My other tactic that I'm trying (no success yet): make an ellipse in CAD. PLot to DWF. Since a DWF is an ascii file (of some form, I think), and I've seen postings somewhere for DWF readers to see the background text of the file, open the DWF in such a viewer to see what exactly is stored for the ellipse in the DWF. But, opening a DWF from notepad is just junk & my web searches for "DWF to TXT" conversion weren't successful last night. Maybe this approach would work...
DXF is the ascii version of a drawing file. Use saveas command and select DXF in the file type box.
How can I get a formula out of a dxf?
Looking at line by line in the ASCII, I don't see enough to extract the parametrics
Fine, to do it the hard way for no good reason you calculate the eccentricity and use that to calculate the first angle, JBJC.
I'm not trying to do anything the hard way (believe me!). I can't program 3 point arcs (start, mid, end). Can you show me how to calculate the radius for an arc using the start, mid, and end points? Because, yes the trammel of archimedes can get those points (really nicely in fact), but I can't see how to get a radius (or center point) out of that information.
The only thing that I can think of is ((arc cord / 2) x (arc cord / 2) + (arc height x arc height)) / (arc height x 2), but the problem is that all of these calculations are on angles to start with, which then means very very complex....help!
In regards to the JBJC angle - the trammel of archimedes can't give that information because as shown in my picture that I posted the center is outside of the pellipse. So I don't know how you meant for me to be able to calculate that amount, but I'm all ears.
I've been in the process of writing a vba program and I needed to be able to programmatically draw the arcs to approximate the ellipse the same way that autoCAD does. I searched and searched and couldn't find anything that says how autoCAD comes up with the approximation that it does. I found this post in my search.
After a lot of messing with the problem I finally figured out how autoCAD does it. So if anyone is still interested in this I've attached a drawing file (it's a 2007 version) that shows the math and geometry behind autoCAD's pellipse =1 ellipse approximation.
Hope this helps someone besides me.
Let me know if you have any questions.
I have tried this (just to confirm), and it works! HA! Well done! I now see the next step that I couldn't figure out before (circles used for P-2 and P-4). The P-2 point can also be found from the mid point of a line created from P-7 at half the angle P-3, P-7, P-1 and extended to line P-3, P-1 (but using a 3point circle is a bit quicker). Thanks!!