Dimension Rounding

[nsinha73]

Is this a normal behavior? or is it just me.....
An Architect in out office expects an accuracy of 1/256" .....If something is off by that, he throws a fit ;-)))

[twiceroadsfool]

I throw a fit too. Because with the way Revit rounds, you almost HAVE to keep the accuracy on 1/256", or you have to be very careful to only run one string of dimensions down any one side of the model. Because the dimensions WILL bust if theyre set to rounding.

So i keep all of my settings on 1/256". The only reason i go ballistic is when people cant model clean enough to still keep things down to a 1/4 inch or 1/8 inch number. Just because its ACCURATE down to 1/256 doesnt mean you need to screw it up down to 1/256 inch... LOL

[Andy.88917]

I absolutley agree with him! I set all my units to 256ths and expect everything to be accurate to that (or more). I have found when you aren't accurate it throws something off a little here and a little there until there is a big problem down the line. It makes things work out better when things are done accuratly!

[charliep]

1/256" = 0.0039" or 0.099mm (give or take a smidgeon), less than the thickness of a pencil line. Could it be achieved on a drawing board? While Revit may be able to produce that kind of accuracy can it really be done on-site? I worked as a carpenter for best part of 40 years before coming into the world of architecture and producing accuracy of 1/16" was considered to be as good as you will get.

Just had a similar discussion about brick dims, we were 5mm adrift somewhere. The construction manager said "Why worry about it, the brickies will lay the required number of bricks regardless."

[clog boy]

1/256" = 0.0039" or 0.099mm (give or take a smidgeon), less than the thickness of a pencil line. Could it be achieved on a drawing board? While Revit may be able to produce that kind of accuracy can it really be done on-site? I worked as a carpenter for best part of 40 years before coming into the world of architecture and producing accuracy of 1/16" was considered to be as good as you will get.

Just had a similar discussion about brick dims, we were 5mm adrift somewhere. The construction manager said "Why worry about it, the brickies will lay the required number of bricks regardless."

Imagine this. You can draw something perfectly well into position measured from a grid line. But give 0.1mm here, another one there and an area might not appear sufficient in size. Mind you 0.1mm over a 100meter length can sum up to quite an area.

Quantities - can a drawing bord do that for you? No sir! We live in the digital age today. Calculate 5% risk and a 2% margin of profit, and that 0.1mm over the length of ten walls over fourty stories can sum up to the kind of money I'd take any day for free.

You would expect Revit to be dead-on, the kind of program it is. The sad truth is that it tends to 'move' or misclick by 0.1mm and this has annoyed me too to no end. But aside from that, building industry as it is today, the virtual reality has to be 100% perfect. (and given how Revit calculates quantities I'm mostly talking floor plans and sections - construction documents and stuff)

[twiceroadsfool]

1/256" = 0.0039" or 0.099mm (give or take a smidgeon), less than the thickness of a pencil line. Could it be achieved on a drawing board? While Revit may be able to produce that kind of accuracy can it really be done on-site? I worked as a carpenter for best part of 40 years before coming into the world of architecture and producing accuracy of 1/16" was considered to be as good as you will get.

Just had a similar discussion about brick dims, we were 5mm adrift somewhere. The construction manager said "Why worry about it, the brickies will lay the required number of bricks regardless."


The issue at hand is not whether that level of accuracy can be built... It doesnt matter. It actually has nothing to do with the conversation. The issue at hand is that if you set Revit to round at ALL, dimension strings with differing numbers of witness lines will be busted, next to each other.

See this thread for an example:

http://forums.augi.com/showthread.php?t=7333&highlight=revit+dimension+rounding

In fact, itll happen even at 1/256" if youre not careful, becuase you cna still move something 1/512" if youre close enough to it, LOL.

It has nothing to do with constructability, and everything to do with putting out documents that are mathematically sound. Its almost a certainty: Set your dims to round, and break out your calculator, becuase theyre going to be wrong somewhere.

[nsinha73]

Thanks guys....If so many of you agree at this, then I better bring myself down and respect that decision. Now for me, my BIM heart is convinced to be as accurate as possible....

Thanks, this has eased lot of ache :-)

[greg.mcdowell]

It has nothing to do with constructability, and everything to do with putting out documents that are mathematically sound. Its almost a certainty: Set your dims to round, and break out your calculator, becuase theyre going to be wrong somewhere.

