Thursday, 4 August 2011

Beware of the Dimensional Rounding


This is an interesting issue i have come across on a number of projects now and therefore one i thought worth posting about. Its symptoms include not being able to dimension elements horizontally within sections. For example, your section cuts perpendicular through a wall however for some reason you cant dim to the wall edges.
To make this worse, you can see your Gridlines in section but if you try and dimension to them you get the following error message:

Crash

As the message suggests, this can be pretty serious.

This pretty much ALWAYS caused by the section not being perfectly perpendicular to the gridline. What is odd about this is the fact that Revit actually displays your gridlines even if this is true.. How can it if the view is not perpendicular? My guess is that Revit has some element of tolerance when it comes to this. The problem seems to come because the dim tools don't appear to have ANY tolerance.

So lets check the theory:

Within the file above the first thing i looked at what the relationships of the grids to each other. I threw a few dims on and it all appeared fine.

Check 01

The thing is, i have seen this a few times before so i knew what was happening here. The key is to be aware of the decimal rounding which the dim styles is applying to the angular dims in the image above.
To check I went into the dim settings via the route highlighted below:


Dimstyle -1

Initially the dim style was set to use the project settings. This was rounding to 2 decimal places.
To check the true angles of the dims i set the dim style to Degrees, Minutes and Seconds.

The results were as below:

Check 02

Oh dear..

This is the equivalent of the being 0.0001 degree out.. Seems a bit harsh doesn't it but the fact is it is incorrect.
Now the fact that the grids are not perfect is not strictly the problem here but it is the cause. In truth the crash is caused by the fact that the section is not at exactly 90 Degrees to the intersecting gridline.

Of course the correct thing to do here would be to check and adjust the gridlines and all the other geometry within the model but this is not always viable. If this error is caught early enough correcting the grids may be viable. However as is more often than not this error is not found until weeks into a project.

I guess the lesson learnt from this would be to use Degree, Minutes and Seconds as a basis for angular dims, at least for the initial project setup.

I do have a work around to ensure that sections are perfectly perpendicular to a gridline. I will try and cover this in my next post.

2 comments:

  1. I have super-precise dim styles set up for aligned dimensions and angular dimensions. I don't use deg/min/sec but I use Decimal degrees with like 6 decimal points or something.

    Dirty hack to bypass the grid issue:
    Workshare
    Put problem grid line on invisible workset
    Make new grid line that is correctly aligned.

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  2. Yeah, I think the super precise dim style is the way to go, but I'm all for your Dirty hack fix as well.. Thanks

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