Tag: accuracy

A really small piece of Revit pi

Luke Johnson

This post was prompted by this from Darren Snook.

Firstly, let’s see actual pi to a decent level of precision:
3.1415926535897932384626433

Can Revit handle high precision pi calculations?

As a test, let’s use a circle with a diameter of 2000mm (radius of 1m).  Using the above precision, the area should be equal to pi in m².

Make a new project in Revit, no template, Metric.  Draw a Filled Region with radius of 1000mm.  The Properties Palette reports the area as:

3.142m²

Ok, obviously we need to increase the precision of Area in our Revit project.  Into:

  • Manage – Project Units – Area.  
  • Set Units to square millimetres, Rounding – Custom, Rounding increment:
    0.000000000001 (I couldn’t get Revit to accept any higher – it would just say ‘Invalid Input’, which I take to mean “what you’re asking for is a bit ridiculous”.)

Properties box for the filled region NOW reports:
3141592.854703690900 mm²
3141592.653589793238 (same amount of digits from the definition of pi)

The percentage of inaccuracy between the two?
0.0000000640165425247805 % (the Revit value is slightly larger)

This was a value difference of 0.201113897662 mm²

However, I am prepared to accept the above level of precision, at least in 99.999994% of my projects 🙂

The lesson?  If you are going to use Filled Regions for areas AND you want good precision, make sure you adjust your Project Units accordingly.

In response to:
Revit @ Waterman: Revit Needs to Go Back to School


Revit 2013 and Improved DWG Import Accuracy

Luke Johnson

Steve made a few interesting points today about Revit and DWG (Import) extents.  I previously posted about Revit accuracy and precision here.

However, did you notice this new setting when importing a DWG in Revit 2013?

If you UNtick the box that says Correct lines that are slightly off axis, then Revit 2013 will bring in the DWG as-is.  It won’t try to straighten things up to make the DWG more Revit-friendly.

Obviously, use this with care.  But for me, 9 times out of 10 I want the DWG to come in EXACTLY as it was drawn.

Revit Accuracy and Precision

Luke Johnson

Do you ever struggle with lines jumping around at extreme zoom levels, or weird numbers appearing when you are using many decimal points in your dimensions (like 1000.0000034)?  This may be why:

Revit uses double precision numbers for all calculations (as do all CAD systems), and they are only good to around 14 or 15 digits.
http://forums.augi.com/showpost.php?p=134341&postcount=4

(the above post was by irwin, Revit Founder)

And:

Other programs allow more decimal digits but they do try to fake (unsuccessfully) more precision than they can really provide.
http://forums.augi.com/showpost.php?p=134356&postcount=5