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Units and rounding: downsides of excessive precision?


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I tend to draw with millimetres as my unit, and with "decimal precision" set to 1.

 

That's because, for architectural work you don't generally need to be more precise than a millimetre.

 

However - there's one issue I run into repeatedly which is (at least in the UK) a standard brick is 102.5mm wide. Of course, no brick is actually exactly 102.5mm wide, but this is the dimension used for various co-ordinating dimensions, and the 0.5mm will add up over multiple bricks.

 

It comes up, for example where I am drawing a cavity wall which is 102.5mm brick, 100mm air gap, 102.5mm brick. The overall width of the wall should therefore be 305mm. But when I make that wall style, because I can't use half millimetres I have to make it as 103+99+103, or 102+101+102 or 102+100+103 which is annoying, especially when I come to dimension things.

 

So - my question is, is there any downside to me changing to drawing with "decimal precision" set to 0.1 instead? Somehow I worry that I'm going to start finding all sorts of things that measure 99.9mm and so on, but I'm not sure this is rational. When I come to dimension things, I can set the dimension precision to "1" (because I don't want to produce working drawings with dimensions like 104.6mm all over them) and then for any dimensions that I do want to show as eg. "102.5" I can change the precision individually. Is that going to work?

 

Final question: if I change an already existing drawing from precision of 1 to precision of 0.1, is anything bad going to happen? Or does nothing actually change 'behind the scenes" and all that changes is what is shown to me?

 

 

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I have come up against the same issues. I always used to set decimal precision to 1mm then occasionally would need 0.5 of a millimetre (12.5mm plasterboard for example) + would change to precision accordingly, only to find that existing objects I assumed were sized to whole mm had in fact dims to fractions of a mm. Then I saw @zoomer's post below + have now set precision to 6 digits + works well:

 

 

(Thank you @zoomer)

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5 minutes ago, Tom W. said:

I have come up against the same issues. I always used to set decimal precision to 1mm then occasionally would need 0.5 of a millimetre (12.5mm plasterboard for example) + would change to precision accordingly, only to find that existing objects I assumed were sized to whole mm had in fact dims to fractions of a mm. Then I saw @zoomer's post below + have now set precision to 6 digits + works well:

 

 

(Thank you @zoomer)

 

Ah, thanks.

 

That other thread outlines the same worries that I have!

 

I've never fully understood what actually happens behind the scenes with accuracy and so on... probably some maths beyond my comprehension.

 

For example if I draw 2 lines at right angles, each length 1 unit and then a diagonal between them, I know that's going to be a number with an infinite number of decimals, so somewhere it must get rounded up or down, but if I then add lots of those diagonals together, don't all those rounding errors add up and cause problems.

 

Probably best just not to worry about it.

 

Going to try @zoomer's method on a new drawing and see if it presents any problems.

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Although... I see that actually I can use these settings to draw to the nearest half millimetre:

 

(Changing "decimal rounding base")

 

1288794693_Screenshot2021-05-24at12_07_53.thumb.jpg.2a7cb058c82a6e692285b5668856e460.jpg

 

Do the settings above, in theory, make it impossible for me to draw a line that isn't in a 0.5mm increment (unless I snap to something else)? If so, then it seems a good way of avoiding accidentally not-in-0.5mm-increment dimensions.

 

I guess my question is still whether there are any downsides of setting the precision higher.

 

If there aren't then it seems to make sense to do what zoomer suggests.

 

 

Edited by line-weight
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Actually, fiddling around with things, I see that in fact even with precision set to 1, if I draw something 10.5mm long (by typing 10.5 into the OIP) then although the OIP tells me its rounded to 11mm, in reality it *is* 10.5mm because if I duplicate it, snap the two things together and measure the total length, it is 21 not 22.

 

Same applies with a wall style for example - I set components at x.5 width and it looks like they've all been rounded up or down, but when I check the overall width of the wall it's correct.

 

This now makes me wonder why there's an option to display less precision than is available, because all it does is make small errors invisible to the user.

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1.

I am not sure if using Millimeters is more accurate,

but more limited in usable size, than Meters in VW.

 

2.

Precission Setting is only what you want to see,

not more precise or more tedious to calculate.

VW will calculate with all digits available.

a)

you can nevertheless enter more digits than shown

and these will be used internally - just that you can't see

them after input anymore.

b)

It is just that it looks nicer if you will see only

the reduced resolution that you need.

