Offset Tool accuracy issue.... and alternatives?

For years I’ve been using the Offset tool to define all sorts of things; widths for roads and paths, edges of streams, locations for planting trees, roof lines around buildings, etc.

Now I am seriously doubting the accuracy of a lot of my prior work, which relied on offsets.

The current thing I’m working on is an offset of a Coastline. It’s been modelled by Intersecting a flat plane ('the sea") with a 3d mesh which I created in Sketchup based on a point cloud scan of the land. So it’s a native SketchUp object, nothing fancy. I need to model a 20m offset of the coastline (this a legal requirement of the site’s development).

I welded the edges of “The Coastline”, cleaned up any strays or overlaps, and then used offset tool.

Since this didn’t work well, I then used a manual method (20m radius circles placed on each point around the Coastline, and joined their extents together, with a bit of manual interpoloation).

Below I show the results compared.
What can I do to achieve a true offset in SketchUp? The manual calculation method is very time consuming.

From Qcad offset…

… dwg imported in SketchUp

If you have a skp file, upload it, and it is easy to analyze the exact cause and suggest alternatives.

That seems to be a strange way to create the offset, joining their extents together. You’ll need the offset of neighboring segments the way SketchUp creates them.

SketchUp’s offset is accurate, considering you measure from each segment perpendicular towards its created offsetted segment. That is what offset is about, not from vertex to vertex.

See image below (input offset 400mm). It’s like a marble that roles over the original curve:

B) Convex angles are not so much of a problem. Short segments may even disappear in the offsetted curve.
A) Concave angles maybe are. SketchUp extents the segments and here you measure values that overshute the offset value you wish. At these corners you could correct the curve to be smooth and keep the offset “around the corner”.

I tried to do something with your shared image but it would be better to examine the coastline curve itself from a file.

offset-test.skp (522.4 KB)

I would use Fredo’s Curvizard to cleanup and simplify your coastline before offsetting.


I would guess from the screenshot that Offset is doing what expected, quite accurately. The coastline has very small segments, some of them visibly differing from the general appearance of a smooth curve. Simplifying the curve would probably work better.

Note that when offsetting arcs, the result is not an arch with a radius of the original arch plus the offset value, but a curve with segments offset from the arc segments with the offset value.

With isolated arcs (single selection of the entity) you have the choice to:

  • offset its segments accurately but the result is a curve.
  • use the vertices to offset accurately (vertex to vertex) resulting in a new arc.
    In the second option offset does not result in a value as entered when measuring perpendicular to segments. But you can’t have it both ways.

Could it be Length snapping activated in the Units that is causing a problem?