Metric or Imperial for accuracy

[paulcroft]

Hello
Just been browsing the warehouse section and came across interesting info that SU is actually programmed to only work in the old imperial (inches) measurement environment. Interestingly some time ago (google forum I think) someone posted that working in Metric with SU was inherently inaccurate and likely to throw up scaling anomalies when viewing models Ie objects would be smaller or bigger than they actually are corrupting the perceived view. Is there any mileage in this opinion? Personally I would have thought that SU would simply convert the scale back to its native format maintaining accuracy.
Kind regards Paul

[john_mcclenahan]

As I understand it, ALL lengths and internal calculations in SU are done in inches, with a floating point precision equivalent to (approximately?) one thousandth of an inch. Metric dimensions are used only for input and display, but internally converted directly to the inch equivalent. So not inherently significantly less precise than imperial units. Certainly not to the extent of ‘corrupting the perceived view’ unless someone modelled in inches and called them cm or vice versa. It can cause problems for import and export from/to some formats if the units are not, or cannot be, specified to the converter program, but not because of internal issues.

@paulcroft

Personally I would have thought that SU would simply convert the scale back to its native format maintaining accuracy.

I agree.

[paulcroft]

Thanks for that John puts my mind at ease. Ever since reading that original post I have often scrutinized my efforts and just wondered if that table and chair was JUST a little too big for it’s context.

[DaveR]

If you are seeing things like the downloaded model/component being too large or too small in your model, you can put that directly on the author. (Or on yourself if you happen to be drawing with the wrong units. ) Although things are getting better in the Warehouse, there are still models that were created by users who either intentionally or unintentionally worked at the wrong scale. Not too long ago I was looking for a computer mouse to put on a desk I was drawing. The component was larger than my house.

The general wisdom is to download components from the Warehouse to a blank SketchUp file so you can check it out and make adjustments if needed.

[Wo3Dan]

@paulcroft

… Ever since reading that original post I have often scrutinized my efforts and just wondered if that table and chair was JUST a little too big for it’s context.

Could you give a link to that post?
I’ve never had any issues with working in metric dimensions, mostly in millimeters.

@john_mcclenahan

As I understand it, ALL lengths and internal calculations in SU are done in inches, with a floating point precision equivalent to (approximately?) one thousandth of an inch.

I disagree with that.
SketchUp’s limitation may be 0.001 inch. (see * below) But precision is much higher, no matter whether working in inches or mm.

*) This limitation means that endpoints in one and the same level (whether basic or grouped environment) can’t be less than 0,001 inch. Otherwise they will merge into one endpoint or the middle endpoint will merge with the connecting edges on both sides, resulting in two collinear edges.

As an example: you can input 0.000001mm to have geometry in different groups separated by 0.000001mm, Groups and components separate geometry, making smaller input and dimensions possible.

[slbaumgartner]

Expanding on what @Wo3Dan wrote, SketchUp uses a floating point representation for values. The limit on how small a value can be captured is so small as not to matter unless you are trying to model sub-atomic particles at real size (i.e. not 0.001").

But floating point calculations are subject to error due to the computer using a finite amount of data to represent values. Some values can’t be represented finitely (e.g. 1/3 = 0.33333333…). So each time edges and faces (“geometry”) are added to a SketchUp model, there is a chance that two vertices that should have been the same actually differ by small amounts produced by calculation error. To manage this phenomenon, SketchUp examines the vertices of the added geometry to see whether they are so close together that the modeler probably meant them to be the same**. When it finds two vertices within 0.001" of each other, it concludes that they should have been the same, so it merges them. This “clean up” is quite different from actually limiting the representation to 0.001.

** This oversimplifies a bit - SU actually looks for more than just vertices, e.g. possible intersections, duplicated edge segments, etc. Also, it examines only geometry within the current edit context (model or group/component open for edit) since vertices from different contexts can’t merge.

[john_mcclenahan]

Thanks, Wo3Danand Steve - I had got myself confused into thinking that the thousandth of an inch tolerance for merging was also the limit of precision that SU cared about. Glad to be corrected.

[paulcroft]

Unfortunately I no longer have any link references to that post as it was quite a while ago maybe as far back as AtLast’s tenure. Only thing I do know is that it had a lasting impression! Thanks for all the input. Have to say this is one of the best and most responsive of all the SU forums I have encountered.