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Draw a parabola


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Discussed in this thread, with link to an earlier thread.

http://www.cadtutor.net/forum/showthread.php?t=28775

 

"Vertical curves are parabolas, you can draw them by drawing a polyline from endpoint to VPI to endpoint, then PEDIT>>Spline. For it to be a proper parabola, set SPLINETYPE to 5 before pediting, and to get a smooth curve set SPLINESEGS to 15 or higher. (Thanks to rc McSwain for the Spline/parabola info)."

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i have the equation of the parabola, and I know the begging point of the curve, the vertex, and the end point of the curve. It will be very helpful if I can plot the equation. I tried the polyline, and then changed it to spline. But changes it's positing and became smaller.

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If by "vertex" you mean the high or low point of the curve, then yes the curve will be inaccurate after "pedit". it should be initially drawn as two straight segments, from start curve, VPI (intersection of tangents) (VPT) to end of curve.

 

if you want to plot the curve, you can do the calculations in Excel, with x & y values in adjacent columns; start the pline command, copy & paste values from Excel

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yes, exactly. I mean VPI by vertex. If plot that in excel, then I think I won't be able export to AutoCAD so that I continue the further on based on what I exported. It will be only a raster image.

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To expand a bit on the procedure described by CarlB: If the parabola is symmetrical, i.e., the two end points are horizontal and the vertex is above the middle of those two points, then the poly’s 2nd point should be at twice the height of the desired vertex. See Sample.

 

Additionally, if there exists a sufficient comfort level with installing and running .NET Dlls this thread may be of interest.

 

http://www.cadtutor.net/forum/showthread.php?t=34370

PolyParabola.dwg

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the parabolas that I am working with are symmetric. what is see in the animation will fit my work, thanks. But is it in AutoCAD? because i could read in the command line "para" and when I tried it in AutoCAD, it does recognize this command. and what is NET Dlls?

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The animation demonstrates a custom routine (written in C#.NET) located in this post of that thread:

 

http://www.cadtutor.net/forum/showpost.php?p=233991&postcount=34

 

To install - Download the attachment, unzip, and locate the file ParabolaByUCS\Release\CsParabola.dll.

 

In AutoCAD:

 

Command: Netload

 

Then find CsParabola.dll

 

To run:

 

Command: pbb

 

 

The routine will produce a spline. If you prefer Splined 2dPolylines, or if your IT department frowns on third party customization, the process CarlB described, and I illustrated in the DWG above, should produce useful geometry.

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