I need help with ELLIPSE Please !!

[Mena__99]

Hi , i have small problem but its really important for my collage drawing

 

 

 

i have drawing an ellipse using the ellipse drawing command and now i need to figure out the 4 points that draw this ellipse Points (H,K,M,L) ..

 

how i can draw the 4 points on autocad ? .. i only know the center and both the majority and minority diameter "sounds enough to me but i dont know how to do it on autocad (( i have version 2013 )) "

[ReMark]

I only vaguely remember two methods of doing this and they date back to my manual drafting days where ellipses were not as exact as a CAD program will draw them today. The methods were called the four-centered ellipse and the eight-centered ellipse. The second method was said to yield a better (truer?) result. Your little diagram to the right appears to depict the four-centered method. You should be able to find an explanation of the technique in any technical drafting book or online for that matter.

 

How exact do you have to be?

 

Check out this link. You'll have to scroll down slightly to where two methods (five-center and four-center) are depicted. Click on an image to see an enlarged view.

 

http://etc.usf.edu/clipart/galleries/455-geometric-constructions

Edited March 7, 2013 by ReMark

[JD Mather]

Attach your dwg file here.

As indicated by ReMark, the method you depict creates 4 arcs that were blended with a pencil.

It is not a true mathematical ellipse and I can show you how to graphically/geometrically demonstrate this to your professor if you attach your dwg file here.

[ReMark]

I have a question...

 

As the ellipse behinds to flatten (the height gets less) would an eight-centered approximation really be close?

 

...and an observation.

 

From my very unscientific method for determining how AutoCAD constructs an ellipse (pellipse=1) I drew an ellipse and then exploded it. It appears each quadrant of the ellipse is comprised of four different arcs.

 

Edited March 7, 2013 by ReMark

[JD Mather]

... .. i only know the center and both the majority and minority diameter "sounds enough to me...

 

 

I found several websites that show how to do it, but at least one I found on YouTube shows a wayyyy to complex technique.

 

I thought I would be able to show you a technique in AutoCAD using Parameters, but found AutoCAD wayyyy to clunky and ended up doing it in Inventor.

[neophoible]

I have a question...

 

As the ellipse behinds to flatten (the height gets less) would an eight-centered approximation really be close?

 

...and an observation.

 

From my very unscientific method for determining how AutoCAD constructs an ellipse (pellipse=1) I drew an ellipse and then exploded it. It appears each quadrant of the ellipse is comprised of four different arcs.

 

Did you try overlaying with an ellipse of the same-parameters made when pellipse=0?

 

From JD's post, it looks like the 4-point is much better than a stopped watch telling time anyways.

[eldon]

i have drawing an ellipse using the ellipse drawing command and now I need to figure out the 4 points that draw this ellipse Points (H,K,M,L) ..

 

I think that this is trick question. An ellipse has a continuously varying radius, so that any attempt to draw it with arcs is only an approximation. Therefore the points H, K, M, L are only a figment of the imagination

[neophoible]

I agree that a 4-pt method, etc, will not draw a true ellipse, but I’m not so sure it’s meant to be a trick question. It’s interesting to see how close the approximation comes. Did anyone else try it out besides JD?

 

I got around to actually trying ReMark’s interesting link, but my attempt at following the solution (see below) did not match JD’s. I’m wondering why. JD does not seem to show how he got the centers, at least as far as I can tell, but his approximation looks more even. I’m not absolutely sure I followed directions correctly in my attempt. I think they were worded confusingly. Like ReMark, I very vaguely remember using such an approximation long ago--in high school--so I'm not sure it's the same one.

 

[eldon]

Since the OP stated that the ellipse was drawn with the Ellipse command, then the points H, K, M and L would not exist as they are only involved with drawing an approximate ellipse with arcs.

 

If the OP were to have constructed the approximate ellipse in the first place using arcs, then these points would already exist.

 

So the OP's request to find these points was a non sequiter, because those points are not involved in the drawing of a true ellipse.

