How to: Draw an Arc with a Specific Arc Length?

This question appears in our local forum. Is there any way we can create an arc by defining the arc length? There is no direct way to do this. There are some methods that seem to allow you define length. For example start, center, length. But this is defining the chord length, not the arch length.

chord_length

There is a suggestion to calculate the angle using simple mathematic calculation, comparing the arch radius and arch length. After you get the angle value, then you can draw it. But it is not easy.

One of our member, Ast, suggest a better solution: draw the arch, then modify it using lengthen tool.

AutoCAD_lenghten_tool

Lengthen tool allows you to define the object total length, including an arch. Select the total length option, then type the distance you want.

Command:  LENGTHEN
Select an object or [DElta/Percent/Total/DYnamic]: T

Specify total length or [Angle] <1.0000)>: 1000

Select an object to change or [Undo]:

Now if I try to inquiry or see the arc properties, I will see the arch has the exact length I wanted.

arch_length_properties

I never need to draw arch by defining it’s length. Any body knows why we would need to do that?

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  1. Jessica says

    In a mete & bound description, sometimes you’d see something like “THENCE westerly on a curve to the right having a radius of ___ feet, a distance of___ feet”, in this case I think knowing how to draw an arc with specified arc length is helpful (:
    Twitter:

  2. Mary says

    My example of use: I design heat distribution systems in AutoCAD and we use steel pipes of standard 12 m length. Sometimes we need to bend the pipes a bit if the street is not straight. Then I need to draw an arc which is exactly 12 m long and has a certain radius (same as the street).

  3. Radulf says

    I have to desing arcs using defined material lengths like pipes based on specific dimensions of the project area. Price vs desing. Ej, 20 feet of pipe bended to a certain ° that makes it to 17 feet

  4. Jeremiah says

    I have an instance where I need to draw an arc by arc length and chord length. I’m trying to help the field confirm that a fabricated part is the correct radius. Since we don’t know what the radius is, but we are able to measure the chord length and arc length. They could possibly measure the height of the arc by stringing from the chord length to the center of the arc, however, I don’t think they can do this very accurately. It would be nice if you could specify an arc with a start and end point (chord length) and the arc length. However, I may have to settle for them trying to measure the arc height as best as possible.

  5. Ray says

    One example of when I need to calculate an arc defined by its arc length is when many lots adjoin a road curve. Then the plat/plan shows the total C/L curve data for the entire curve but only the individual arc lenghts along the adjoinging lot lines.

  6. Linda Williams says

    To all Professional Engineers and Professional Land Surveyors who are skeptical of their plan checkers.

    Why the length of a curve, if used in a land description, should be used only as a reference if at all.

    Amazon Books: Land Descriptions including lots of Samples.

    • says

      Run WMFOUT, select the Arc, give it a filename, etc.
      Run WMFIN, pick the file, pick the UL of your viewport as the ins pt, scale X2.
      Explode it.
      There is your Arc made up on line segments.
      Note that since you cannot pick the _EXACT_ upper left of your viewport during the WMFIN command, the arc will not be in the _EXACT_ same place. Use some reference points.
      Twitter:

  7. Fire flame says

    Having worked in Mechanical engineering field for a number of years I have run into a number instances where have worked with limited information when drawing a new part. When drawing arcs you may not have determined all the information you need. You may know the length but not know center. So rather than spending time searching for the unknown work with the drop down that matches the information you do have and move on.

  8. Erika says

    I have to do it all the time: when we work with documents that describes an area, they always describe using the lenght and 2 points. The command "Lenghten" don work properly for me, beacause besides the specific lenght, I also have 2 points that can't be changed. In other words, the radius, unknown, varies…..

  9. Erika says

    I have to do it all the time: when we work with documents that describes an area, they always describe using the lenght and 2 points. The command “Lenghten” don work properly for me, beacause besides the specific lenght, I also have 2 points that can’t be changed. In other words, the radius, unknown, varies…..

  10. joshua says

    man you have know idea how much this tip helps. up until now, the project i was working on was kicking my trash. THANKS!

  11. atiq says

    hello every body

    i want to know .. that

    why dimensions are spread here and there when we move an object ?

    plz solve my problem

    atiq

  12. Odong says

    Edwin, thanks for the info, greatly usefull! I'm laying a subsea cable and need to modify our route. I need to define an exact arch length as our new lay route for the cable end. This is the best answer.

    Greeting from all Surveyors :)

  13. Richard says

    We have to measure "girth" dimensions all the time on ships, so we lay out arc lenths on the screen for verification.

  14. says

    Who needs to draw an arc with a specific length? Surveyors need to do it all the time. Based on a legal description, something like, "Then an arc to the left, having a radius of X and length of Y". I was hoping dynamic input and grips would be an alternative but it looks like you have the best way to do it.

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