Why doesn't this line intersect?

The highlighted region is formed from two arcs and two lines. As you can see from the coords, everything lies in the xz plane.

The outside contour was created using the offset tool on the appropriate edges. (Bonus question - why does the offset tool fail on the highlighted face? I’m trying to offset .75", but it allows only about half that.)

Now look at this zoomed in view:

The selected line, in blue, intersects the upper curve in the figure, but not the lower; selecting the line at the lower endpoint selects the entire line - it’s not two separate segments. In other words, the intersection with the boundary is not being recognized.

Presumably the problem is that something is not truly planar, but I don’t know how to draw this and get it all to lie in the plane.

TopRail.skp (26.7 KB)

Firstly, try rotating so that your linework is flat along the ground plane. Secondly, 3/4" isn’t too small but small numbers sometimes have trouble. Try the DaveR method of scaling up your linework/faces. Perform the offset, then scale back down:

For the line break, again, try working flat first, then rotating vertical if that’s really what you desire. Here I used Eneroth Flatten to Plane to make sure that all the lines were indeed flat, ie coplaner. After that, you can see now that the line breaks as expected.

If you change the model units to mm and crank the precision right up to 6 decimal places, you’ll see that the geometry is ever so slightly out of plane. As Eric suggests drawing flat on the ground plane is probably better. Also drawing your geometry on a large rectangle can help keep everything on plane, just delete the edges of the rectangle when done.


Your model’s text markers lie to us, in two ways.

The first way is that those texts are not the actual coordinates of those vertices! Display the axes and play around a bit with the tape measure and you will soon realize there is no way that the middle points of the curves are at z coordinates in the range of ~ 14 - ~ 16" while the ends are at 0" and ~2 3/4". Clearly you have moved the whole thing after drawing it. Texts are frozen when you create them, they do not update if you move the objects to which they attach.

The second way is a quirk (or maybe a bug) of the fractional units formatting. Values close to but not exactly 0 display as “0”, not “~ 0” as you might expect. As @IanT suggested, if you change the units to decimal with high precision and then redraw the texts (you have to delete the existing ones else the tool will just try to move them), you will see that they are not exactly on y (green) = 0.


Thanks for all the help. A question - is the ground plane treated differently than the other principle planes (eg, the xz or yz planes)?
In the case of this design, I drew in the xz plane do that the front view was oriented as expected. Would things have worked differently in the xy plane in terms of maintaining co-planarity? Of course, as IanT points out, I would have been better off, regardless of orientation, if I had created a rectangular face to work in. That’s what I ended up doing to redraw the design.

I think you’ve done it right to draw in the intended orientation if that’s what’s required, it certainly saves having to rotate it afterwards. I draw everything in place and in the correct orientation so I know parts are exactly where they should be.

Regarding the ground plane theory, I think the SketchUp engine likes to draw flat on the ground plane, assuming then you will pull the faces up. Hence the name “SketchUp”!

But really, you can draw confidently on any plane and once you get a feel for how the inference engine works, you probably won’t need rectangles either.

I work with CAD files a lot which are typically drawn in plan view. But with CAD, you’ll often get stray geometry on the Z axis. It may be hard to tell if everything is coplaner (flat) or not. Check out this extension that will flatten everything in the event lines start giving you trouble: https://extensions.sketchup.com/pl/content/eneroth-flatten-plane

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