Accuracy paradox - a kind of Sketchup cancer

Its difficult to explain but over the years I’m starting to experience an increasing sense of annoyance in using the fundamental tools of SU. I think its stemming from the use of an imported Autocad file to start the process of working up the 3D SU model - something I’ve done regularly for years. I often find myself wrestling with trying to insert simple openings into planes that continually break up (or whatever the SU term is) On many occasions this may not bother most people but for me its now starting to get right up my nose. Is this an issue that fundamentally starts with basing the model on a imported dwg - where the lines may diverge by a fraction of accuracy?

Is this realy a SketchUp “cancer” or do these annoyances/errors happen to be due to what have been created in some other program?
Could you share an example that you imported, before and after importing?

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We use the general term “triangulation”.

SketchUp native tools expect to draw along vectors and planes that are parallel to the drawing axes (and are set by default to the model axes.)

The native AxisTool allows a modeller to change the drawing axes to align with model objects (such as a face plane imported from an external file.)

Perhaps ‘SU cancer’ is a little derogatory to SU as I suspect its the Autocad data thats at fault. I refer to the cancer term as for me its a symptom that appears midway through a project when you realise that all is not as well as it seems and you cant quite place where the inaccuracies lie - its too late to go back and start again. Extension Clean3 highlighted confirmed the errors to me. I confess that recently I have been a bit lax with preparing the dwg fully before importing.

n. 1. (Med.) An indefinite feeling of uneasiness, or of being sick or ill at ease.

My boss always tells me:
"a computer is a computer, the only variable is the person in front of it"
He is always right, apparently…

ah! Doctors can’t be sloppy with their next patients. I guess so can’t you with your next to be imported dwgs. Either create accurate geometry in third patry program or correct the import directly. The topic title doesn’t do SketchUp any justice.

Both Cad applications and SketchUp benefit from not trying to work too fast and trying to pay attention to what you are doing. Especially when you create things using “snap” or “inferencing” it is good to make sure that you really know what the application is snapping to. Also, using the right tools helps - for instance, in SketchUp, the Rectangle tool by its nature will not, at least in normal circumstances, create off-axis geometry whereas the Line tool can, if you are not careful, snap to all kinds of unlikely places.

One reason these small errors happen to me from time to time in various applications is that to me it seems often possible, when you are in a hurry, to point the mouse on a desired destination, and, while pushing down the button, move it slightly so that the clicked point falls short of the intended target.


As my good friend Matt Donley always says - model perfectly. The same goes for any CAD application. I can’t tell you how many times I receive CAD data that is sloppy - lines don’t meet, crazy layering systems etc. I don’t use any ACAD anymore and model everything from scratch in SU with the exception of site information, where I rely on the Topographer’s file to generate most of the site information. Over the years I’ve found two that I can rely on for clean CAD work.


Sadly you are correct, it is from AutoCad and it is a SketchUp cancer that no matter how much you resnap and redraw your model just degrades until you get to the point that as you correct one surface another dissapears. It originates from a bug in SketchUp in importing polylines from AutoCAD, Imagine your SketchUp model snaps say every 5cm to match your AutoCAD drawing which is religously snapped 5cm. What SketchUp stupidly does is to import the line thickness so instead of one line you have two micro lines next to one another (that SketchUp cannot display!) the thickness of the original line is how far they are apart. Initially you will be impressed and all the surfaces look fine and your SketchUP model will grow and then all of a sudden in one bit of your model you wont be able to join 3 lines and make 3 surfaces, you will only be able to have 2 surfaces! Hopefully at this point of reading this, the light has shone and you will have just realised what has happened! What you have is most lines snapped to 5cm with a few that are 5cm less half the line thickness and some others 5cm plus half the line thickness. SketchUps resolution engine thingy will not fix this, believe me, I have wasted hours!
The solution is start again with AutoCad and copy your drawing to a scrap one. Use the purge command in AutoCad to get rid of everything that you dont want in SketchUp.If you have layers for things like Roof, use AutoCad to move the roof onto a blank area at an exact distance away (say 20m), keep doing this until you have no layers and everything is on layer 0. Get rid off all the layers. Select everything, properties, change linetype by layer, colour by layer, etc. Layer properties set linetypes/thicknesses to zero (or hairline depending on version). Back to properties, then scroll down and find global line thickness, set it to zero! Now if you dont see this option, its because something is selected that has no concept of line thickness, its probably text, sometimes its a tiny zit buried deep in your drawing from 10 years ago that will be a dimension, text or anything that is not a line. Persevere and eventually you will see the Global line thickness (even if you have to do it in bits), set it to zero!
Reimport into SketchUp, you will live in peace and harmony forever!
When you get a bit more into it, you will find that its best to divide your AutoCAD drawing into separate little drawings that are all on the same page but not physically touching one another (ie move all your furniture, sinks, toilets, windows, doors out of the house and leave them to one side say 20m above). Import into SketchUp, explode the lot, then convert each sub part into a Group, rename to be sensible and then assemble the SketchUp model from the Groups making components as you go and as approriate. Hopefully you are one step ahead and you can see the trick is to make one drawing lots of little groups, then slide it back down en-masse say 20m back to where it started! This is important because you dont want doors and windows to auto combine with your SketchUp model.

