I have the following dilemma:
I draw a simple cylinder radius=10, height=20. I make it into a group and show the entity info box, the volume Sketchup calculates for this cylinder is = 6211.66 mm³
Enter the data cylinder into Excel and use the V=PI x R2 x H formula and the volume computes = 6283,19
Why is there a difference??
It probably has to do with the fact that SU doesn’t draw actual arcs or circles. They are segmented. If you increase the number of segments in the circle the volume will get closer to the exel number.
Shep
Thank you Shep, that did the trick. I redrew the cylinder with 1000 segments and the volume came close enough for me: 6283.14 mm³ vs 6283.19 from Excel!
Cheers,
I came to report this bug, as well. I understand the problem is with the segment count, but this shouldn’t be a valid excuse for incorrect values, regardless of how many segments the circle has. This means it applies to circle area and cylinder volume. The calculation functions should calculate the area using the circles actual radius (pi r squared). Surely, this can’t be too difficult to fix! “close enough”.isn’t a very good solution.
OP drew a shape close enough (for the eye) to look like a cilinder. And that is what SketchUp uses to calculate the volume of. It is what it is: very accurate!
The user can create the shape even closer to a perfect cilinder. And again, SketchUp will give you a more accurate value of its volume.
Its up to you how accurate you create the/a shape. Result of the volume… : accurate value
I do hope SketchUp will not be fixed to give us the wrong value, that of the 100% perfect cilinder that in fact isn’t there.
Also see @Cotty’s post above.
One potential problem is that a circle is not a fundamental shape in SketchUp, it is a polygon with some attached metadata about the parameters of the circle it represents (view that as a kludge if you like). Various kinds of edits force SketchUp to discard the metadata, at which point a loop of edges is all that remains. That will affect the calculated area and volume, and the ability to change the radius or segment count will be lost I think.
Making a circle be a fundamental shape would most likely require a major rewrite of SketchUp’s geometry database.
I think the issue might come from the metadata system.
When you use the circle tool, you don’t trace a disc (the face). you trace a circle (the lines).
the lines have a circle metadata that allows them to have their perimeter properly calculated. But the disc (face) is simply the result of having a set of closed-loop coplanar lines. The face isn’t therefore a disc but whatever is defined by the circle. Therefore it can’t have an area measured by the formula.
Same for the cylinder, a disc extruded in 3d.
SU is a software that is based on lines, vertices, that generate faces. not the opposite. I get it’s very “egg or chicken, who came first”, but in this case it actually matters.
