Dimensioning an Ellipse

[KC Jones]

For those of you that want to dimension an ellipse,
exactly what is it that you want to measure?

Do you want to measure the radius of curvature of the
ellipse at a specific point? (Which would be
applicable to any well behaved curve?) Interesting
data, but I haven't heard much drafting interest in it.
Or do you want, in the case of an ellipse, to find the
radius of a hypothetical cylinder whose planar section
is the ellipse? (The minor axis dimension of the
ellipse.)

|<-- K.C. Jones --------->|
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[Rick Moore]

I've had the misfortune of having to work with elliptical building masses,
and the only way I've found to convey this to a contractor is to give them
coordinate (x,y) points from a benchmark. The more points, the more accurate
the approximation.

[Steve Doman]

KC Jones wrote...

>For those of you that want to dimension an ellipse,
>exactly what is it that you want to measure?
>

Where I work, we occasionally need to dimension an ellipse. What we do is
simply dimension the major and minor axis. Not much else you can give other
then XY coordinates along the perimeter as Rick Moore already explained. Or
perhaps the equation and the focal points,if someone might want to layout
the shape. Those whom want to dimension ellipse by radius misunderstand the
geometry.

Regards,
Steve Doman

[William H. Geiger]

Craig,

If "in the field" is taken literally, you need to provide dimensions
for the two focus points (the peg locations) and length of "line"
(rope, wire, ...) to be used to layout the ellipse. This length is
the sum of the distances between the foci and any point on the ellipse
and is also equal to the length of the ellipse major axis. Formula
for focus point dimensions relative to the ellipse center are
+ or - (a^2-b^2)^1/2 where "a" and "b" equal 1/2 the length of the
major and minor axes respectively.

Alternatively, to dimension an ellipse in Cartesian coordinates, use
the draw->point->divide command and set the number of segments to some
multiple of four. Use the points generated to dimension the ellipse.
Note that only one quadrant need be dimensioned. If a lot of points
are required, label the points and put their x and y values in a
table.

Hope this answers your question.

Regards,

WHG

On 4 Aug 1997 20:10:20 GMT, "Craig Paxton" <cpa...@mindspring.com>
wrote:

>I am looking for suggestions on how to dimension the new ellipse in R14.
>In older versions of AutoCAD the ellipse was not true and you could simply
>dimension the key arcs and the people in the field could build it from
>that. Now the radius dimension command doesn't let you pick an ellipse and
>you can't explode it. Just dimensioning the four axis points is not enough
>for the people in the field to really build it. Any help is greatly
>appreciated.
>
>Thanks,
>Craig Paxton