On the other hand you can't see when you are a bit off

when snapping and such things. Or if you limit angle unit

display, you may no more see that your Wall is off 0.225798°

c)

As I was always punished by CADs from the beginning, with

any imperfect drawing, like Boolean Operations failing and

such things, I always show much more digits than needed,

just to see that I am still precise.

That is what I proposed in the other thread.

But if I would use Dimensions, I would set precission display

to only the digits needed.

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Thanks @zoomer, that all makes sense.

 

On using mm vs m - I prefer to use mm just because I "think" in mm. I think this is a UK convention, compared to some continental European countries where dimensions are shown in m or even cm.

 

I will always think of something as 200mm long rather than 20cm long, and 1450mm long rather than 1.45m long.

 

I have wondered the same as you though; does using mm instead of m mean that drawings start to show problems from getting too large (objects far away from origin) sooner.

 

Currently my biggest model is about 500m in its largest dimension but drawn in mm and it seems to be ok.

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I am so terribly used to think in meters for buildings.

I can reasonably adapt my brain to centimeters.

(I think no one uses Centimeters by default - beside C4D

 

 

But I still don't get around millimeters,

as soon as we are dealing with more than single digit of meters.

 

But I have to adapt to your UK standard 😉 millimeters sooner

or later. As when I will finally get used to it I would have it so much easier.

(German keyboard's NumBlock comma, Apps Millimeter defaults, ..)

Edited by zoomer
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But likewise @line-weight do you really think of a building as 12,000mm long or 6,000mm high for example, and not 12m + 6m? I use mm mainly but start switching to metres once we’re into 4 digits + definitely once we’re in 5 digits. But then I also use feet + inches a lot – ¾” ply, 8x4 boards, 4x2 timber, etc – I guess it’s just a matter of the right unit for the circumstances. With VW I am set predominantly to mm but I do have a dim standard that has metres as the dual dimension so for large floor plans I can dimension in metres then switch back to mm for the rest. 

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26 minutes ago, line-weight said:

when you think of the thickness of say plywood, it's 0,018m in your head?

 

 

No,

for components I may think in centimeters, but for room or building grid

dimensions I think in meters.

 

Meters for me means always something like typing "2, + 135 m" =  2,135 m

for Centimeters like in components I switch by typing in ", + 125" = 12,5 cm

for Millimeters I switch to a prefix of ", + 00 + 75" = 7,5 mm

That's in muscle memory.

 

Millimeter does never need, but also not offer that separation between units

by such a "prefix" switch.

 

so thinking about 7,5 mm is ok,

100 mm for 10 cm is an abstraction level more but ok.

Even 7000 mm for 7 m would be ok.

But for my brain ends with something like 32,08 m - 32080 somehow doesn't fit.

 

I think that is an advantage of our metric system that we can easily switch from

Nanometers to Kilometers, according to purpose.

 

 

26 minutes ago, line-weight said:

And then half a metre is 0,5m or is it 0,500m?

 

Usually I would get a demand of 50 cm  ....

and would instantly type ,5 (m)

 

But if I would get Arch Plans in scales like 1:100 or 1:500,

there would be written a 0,50 (m) in dimensions

Edited by zoomer
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1 hour ago, Tom W. said:

But likewise @line-weight do you really think of a building as 12,000mm long or 6,000mm high for example, and not 12m + 6m? I use mm mainly but start switching to metres once we’re into 4 digits + definitely once we’re in 5 digits. But then I also use feet + inches a lot – ¾” ply, 8x4 boards, 4x2 timber, etc – I guess it’s just a matter of the right unit for the circumstances. With VW I am set predominantly to mm but I do have a dim standard that has metres as the dual dimension so for large floor plans I can dimension in metres then switch back to mm for the rest. 

 

In my head I'd be thinking 6200mm rather than 6.2m, at least while I'm drawing.

In my head it's "six two hundred" rather than "six thousand, 2 hundred".

 

Of course, we can write 6,200mm or 10,450mm which can make it more easily readable but runs the danger of causing confusion when read by a continental type who reads the comma as a decimal point, so I have got out of the habit of writing the comma.

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20 minutes ago, line-weight said:

In my head I'd be thinking 6200mm rather than 6.2m, at least while I'm drawing.

In my head it's "six two hundred" rather than "six thousand, 2 hundred".

 

 

Maybe that will help me to finally do the switch 🙂

 

But it is still not as logically as thinking about 6 m comma/dot 20 cm

Plus the mnemonic for Millimeter, which needs a 00 behind comma.

