Friday, 20 June 2014

The space between the bit 0 and 1

Coordinates again and again and again and probably more to come

Revit, as you might know, increased it's area of where you may model your project. It used to be a circle with a radius of 2 miles = ( 3.218688 kilometer) and they scaled it up to 20 miles = 32.18688 in Revit 2011.

I opened a file recently we had received and I took a look at their survey base point and project basepoint. First thing I noticed was that the survey point was unclipped... That's odd I never unclip it, because it needs to stay at 0.0.0 (at least for people who work with a coordinate system like the Dutch rd system. The next thing I thought was odd are the values of the survey point.
Normally I don't use so many decimals but something in this file triggered me to set the units this way.
I don't think anyone would on purpose enter those survey coordinates. So what happened here? As there are some mathematical rules every cad program has to abide to I was getting suspicious. Could it be that we are dealing with rounding? 

To test this theory I created this spectacular drawing in Autocad: a square, lower left corner at coordinate 100km, 400km and each side is 100 meter. 

I linked this drawing into a Revit project. Next I moved and rotate it in the right position. The culprit here is the rotation... I rotated the drawing 60.389 degrees clockwise. So when the drawing was in the right position I acquired it's coordinates. 
Notice the N/S value! I don't think that Revit is here to blame this is mostly a mathematical issue.

My first advise don't rotate dwg files to align it to be able to draw orthogonally so don't use project north and true north. Use a rotated scope box as I have explained in this blogpost:

My second advise be careful with acquiring coordinates from a dwg.

My third advise is to manually create a point in an Autocad file. 
Make sure that you move this point to an integer xyz value (rounded) what is an integer
Use these values an insert them into your project base point. Leave the rotation to the scope box.

If you do start to use the rotation trick, stick to one methode. I ran into a file where they had rounded the rotation but started to pick lines from autocad that had apparently the same rotation. The revit rotation was 60.389 and the autocad rotation was something like 60.3887274766673 You might think that is trying to be overly precise. But this has the following effect: The same point 100 meter away from the project basepoint is almost 1/2 a millimetre apart between these rotations. At a thousand meter is't 4.5 millimetre apart. That's still no problem if everyone uses the same rotation. But what if someone uses the autocad file and the other one uses the rounded rotation in Revit. This can seriously screw up you Navisworks clashdetection!

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