 1.

No matter how I try and move, offset or change dimension I can not get rid of the trailing decimal figure.

I know this is such a minor point but I thought Revit was accurate and this sort of thing justs bugs the hell out of me - if it happened with angles, over a long distance you could be a fair way out.

[J. Grouchy]

Perhaps you need to change the unit accuracy within the level family?

Maybe you can post the RVT file?

[beegee]

Paul,

Are you attempting to move one of the level lines by this minute amount ?

Instead, have you tried deleting the offending level and using offset to place a new line ?

[irwin]

Can anyone tell me why I can't get an even distance of 1000 between levels R.L. 0.00 and Level 1.

No matter how I try and move, offset or change dimension I can not get rid of the trailing decimal figure.

I know this is such a minor point but I thought Revit was accurate and this sort of thing justs bugs the hell out of me - if it happened with angles, over a long distance you could be a fair way out.
The difference you are seeing is approximately one part in 10 to the 14th power -- it is around one ten thousandth of the diameter of a hydrogen atom. Revit uses double precision numbers for all calculations (as do all CAD systems), and they are only good to around 14 or 15 digits.

Regarding your concern about angles, if an error of this magnitude were introduced into the angle between two walls, and the length of the walls was the distance from the Earth to the Sun, then the error in the distance between the ends of the walls would be around 1 mm.

[LRaiz]

As Irwin pointed out programs that do numerical computations in double precision are limited to about 15 significant numbers of decimal precision. This limitation applies to all programs not just Computer Aided Design.

It is interesting to observe how different programs deal with inquisitive users who might be alarmed by the apparent accuracy limitations when numbers are printed with many significant digits. Revit does not try to do anything too sophisticated but it does limit the number of printable digits after the decimal point to 12.

Other programs allow more decimal digits but they do try to fake (unsuccessfully) more precision than they can really provide. Consider Excel. You can display a number with many significant digits and if you ask for 20 digits after the point then you will see that last digits are zeros. This is fine for a decimal number that user typed. However use a formula like "=10/3" and you will immediately see that instead of 3.33333333333333333333 Excel is displaying 3.33333333333333000000; it went beyond the limits of precision but added zeros to the end of display in an attempt to hide accuracy limitations.

[patricks]

so all this to say, just reduce the accuracy display of your dimensions and don't worry about it. :)

[PaulB]

I can't believe I got the two founders of Revit to answer my question, that's what I call service.

With the answers given, I don't know what I was worried about. Just goes to show that there is always a reason or an explanation for most things.

Did I say "It bugs the hell of of me" - it was just a minor irritation that seems to have gone away. I'll make sure I keep my units of accuracy down and stop being an "Inquisitive User".

Thanks for the replies, reading them was the best laugh I've had for a while even if it did make me feel silly for casting doubts on Revit's Accuracy.

[Damo]

The difference you are seeing is approximately one part in 10 to the 14th power -- it is around one ten thousandth of the diameter of a hydrogen atom.An Irish builder said to me once after we changed from Imperial to Metric "Never mind about these millimeters things, on this site our setting-out is 'Spot-On' ".

[Wes Macaulay]

I've had many people ask me how accurate Revit is and whether it is as accurate as AutoCAD. Thanks very much for the clarification, Irwin and Leonid! <applause>

[FK]

Revit, AutoCAD and very many other applications use the same IEEE 754 double-precision floating point arithmetic that's natively implemented in the CPU, so precision is exactly the same.

[gravelin]

just a question ?

So, with such an accuracy, why do we have difficulties to model very small things ( for exemple ; cylinders smaller than 1mm diameter in shading mode)

[Roger Evans]

Just a question from me ~ Who checks this accuracy, how & what with?

[beegee]

Revit is set up to only allow a line or object larger than approx 0.2 mm in order to prevent inadvertent or unintentional movement or placement of objects or linework. You can however input dimensions for objects that are less than this over-ride



This over-ride feature is quite different from reporting the length of a line / object however.



As far as the original question is concerned, and realizing that the program does not accurately report this degree of accuracy,( and who would want it ) I tried to reproduce the problem, but I have no trouble getting lots and lots of zeros behind the decimal, for levels.

[rod.74246]

If it makes you feel any better i have seen the same problem with ArchiCAD, AutoCAD and Microstation so its an inherent computer issue. But in reality the issue isn't that big a deal in my mind. If you are getting into points of a millimeter it is a worry. No matter what you dimenion it as, the builder will only be within 5 to 10 mm of it anyway. Constrution tolerances are nowhere near the accuracy we are talking about here.

[Roger Evans]

I was really after the name of manufacturer of the tape measure

[MikeJarosz]

Many CAD users are not computer scientists, and understandably don't know what's goin' on inside. Shocking truth: computers are inherently inaccurate! Visit this site:

http://www.lahey.com/float.htm

The arithmetic might be a little tough, but the concepts are essential to anyone who wants to understand why they can't get an exact number.

[PaulB]

Beegee,

My problem with the levels that I posted only happens when I use a certain template file. The template is a custom one supplied by someone else and already has the levels set-up. I have deleted the levels and put in new ones but still get the problem. I have even picked a level and offset from that level by 1m and still get the trailing decimal point.

It seems strange that it is doing this and I wouldn't have picked it up but the template was also set to display the no. of decimal points that I had on the pdfs.

It don't believe that I have had this problem when using the standard templates.

In any case it is not that I ever work to those tolerances, obviously, but as this was how it appeared on the screen and then I couldn't correct it I was getting frustrated that no matter what I did I couldn't lose those decimal points.

After the explanation I felt a whole lot better, not that I was about to change my accuracy, but that it wasn't any deficiency on Revit's part.

In normal practice I would have my settings on 2 decimal places just as a check as I go along placing objects and my dimensions are always whole numbers unless working on some large scale intricate details.

Although in reality the contractors are not going to work to anything less than 5mm, they will be the first one to tell you that you have made a mistake in that your dimensions don't add-up (due to rounding).

Anyway back to the drawing board.

Regards,

Paul