(which is only needed for values smaller than 10 mm)

Edited by zoomer
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That is one of the great Apple User-like behaviors of VW. (less is more)

You just set the Units you like to work with.

 

Microstation was much more complicated but also just had a "Working Units" setting.

But Autocad compatible Bricscad has preferred INSUNITS = File Units on top of

the Unit-less DWG internal Units.

You can choose the Units that your DYNDIMS etc. (number fields) will show,

or even dynamically depending on value which is even more distracting,

but for all your inputs it will only work in INSUNITS ! nevertheless (WTF)

 

So you can start by your Units of choice by a Template File,

everything imported will be adapted in size to fit Units.

But if you ever want to switch units later - it would basically need to scale

everything manually.

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I've seen a couple of different conventions here in Canadian construction documents where we are dealing with a blend of materials arriving from Europe, Asia and the USA. The one that seems to keep projects operating smoothly with less errors is straight up mm. This avoids the pitfalls of the American Imperial system of nominal verses actual size of products especially in tightly regulated areas of construction like exit stairs, safety rails, and handicap provisions.

In this convention the accuracy is always at least one decimal place greater than the dimension number presented on the documents.

Information which would be subject to the jurisdiction of a surveyor, lot lines, contour elevations and building heights are presented in metres as thus 10.56M, almost never showing accuracy to the millimeter. Note we use a period separate whole units from fractions of units (decimals) and commas to delineate every third order of whole units in banking. In construction documents our comma is often not used even when displaying 10000 which would be ten metres outside of a site survey document.

All other dimensions we dimension in millimeters.

But in the object information palette you will see and enter the value to the nearest tenth of a millimetre where applicable. This helps reduce rounding errors where the sub-dimensions do not add up to the overall dimension.

In presenting building elevations I've seen two systems, the surveyor's method 7.30M and where more accuracy is required 7 305 next to an elevation target mark (more European like). Implicit when no measurement indicator is present, millimeters are assumed.

So it the case of stairwells we do see cases where dimension between faces of framing is 1125 but a clear dimension would show 1099 because the detailer rounded up the 1/2 inch gypsum board to 13mm instead of entering it in at 12.7mm when producing the wall assembly.

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21 minutes ago, zoomer said:

 

 

Maybe that will help me to finally do the switch 🙂

 

But it is still not as logically as thinking about 6 m comma/dot 20 cm

Plus the mnemonic for Millimeter, which needs a 00 behind comma.

(which is only needed for values smaller than 10 mm)

 

But it seems to me that it is more likely that a dimension will be 6202mm or 6204mm or 6208mm than 6200mm. And then what do you say, 6m comma 20cm-and-2-mm?

 

I just say, six, two hundred & 2. Or six, 2-0-2.

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1 minute ago, LarryO said:

This avoids the pitfalls of the American Imperial system of nominal verses actual size of products

 

We still have that here in the UK, you'll commonly ask the builder's yard for a 2" by 4" but what shows up is not 2" by 4". It might also be sold as "nominal 50mm x 100mm" or something like that, and again what shows up won't be that. Unfortunately it can be all sorts of things, like 45mm x 95mm or 45mm x 90mm or 47mm x 97mm and so on, and I'm often not 100% sure what's best to actually draw. In some cases it's not really critical and in some it is.

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41 minutes ago, line-weight said:

But it seems to me that it is more likely that a dimension will be 6202mm or 6204mm or 6208mm than 6200mm. And then what do you say, 6m comma 20cm-and-2-mm?

 

 

 In this case everything is breaking down for me ... 🙂

 

It may be more like more like a 6 comma 20 (cm) and following millimeter digits ...

 

 

42 minutes ago, LarryO said:

The one that seems to keep projects operating smoothly with less errors is straight up mm. This avoids the pitfalls of the American Imperial system of nominal verses actual size of products

 

As if fractions wouldn't be abstract and complicated enough.

But the real horror comes with Volume units.

(Or using manufacturing processes, which change and develop

further over time, as Units like "gauge" for wires)

 

 

Our Dimension's Units are defined according to realistic on site

precision, usually half of a centimeter. Like for Masonry.

Although any Carpenter will laugh at you for any Precision under

a Centimeter, which his Wood will work alone for humidity or

temperature.

Therefore, for residential buildings we have Standards like 0,88(5) m

(With a smaller + elevated 5 at the end) for Window Openings,

coming from Brick Sizes.

