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@GrantsPirate wrote:
The isocircle diameter will be the longest distance between quadrants.
Actually, no, it won't. See the original image, in which they drew a vertical [pinkish] axis Line [or Xline], and Circles [also pinkish] going through the points where that vertical axis intersects the Ellipses, and they're showing the diameter of that Circle [for only the inner one], which is the "real-world" diameter of the inner isocircle. That is the circle of which the Ellipse is an isometric projection, and as can be clearly seen in the image, the corresponding Ellipse is longer along its major axis and shorter along its minor axis than the diameter of that circle. Only a distance parallel to one of the isometric axes [such as the distance across that Ellipse along that vertical axis] is a "real" length in the "world" that the drawing represents in isometric projection. The Ellipse's major and minor axes are not parallel to its isometric axes, and therefore the distances between quadrant points are distorted by the projection, and don't represent "real-world" lengths on the represented circle. [The same is true for axonometric projections at other than isometric viewing angles, with isometric just being a special case of axonometric projection.] That's what the "-metric" in axonometric and isometric means -- you can measure such a drawing only parallel to the axonometric/isometric axes, if you want to get "real-world" dimensions.
Fortunately, parameter values in (vlax-curve) terms going around an Ellipse correspond to radians going around the virtual circle of which the Ellipse is a projection, which is why my routine above is fairly simple -- it's looking at multiples of 45 degrees [1/4 of pi radians] around the represented circle to find the axonometric axis enpoint locations, between opposite ones of which the diameter can be measured.