And/or don't close your dimension strings! :lol:

[twiceroadsfool]

Over large distances with some items in the string that are basically indeterminate, i agree with you in principle... BUT- Ive literally gone to dimension an 2'-6" wide architectural pier, with a structural column in it, using two strings. One to set the column, and one to give the width of the pier.

Now, i get what your saying, the string for the Column doesnt *need to be* closed, but jesus, why shouldnt i? Its just another check. Leaving a dim opened is great when there is a *space* in a room that can basically be indeterminate, but you know what? The pier is 2'-6", and the column is 8"" from the CL of col. to the inside of the piers wall (back of track. Having the dimension to the back of the OTHER wall shouldnt be a liability, over a setting in the dimensions of the program. Yeah, maybe leaving it out can put the onus on the contractor to not screw up HIS math, instead of OUR math being wrong, but thats not good enough for me.

Im using Revit, and modeling, and BIM to help the entire PROJECT, not just me. If having that dim there is one more step towards getting a better built (less mistakes) project in the final condition, then i want the dim there.

NO ROUNDING DIMENSIONS! LOL, cheers :)

[greg.mcdowell]

You leave an open string for the contractor more than for you.

The overall constructed dimensions will not be exactly as you have them in your drawings (people aren't robots and construction is messy and inaccurate) and the contractor needs to know where to take out the excess (or add back in the under). If everything is tied down then he'll just deal with with it where ever he pleases (or she of course) rather than where you might have prefered.

Surely there's one dimension you can leave off... and if you can't, if you really need it that tight (in a lab for example), you better let the contractor understand your expectations so they can add the extra work necessary to achieve that level of accuracy into their bid and you might also want to revise your specs to pull or change the allowable dimensional deviations.

And, not to pick on semantics, but if you're rounding to 1/256" you're still rounding. If you've got angles or curves in your project that's not going to be good enough. Pi after all is an irrational number.

Listen to the builders. They know what they need more than we do. Architect as Master Builder hasn't been substantially true in decades.

[twiceroadsfool]

Surely there's one dimension you can leave off... and if you can't, if you really need it that tight (in a lab for example), you better let the contractor understand your expectations so they can add the extra work necessary to achieve that level of accuracy into their bid and you might also want to revise your specs to pull or change the allowable dimensional deviations.

And, not to pick on semantics, but if you're rounding to 1/256" you're still rounding. If you've got angles or curves in your project that's not going to be good enough. Pi after all is an irrational number.

Listen to the builders. They know what they need more than we do. Architect as Master Builder hasn't been substantially true in decades.


In a room, or across a building, i agree with you. But i reiterate: Over a 2'-6" wide object, i shouldnt have to fear closing a dim string.

And really, the issue at hand isnt even that. With ANY of the units or objects set to round in Revit, you get issues. Area scheduled dont total correctly, dimensions break, etc. The safest option you have is to leave them at 1/256".

And im not saying that that solution ISNT rounding, its just the safest option we have. If it were up to me, i wouldnt have them round at all. You can still have your drawings to the quarter inch, half inch, or whole if you really want. Just build the model correctly. <shrug>

And i dont mean to climb on a soap box, but just because "it hasnt been true for decades" isnt good enough for me. We owe it to the world to do better than the industry has for the last few decades. To quote a professor of mine from college: "If you want to lick the lollypop of mediocrity all the time, its the only taste youll have."

[patricks]

Our dimension types are set to display an accuracy of 1/32". But I will personally throw a fit if I see 256's pop up on temporary dimensions. I try to not have 1/8" or 1/4" or even 1/2" dimensions but sometimes that's unavoidable. I won't stand for any 1/16" or smaller dimensions. Keeping the dimensions set to 1/32" will make those discrepancies show up easier so we can fix them.

I can't stand when people "loosely" model, that is, without using snaps, or without typing in exact measurements. That's a personal pet peeve of mine, and I'm not even the boss around here. :p

[twiceroadsfool]

I won't stand for any 1/16" or smaller dimensions.

I can't stand when people "loosely" model, that is, without using snaps, or without typing in exact measurements. That's a personal pet peeve of mine, and I'm not even the boss around here. :p

I dont tolerate seeing less than an 1/8th, unless its on something very complex and necessary. Your second point, is the point im trying to make.

I dont want to see anything smaller than an 1/8th on a dim, period. BUT, i want that achieved, while the dims are set to 1/256 so i dont have issues later.

It sounds worse than it is. If you START the project that way, its never an issue.

[patricks]

We normally don't have any problems with dimensions set to 1/32". However I'm working on something that was recently started by a very novice user (summer help) and I found several discrepancies. So I changed the dimensions to 1/256" to make sure I found and fixed everything.

