Move/Copy/Divide,Tape Not Working as Expected

I am a new Sketchup user & I find I am very confused how things work. For example; (using inches) I created a rectangle measuring 2’ wide x 1’ 10 5/8” long x 3/4” thick. I then made it a component. I then used the tape measure tool to mark off 2” on each end of the 1’ 10 5/8” sides. I then edit the component (double click) and added a small rectangle about 3/4” x 1 1/4” and placed it on the 2” mark at one end. I then made a copy of this and moved it to the other 2” mark and divided this by 9 to have sketchup equally place copies along that path. Sketchup completes this task as expected. However, when I measure the distance between these items (so I can build it), the measurements between items is not correct. It is off by 1/4” from one end to the other. No matter what I do the math is wrong. I can understand Sketchup not being able to equally divide this number when placing these on the path, but the final placement of objects & the measurements between them should at least measure up. What the heck am I doing wrong?

How about sharing the .skp file so we can see exactly what you are working with?

I tried to construct sort of what you describe although I don’t know exactly what the orientation of your “about 3/4” x 1-1/4" rectangles is supposed to be. I get the exact same distance between them when I distribute them between the guides, though.

I’d be inclined to think it’s possible that you have Length Snapping enabled and that’s getting in the way or Precision is set too coarse, or it’s just pilot error. Sketchup is not doing the math incorrectly.

I’m new and I don’t see an option to include a file in this post.

There’s an upload button.
Screenshot - 3_12_2021 , 9_25_15 AM

Or just drag and drop the .skp file into the reply.

My measurements say the distance is 7 3/8". I used the measure tool and I also try measuring using the tape tool and both say 7 3/8" which is not correct of course.

Again, share the model file so we can identify what you’ve missed.

Example.skp (193.7 KB)

In your first post you wrote:

When I look at your model, it shows as 5’ 10-5/8 in. long.

You have Display Precision set to 1/16 in. and you have Length Snapping enable. The former limits how precisely the dimensions are displayed and the latter can lead to accuracy problems.

With Precision set to 1/64 in. the dimensions are displayed more precisely. The math is done correctly in SketchUp however.

When you start a new model using feet and inches for the units, Display Precision will be set to 1/16 in. and Length Snapping will be enabled. You can refine the Display Precision and turn off Length Snapping in Model Info.

You could also use decimal inches if you feel you need more precision.

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That’s what I figured was the problem, but ultimately Sketchup should in my opinion not be off my more than 1/16"(my setting), but it is off by a 1/16" from point-to-point making accuracy over a long length impossible no matter what the accuracy setting is set at. I can understand being off by 1/16" from one point to another, but not from one end to the other. Does that make sense? Example; measuring from Point A to Point B should not be off by more than 1/16". And from Point B to Point C should also not be more than 1/16" off which Sketchup is. The bad thing about this is that the 1/16" that Sketchup can’t compute correctly is then added to the end making the next measurement off by 1/8" and it continues on and on. In this case Sketchup held on to this 1/16" 9 times making the overall measurement off by 1/4". This simple measurement issue can never pass to a production environment no matter how small the accuracy setting is set at. That’s just my opinion of course.

What you are seeing is only the dimensions at the Display Precision you have selected. The model you have uploaded is set to display the dimensions to the nearest 1/16 in. The math is still done correctly and you can see it with more precision if you wish. That part is up to you. I showed inches to 6 places past the decimal which I’d challenge you to measure in a production shop. If you want you can use millimeters with six places past the decimal.

Although this hasn’t anything to do with SketchUp’s displayed precision, you of course wouldn’t be using SketchUp Free in a production shop.

I get the fact you can change the settings to mm and be as precise at you wish when measuring, but overall that does not fix the problem Sketchup has. Even using mm the problem is still there. The problem is how Sketchup does the math when it equally divides to a path. This is an error (bug) in my opinion because the math just doesn’t add up no matter how precise you have your settings set to. Sketchup will always compound the fractions over the total times you divide it to a path. As a human I can easily see I can not divide this equally and I must decide what to do with the results. Sketchup simply forgets about the difference it lost between the divide instead of remembering what it lost/added to the figure. Do you get what I mean? Sketchup should remember when it does it’s math so the real margin of error is indeed equal to the settings you have set in Sketchup. I don’t like it when my math is more accurate than a computer. Luckily these were not cut lines or we would be in real big trouble. Being off by 1/4” in only 1’ 10 5/8” length is terrible. Again, that’s just my opinion…

The actual dimensions are not off by 9/16 in. Only the displayed dimensions and that’s due to the coarse Display Precision you have chosen.