[neophoible]

I think we agree here, as I said already. And, yes, it is always possible that there is a trick involved and it’s OK to mention it. I just wouldn’t assume it and then not give any further help. The OP is not a native English speaker and may have trouble giving the whole story, so, no need to assume a trick, just an investigation into a 4-pt method, which ReMark provided a way to do.

Now, if anyone wants a trickier assignment: Define the Universe (in full, in detail) and give three real examples. Obviously, at its deepest level, this one’s rather beyond us to fulfill.

[ReMark]

How dimensions do you want that defined in?

 

Note that Mena posted his question on the 7th of March then never returned to follow up. So I guess that the word HELP and the two exclamation points, in the thread title, were just window dressing.

[neophoible]

How dimensions do you want that defined in?

 

Good Q! As many as you are able to handle, I suppose. I'm really only used to the few we live in (3-1/2 the way I count, though not everyone agrees on that, so I've been told). Of course, I'd be very skeptical of any claim to a correct answer that actually contained useful information (Holmes & Watson in a balloon). And then of course there would have to be someone qualified to evaluate it. But I don't mind anyone giving it a whirl. God knows there's been a lot of folks try that sort of thing. Still, stretching the imagination beyond reality, or beyond our ability to perceive can be useful, and fun.

 

Note that Mena posted his question on the 7th of March then never returned to follow up. So I guess that the word HELP and the two exclamation points, in the thread title, were just window dressing.

 

Yeah, I noticed the OP seemed to have dropped out, perhaps from the beginning. Anyway, I don't necessarily need him to have a good time! It would be interesting to see if it was a trick Q. It's also possible, as JD may have implied, that the instructor really did not know that finite arcs can't really define an ellipse, at least not in my universe. Hey, maybe another universe took Mena, one where an ellipse can be defined by arcs! Or maybe...

[ReMark]

If an ellipse cannot be defined by an arc then what do you make of the way AutoCAD constructs one?

 

I believe there are 11 dimensions.

[eldon]

AutoCAD might be clever enough to use the basic ellipse equation which only has two variables.

 

But then, the way AutoCAD works sometimes is not easy to follow

[neophoible]

If an ellipse cannot be defined by an arc then what do you make of the way AutoCAD constructs one?

Well, I’m thinking that your exploded image of an AutoCAD ellipse created when PELLIPSE=1 looks like a very good 12-ctr approximation. It may be way over my head to understand how they generate what is essentially a true ellipse (like what Eldon is talking about) when PELLIPSE=0, but I'm guessing that the mathematical error is essentially negligible, that is, akin to circles, etc. I did compare the two variations and they are pretty close using JD’s dims of 100 & 50, but they do not look exactly the same. Of course, if memory serves me here, AutoCAD hasn’t always had the true ellipse option.

 

I believe there are 11 dimensions.

Yes, I’ve heard from men I respect that there must be at least that many spatial-temporal dimensions just to have what we notice daily. Of course it doesn’t mean that that’s the limit, nor does it mean we have direct access to those beyond the basic 4, or, as I called them, 3-1/2, and that was because traveling back in time is still fantasy and has no humanly foreseeable future. I can still have fun watching sci-fi stuff that includes it, though.

[ReMark]

What do you mean traveling back in time is still a fantasy? I'm the Terminator. I traveled back in time as a cyborg assassin.

[eldon]

I am always interested as to how far one can wander off-topic with a bit of deep lateral thinking

[neophoible]

What do you mean traveling back in time is still a fantasy? I'm the Terminator. I traveled back in time as a cyborg assassin.

 

Ah, yes, but from what I heard, it wasn't so successful, depending on your definition of that.

 

I am always interested as to how far one can wander off-topic with a bit of deep lateral thinking

 

Well, even things like that may still need some sort of elliptical 'garden path'. AFAIK, he still may be caught in some sort of elliptical time loop! No doubt that no matter how far he wanders, still he'll be back!