(n*Brick net +0, 1 or 2 mortar fugues)

 

But especially Steel Works, with its higher precision, was always in

Millimeter.

Edited by zoomer
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More accuracy never hurts it may only slow down the process due to the research required to find out the truth of the product size.

A good example here that can cost sub-contractors $ is when imperial measures are use to size handrails in stairwells and other exits. I've seen 1 1/2" in the description of the required size. But there is no such product with a true outside diameter of 1.5 inches [38mm]. The closest size would be 42mm, nominally referred to as 1 1/4" and if the builder followed the document's description and supplied 1 1/2" nominal tubing or pipe the actual size would be 1.9 inches [48mm] which would be rejected by most building code inspectors as exceeding our building code specifications, unless it is on federal land where they often get away with ignoring municipal and provincial jurisdictions and their regulations. But really whose fault is it; my assumption is it's the architect/designer for not being accurate in their documents. Either way its the final owner who pays the price of these omissions and errors, because an observant contractor when bidding will add overhead into a project just in case he guesses wrong rather than submitting a time consuming request for information.

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In this area the mills produce planed framing lumber in 38x89, 38x140, 38x190.5 (rounded to the nearest 0.5mm), but the tolerance for straightness and true leaves something to be desired and with the almost non-existence of 200 plus year old-growth in the market place these days the engineered products are preferable but again with their own sizing for small beams and joists.

And steel studs are another matter altogether they can be 38 or 41 x63.5, x76, x89, x92, x102, x140, x152 depending upon the brand.

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11 hours ago, Tom W. said:

I always draw 4"x2" as 95x45. It's sawn size is 100×47 then it gets 5mm planed off width + 2mm planed off thickness. So 3x2 is 70x45, 2x2 45x45, 6x2 145x45, etc.

 

My experience when I did a bit of building work myself was that it's not as consistent as this! I'd get timber from one source and then some more from another, and then the studs would be 2 or 3 or 5mm different in one or other dimension, which is a problem if you are using them to frame the same bit of wall or roof. I learnt to be very careful when ordering and check what their 'actual' rather than nominal size was.

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11 hours ago, LarryO said:

 But really whose fault is it; my assumption is it's the architect/designer for not being accurate in their documents.

 

It is sometimes, but then as a designer you can waste a lot of time trying to second-guess contractors' preferences, availability of materials/products and so on. You have to find some kind of middle way between insufficient and excessive 'accuracy'.

 

Often the skill is in how you present the setting-out dimensions. If you are thoughtful about it, you can try and do it such that if things like actual timber sizes are different from your assumptions, then it'll be a non-critical rather than critical dimension that changes.

 

Of course many of us have plenty of experience of working with builders who have no intention of looking in detail at the drawings anyway!

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10 minutes ago, line-weight said:

 

My experience when I did a bit of building work myself was that it's not as consistent as this! I'd get timber from one source and then some more from another, and then the studs would be 2 or 3 or 5mm different in one or other dimension, which is a problem if you are using them to frame the same bit of wall or roof. I learnt to be very careful when ordering and check what their 'actual' rather than nominal size was.

 

In my experience since the introduction of regularized timber I find the actual dims pretty consistent but of course there is some slight variation due to the nature of the material: moisture content, where in the log the timber was cut, etc. Or occasionally you'll just get a bad batch (human error at the mill?). But certainly nothing like the territory you enter with things like handmade bricks for example. I was involved in a job where the (very well known) architect specified a certain handmade brick that was made from the same clay as the Georgian bricks on the Grade I listed building our new building adjoined. They'd set out the coursing + joint size based on the samples they'd received for the new handmade brick but when the contractor had them delivered to site found they were often as much as 5mm bigger in height, making them unusable for the coursing + joint size. Every brick had to be hand sorted on site + for every 3 pallets delivered only one pallet could be used... We even went to the brick yard to try + persuade them to make the bricks a bit more uniformly!

 

All being said it did look very nice in the end...

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It's interesting reading this thread with regard to architectural building materials.

 

Even in precision manufacturing industries, there is the design size and a specified tolerance.  Modeling is done to design size.

Architects need to remind themselves of the allowable deviation/tolerance with building materials and draw building systems to design size that accounts for this and follow regional standards and specification with regard to acceptable tolerances.  Designing to the mm without relief in a system is a good way to induce heartburn 🙂

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