You are expecting micrometer dimensions from a carpenter’s tape measure.

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As Dave says: SketchUp math is correct. You are choosing to look at rounded numbers.

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I get what you are all saying, but… when Sketchup places objects using the divide function is not how I would do it. It failed ONLY because it tries to be equal in it’s division. If it can not divide this equally it will place the first item at position A (maybe only a -1/16" short) and then it places piece B (maybe only a -1/16" short), then places piece C (maybe only a -1/16" short) and finally places piece D (maybe only a -1/16" short). The result is that piece D is actually placed -1/4" off. In a perfect would Sketchup should have done this instead;

Place first item at position A (maybe only a -1/16" short) and then it places piece B (remembering the -1/16" short on A so now adds it back), then places piece C (maybe only a -1/16" short) and finally places piece D (remembering the -1/16" short on C so now adds it back). The result is that piece D is in the exact position you would have expected it to be in. I get it just a visual thing and that these are just measurements, but can you imaging this being a computer chip with billions of resisters in an inch square? The measurements would be so far off it would be impossible to construct no matter how small your settings are. The math just doesn’t work no matter how big or small the project. Sketchup compounds your measurement of accuracy as many times as you divide. The results of copying multiple objects to a path is not something I will ever do in Sketchup.

SketchUp didn’t fail, you did…
The Division SketchUp makes is equal!
Dave already told you that.

You need to turn off length snapping and set the display precision to 1/64 if using fractional. If you want to build an object to a precision of 1/16th you can. The math in SketchUp is only as precise as you set it.

Well, I guess I’ll try one more time. Nothing is “off”. SketchUp precision is about one millionth of an inch. SketchUp math is correct, for division of multiple object spacing or any other thing. In the example you posted all of the arrows are exactly 7.402778" apart from each-other, they are evenly spaced. There is no compounded error. If you open that example, go to model info and set the viewing precision to decimal inches and the most decimal places you will see the same. If SketchUp really could not do math do you think we would use it? I design machine parts to the .001" all the time. I can assure you, SketchUp works.

In a real production environment we have tolerances. Let’s say the drawing states it is within 1/64". That means the measurements between Point A and Point B will always be within 1/64". This is also true if we were to measure from Point A to Point Z and it is also true from Point A to Point B to Point C all the way to point Z. Drawings produced by Sketchup will either gain or lose 1/64" (assuming that was my fault tolerance setting) for every single object placed within the division. I already showed you how a human would solve this problem. S/He would do the math first. Then realize it could not be divided equally and then the human would place those objects to achieve the best results. Sketchup does not use this approach. It simply drops/gains what ever your tolerance setting is and then compounds this throughout the placement of objects.

I don’t know why you guys don’t see this? At some point you have to print out your Sketchup and give it to someone to build it. If you have your settings to “.000000000000000000000000000000001” tolerances, it still doesn’t change the compounding issues during a divide. And it would be impossible to read the drawing and actually build it.

What’s that on a tape measure? Who does this? I’m using a tape measure that reads down to 32nd. Take my example and a piece of wood and measure from each end 2" and then measure 7 3/8" between them. This can not be done because the math that is “display” is NOT correct. We are only working with a 1’ 10 5/8" piece of material. We simply want to divide this up as equally as possible. A human can do this within 1/16" from Point A to Point B and every other combination and all I would use is a tape measure.

My point is that SketchUp has done what you asked it to do. It sounds like you asking SketchUp to do the impossible, to divide 10 by 3 in only whole numbers. You want the spacing of your blocks to come out evenly to 1/16" of an inch even though the total distance is not divisible by 1/16"? Yes, I work in a production environment, I make drawings and people build them with tape measures, I get tolerances. What you are saying is that a human would just fudge it, we would put the blocks slightly over or under the tape measure mark until it looked evenly spaced. I understand that the act of rounding numbers down to the nearest 1/16" means that the individual distances will not always equal the total length, This is the nature of rounding and not a fault in SketchUp. If you design an object that does not have an exact measurement of 1/16" but you want to only see things in 1/16" increments then you need to live with rounding. 10 divided 3 in whole numbers only is 3.0, that measurement won’t yield even spacing of two objects in a ten foot space, it’s rounded and incorrect, we could get more precise but the truth is it is impossible to be absolute correct , eventually the placement will be a compromise.

A human can do this because a human can make value judgments about what is “good enough” right?

or we do this:

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