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Thread: Revit Accuracy

  1. #1
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    Default Revit Accuracy

    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.
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    All AUGI, all the time J. Grouchy's Avatar
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    Default Re: Revit Accuracy

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

    Maybe you can post the RVT file?

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    Super Moderator beegee's Avatar
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    Default Re: Revit Accuracy

    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 ?
    Bruce Gow.

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    Revit Founder irwin's Avatar
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    Default Re: Revit Accuracy

    Quote Originally Posted by 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.
    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.

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    Revit Founder LRaiz's Avatar
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    Default Re: Revit Accuracy

    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.
    Leonid Raiz

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    Certified AUGI Addict patricks's Avatar
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    Default Re: Revit Accuracy

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

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    Default Re: Revit Accuracy

    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.

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    I could stop if I wanted to Damo's Avatar
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    Default Re: Revit Accuracy

    Quote Originally Posted by irwin
    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' ".
    Last edited by Scott Davis; 2005-06-04 at 05:01 PM.
    Damian
    I know it can be done, I just don't know how to do it yet

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    Revit Arch. Wishlist Mgr. Wes Macaulay's Avatar
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    Default Re: Revit Accuracy

    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>
    Wes Macaulay LEED AP
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    Autodesk Revit Developer
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    Default Re: Revit Accuracy

    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.

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