[tip]How to graphically find out focus of an ellipse.

[Anonymous]

Someone in the office today asked how he could communicate to the contractor how to lay out an ellipse in the field. If you remember from high school geometry an ellipse is a shape you can draw with a string of some length (Equal to the major axis of the ellipse.) attached to two points. Use a pencil or chalk to trace the ellipse with the string. (Visit http://mathworld.wolfram.com/Ellipse.html for a nifty animated gif.)

So ArchiCAD can draw an ellipse, but how do you construct it in real life? Sure you can trace it in AC with the arc tool and give the contractor centerpoints of dozens of circles, but you can also give the contractor the major axis and the location of the two foci. Then he can get some string, have two guys hold down the ends on the foci and just draw it out himself. We did it on a job recently. The contractor loved it. Soo much easier than doing the same thing with 10 or more circles. So how do you get the foci?

You can create an object, which I did a long time ago. (No ellipse with foci in the AC library.)
You can figure it out mathematically. Distance from center to focus = square root ((majoraxis/2)^2 - (minoraxis/2)^2)
You can figure it out geometrically. Only takes a sec. It's a lot easier to show than to describe. Just lookit the pic. It looks a whole lot more complex than it is. Really just draw a circle with radius equal to half the major axis at the top of the ellipse. Intersection of that circle and the major axis is the focus. Brilliantly simple.

Give the contractor the position of the foci. He also needs to know the length of the major axis. That's the length of the string. If you want you can give him the minor axis and the center too, just to check, but it's not essential.