Many thanks for these recent comments. Its reassuring to know I’m not the only one whos been traumatised by this. I had become rather lax in the depth of purging recently and wasn’t aware of the importance of the Global line thickness. Looking forward to a bit of ‘peace and harmony’!

That process sounds really painful. I would do all the layer management in SU. Much easier to isolate the geometry into groups, then edit the groups to place entities on Layer 0, then assign the groups to your SU layering system. Then you can forgo the potential errors involved with moving things around from their original locations.

Fortunately I only deal with Topographic surveys and all my architectural work is done inside SU. It is much faster than going from ACAD to SU.

I commented on a similar topic here.

I don’t use ACAD. I use PowerCADD and it doesn’t use polylines to make thick lines. I have seen that in ACAD files from others, like surveyors. I think corruption creeps in by means it’s often hard to figure out. Accidentally nudging something? It is utterly frustrating when it happens. As I mentioned in the other post, I keep a trail of saved files to go back to and retrieve unspoiled geometry from, but even that has proved tricky.

Why not do it all in SU? It is so much more efficient.

It sounds a lot, but it only takes a few minutes. Yes you are right on layer management, I am very methodical on my AutoCad layers so when I import a block, I always explode it in a temporary drawing then remake the block before I import it, but many architects like to import things like people, trees, cars & bushes and finish up with loads of junk layers that cannot be purged because they are on unknown layers nested in blocks, in which case,as you point out, its easier to get them into sketchup, move them to an empty space, delete the layer and move onto layer 0 and then see what you got left, delete the unwanted junk, regroup, move back.
The main thing is you want to finish up with as many groups as you can when you import into sketchup and ultimately finish up with everything on layer 0 but in a well managed way (all the groups having names etc) because what you don’t want is for SketchUp to auto combine a door with a wall.
Ultimately, its very easy to make layers for your animation scenes / layout interface in SketchUp if you have a very well managed series of nested and well documented groups / components.

Do not use the polyline width feature in AutoCad. I haven’t used it for years, now that line weight can be controlled either individually, by layer, or with a plot style table. Polyline widths even do not respect your output scale setting.


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You are not alone in your frustration. It’s perhaps unfair to say it’s the fault of sketchup, but it feels like a symptom when it manifests halfway through a project. “Do a better job modeling”, is not practical advise for those of us crunching lots of projects with many collaborators, sometimes you have to work with what you have and forge ahead. Lots of ideas on mitigating the problem in this thread, I thought I would add one more trick that has saved a few lost causes for me. Select the offending batch of “triangulated” lines and use fredoscale to scale them to 0. This forces any batch of lines into a plane, it’s a cumbersome solution for an entire complex model, but perhaps useful from time to time. Good luck.

I have had that happen just in sketchup all by it’s lone some. I was drawing a simple 4/12 roof intersecting a wall the other day and though I used the tape measure tool to lay it out, when I drew over the guide lines with the pencil tool - they would not make cohesive planes.

These sort of things make the program frustrating.

A related question - I want to make an array of rafter tails - they have to be 24" on center but the roof length is not divisible by 24" Can I make an array, then slide it right or left until I find the best looking spacing?


Yes. What do you plan to use for spacing? 24" OC except at each end? There’s no reason you can’t move the components individually or en masse after distributing them.

With plywood (or whatever is going) to cover the rafters and their fixed measures I can’t see why you would want all the rafters to be equally spaced an odd dimension around 24". I would say, make the array with 24" and only move the last one or both at the beginning and end.

p.s. you can create the array by typing /10, then /11 or /9 (and so on) till they fit best, closest to 24". But see the top of my response.