topic Re: How to give different z values to start, mid and end of a sloping arc in AutoCAD Forum

How to give different z values to start, mid and end of a sloping arc

Anonymous — Sun, 11 Jan 2015 02:49:20 GMT

Can anyone please advise how to give different z values to the start, mid and end of a sloping arc, as I need to add 2 more points and levels. It is for a long sweeping radius kerb. When I give the start a z value, then go to properties, it gives the same value for start, centre and end and they're 'greyed out' so can't be changed. I've attached the arc to points with z values but to no avail. Please see attachment

Re: How to give different z values to start, mid and end of a sloping arc

Ajilal.Vijayan — Sun, 11 Jan 2015 05:51:22 GMT

Hi,

This seems to be the solution.

http://forums.autodesk.com/t5/autocad-civil-3d-general/arc-with-2-different-elevations/m-p/3246054#M164643

First draw the Arc, then use the Align command(with 3 points) option to align with the points.

Re: How to give different z values to start, mid and end of a sloping arc

parkr4st — Sun, 11 Jan 2015 13:29:03 GMT

how about draw a helix with bottom and top diam equal, & 1/4 turn (Based on 90 degree angle), twist CCW , height 832 (Delta of y1 y3)

this results in a smooth radius, constant slope arc? this does not line up with your three points so it may not be what you want.

you can also do 1/8 turns. heights 584 and 248. see attached

dave

Re: How to give different z values to start, mid and end of a sloping arc

Kent1Cooper — Sun, 11 Jan 2015 14:09:02 GMT

@Anonymous wrote:

Can anyone please advise how to give different z values to the start, mid and end of a sloping arc...

The way AutoCAD stores data about an Arc involves not start/mid/end points, but a center point, a radius, start and end angles from the center, and an extrusion direction. For an Arc that does not lie in or parallel to the current drawing plane, that extrusion direction will not be straight up the Z axis. That means the Coordinate System must be different for that Arc. You can use your three points to establish a new User Coordinate System in which to draw the Arc. If you're in the World Coordinate System, and the start/mid/end points in WCS are pt1/pt2/pt3, in simplest terms:

(command

"_.ucs" "_3point" pt1 pt3 pt2

"_.arc" (trans pt1 0 1) (trans pt2 0 1) (trans pt3 0 1)

"_.ucs" "_previous"

)

[Turn object snap off if appropriate, either beforehand or incorporated into the routine.]

For some reason, in 2015 the 3point option doesn't show up in the UCS command's base prompt. Neither does the New option. If you type N for New anyway, its sub-prompt offers 3point as an option, but you can also use 3 directly from the base prompt, as above.

Re: How to give different z values to start, mid and end of a sloping arc

Anonymous — Mon, 12 Jan 2015 08:17:23 GMT

Kent1Cooper,

Thanks for your suggestion, I copied and pasted your command prompt, and it
returned a message saying
"; error: bad argument type: 2D/3D type: nil" so I tried it anyway, I drew
an arc, attached to 3 nodes with different z values but when I go into
properties it still has the same values for all 3 z points.

Brucey

Re: How to give different z values to start, mid and end of a sloping arc

Ajilal.Vijayan — Mon, 12 Jan 2015 08:53:33 GMT

You need to define those points first inorder to run the lisp.

try this ( Also find the attached drawing).

command name is 3ZARC

(defun C:3ZARC (/ pt1 pt2 pt3)

(setq

    pt1 (getpoint "\nFirst point: ")

    pt2 (getpoint "\nMid point: ")
	
    pt3 (getpoint "\nEnd point: ")

  ); setq

(command

  "_.ucs" "_3point" pt1 pt3 pt2

  "_.arc" (trans pt1 0 1) (trans pt2 0 1) (trans pt3 0 1)

  "_.ucs" "_previous"

)

);defun

Re: How to give different z values to start, mid and end of a sloping arc

Ajilal.Vijayan — Mon, 12 Jan 2015 08:59:55 GMT

Kent1Cooper,

While using the lisp suggested by you or the suggestion from Allen to use the Align command, the z value for the midpoint of curve is not exactly the same value as the point.

Attached drawing is with the result after running the lisp suggested by you.

Re: How to give different z values to start, mid and end of a sloping arc

Anonymous — Mon, 12 Jan 2015 11:23:41 GMT

Ajilal.Vijayan,

Your suggestion worked spot on. Thank you very much.

Bruceybee

Re: How to give different z values to start, mid and end of a sloping arc

Kent1Cooper — Mon, 12 Jan 2015 15:26:48 GMT

@Ajilal.Vijayan wrote:

....

While using the lisp suggested by you or the suggestion from Allen to use the Align command, the z value for the midpoint of curve is not exactly the same value as the point.

....

It will be the same only if the point you give it for a so-called "midpoint" is actually geometrically the midpoint of the resulting Arc. The Arc command with 3 points draws an Arc that goes through those 3 points, but that doesn't mean that the second one will be the geometric midpoint of the result. You could give it a "midpoint" that's only 1/100 of the way between the start and end, and it will draw an Arc through that, but obviously the midpoint in geometric/object-snap terms of the resulting Arc won't be the second point given. That's equally true of Ajilal.Vijayan's function [which is just mine with asking for points added] -- perhaps its prompt asking for pt2 should say "Second point" rather than "Mid point."

Consider whether you should be using a 3D Spline, instead an Arc. That will go through the 3 given points exactly including having the correct Z coordinate at the second point, though that point will not always be the geometric midpoint of the Spline.

Re: How to give different z values to start, mid and end of a sloping arc

Anonymous — Mon, 12 Jan 2015 15:43:30 GMT

Many thanks for your assistance Kent Cooper, although between you we have found a solution, I'll have a look at your 3D spline version aswell later on, as this may be of use some other time.
Kind regards,
Bruceybee

Re: How to give different z values to start, mid and end of a sloping arc

Ajilal.Vijayan — Tue, 13 Jan 2015 04:29:41 GMT

Hi Bruceybee,

Glad to hear that now you can draw the arc with different Z values.

But the excat solution came from Kent Cooper, I just added the code to define the points.

Kent Cooper,

Thanks for the code and detailed explanation to help understand the second point elevation difference that I have asked (+ Kudos)