# 2-point arc fails to match points

Given a center defined by two intersecting circles, each circle centered on two independent points; drawing a two point arc from this center to each of the two reference points fails to match both points. Point one is accurate while point two is off a millimeter or so.

It might help to post a picture of what you are seeing. As of now, it is hard to imagine exactly what you are describing.

One thing to bear in mind is that SketchUp’s arcs and circles are represented by sequences of straight edges. Only the endpoints of the edges are actually on the theoretical circles.

And if you have drawn your circles off axis those endpoints will be slightly rotated.

What you are trying to do is what I know as triangulation. In olden times, you would have used a compass, hence why you are now using arcs. It doesn’t work too well in SU because arcs & circles are not true circles but multifaceted polygons. You could possibly achieve what you want with sufficient accuracy by increasing the number of sides to your arcs/circles. I think I came across a plugin that does this another way but I cannot find it.

I recently published an extension named “Circle Intersect” on sketchUcation that places guide points at the mathematically correct intersections of circles and arcs. TIG’s “True Tangents” also has a similar ability.

Yup, that looks like just the kind of thing.

Hmmm… the way I actually solved the problem was figuring out the height (bulge) of the chord. Since I know the Radius ® and the chord Length (L), the Height (bulge) = r ± (r^2 - (L/2)^2)^1/2. Now I use the 3-point curve and supply the bulge value. This works much better. BTW - the piece is ON axis.

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Yes OLDEN times works for me. Geometry is what it is and it works. BTW - I am olden times so it is appropriate 