[twiceroadsfool]

Exactly. :)

[dbaldacchino]

I cannot tolerate inaccurate work. It's just sloppy and leads to document clutter. I wholeheartedly agree with Greg about leaving certain dimensions out. Just tell the contractor what you really care about the most and it'll make life easier for everyone. I also like to keep the rounding to 1/256 to increase the "sloppyness index"! if you mean to have a dimension of 2'-6", then don't model a 2'-5 253/256" object! Being minimalistic in your drawings helps with avoiding too many questions in construction and making your drawings more clear. Sometimes relationships between objects is more imporant than overall running dimensions. For example something I saw yesterday that made me flip was a storefont. We had a column fur-out with some frosted glass to obscure the column (was not centered due to existing conditions) and the frame butted up on both sides against the fur out. Because there was a bust for some reason in the fur-out, they maintained the frame in the same spot and buried the mullions flush with the wall. Needless to say it looks horrible and WILL be fixed.

If you are working in the field with a contractor that gives you up to 1/4" accuracy, please pass on their info so I can recommend them to our clients ;) Here's the state of some construction companies....we have the building modeled in Revit and output as typical dimensioned 2D drawings. You know how glass is ordered? No, Robots don't take my Revit dimensions and fabricate a panel, but "Joe the Glazier" gets some sciccors and some cardboard and "accurately" creates templates to send in for fabrication. BIM anyone?

[twiceroadsfool]

I dont disagree with you guys on the accuracy in the field, but its just a cop out, IMHO.

Dave, im almost spot on with everything you said, but you know what? Gyp comes in 5/8 pieces. 1/8" accuracy happens. I absolutely have no tolerance for the "heres the level of accuracy we build to, if you dont like it tough ****" attitude. News Flash: Dimensional tolerances and quality control at final assembly is the reason Toyota builds a better car and isnt bankrupt, like detroit is about to be. LOL... I mean, i get the argument. Saws cut material, things arent all done by laser, yadda yadda.

But really, the construction in the field has LITTLE to do with this thread. Heck, the firm im at now likes everything in whole INCHES. But guess what? You make REVIT round those inches, youre drawings are going to be WRONG somewhere. I'll bet large bills.

You put the accuracy at 1/256 and YOU make it whole inches, and the chances are much less likely that youll be wrong. Case closed. :)

[dbaldacchino]

No it's not closed :p haha We're talking the same points from different directions and it really boils down to the difference between the Digital environment we design and document in VS. the Analog one in the built environment. In the grand scheme of things, half an inch here and there is not even perceivable to the occupant on large distances. if we were getting all components digitally fabricated based on our models to the jobsite, than accuracy takes on a whole other level, although there is still a level of "fudge" that is possible in putting things together. We don't build "space stations" on earth in most cases! As long as a measuring tape is being used, an inch to a half inch is probably the most you can aspire for. I'd rather build accurate models and have the contractor mess up a bit, instead of the other way around.

[greg.mcdowell]

You know, you can only be accurate to half the sensitivity of your measuring device... and the width of your pencil lead too I suppose.

Or so my 8th grade physics teacher told me.

[Steve_Stafford]

Dimensions are a "monologue", not just something we add to drawings because we are supposed to. They are supposed to tell our story, the one we actually intend to tell. They shouldn't cause the reader to guess or doubt us.

[tc3dcad60731]

I do not show any dimension smaller than 1/8" when detailing unless it is a fine detail object that is to be custom made. What I mean is that for wall, window, door locations etc that 1/8" is the smallest dimension that the builders want to see. You start showing 1/16" and 1/32" and comments will fly that are not nice for these forums. Having said that I will model exact and will not tolerate something being off. If you want 2'-6" then make it 2'-6" is my opinion.

[jsnyder.68308]

We are actually talking about two different ideas here: accuracy and precision. If people model accurately, precision takes care of itself. If the model is constructed accurately, then 256ths of inches won't really matter.
Dimensioning a project is a bit of a lost art IMO. CAD and now BIM are so precise that people just take for granted that putting more dimensions on a drawing will make it easier to build. They close dimension strings and drink the coolaid without actually checking whether lower strings add up to the overall dimensions. When projects were hand drafted, you actually had to THINK about your dimensioning system and how it tied back to known references contractors could use as the project was built. Computers are good at adding numbers up, but they are not a subsitute for the designer's responsibility to communicate design intent clearly and rationally.