Help With Arcs, Points & Tangents For a Walkway Alignment

Hello everyone;

I am currently using autocad civils 3d (which I am not very good with). I have a problem which I was hoping someone could help me with.

I have 154 x and y co-ordinates which I have imported into civils 3d. The points represent the alignment profile of a walkway.

I am trying to create a new alignment such that:-

· I have a series of arcs that best fits the existing points.

· The offsets to the arc from the points are as small as possible

· The arcs are connected tangentially to each other either directly or by a transition.

· The arc(s) must pass through the first and last point.

I have attached the autocad file and below are the points I am working with. I was hoping someone could help me get started and explain how I can solve this problem.

Actual examples will be greatly appreciated.

Thank You.

Walkway Alignment.dwg

In your post, your data seems to be spread out a bit. T'would have been better to attach a text file or similar.

To your problem. It does depend on what you are going to do in the end. Are you trying to have a theoretical mathematical adjustment?

Are you going to build a walkway? What material would be used? Are you looking to get some standard radii? Are you trying to minimise the number of curves? What is the tolerance to miss the points? etc., etc.

The AutoCAD way is easy. Just connect all your points with a polyline, and then use the option Fit. This creates an arc-fit polyline, a smooth curve consisting of arc joining each pair of vertices. The curve passes through all vertices. But you would probably have 80 arcs (or so). Is that acceptable?

I presume that your units are millimetres. If you join the first point to the last point, then the maximum offset is 155, and the average offset is 102 - all one way.

More information on what is the final purpose would be of interest. I would probably be working on the geometry manually for something like this.

I took a look at the file and as Eldon said, I would also manually do it. Assuming the units are in millimetres, not metres, then the walkway is going to be 65m or so long. While you could (theoretically) have many tangential curves and short lines in the alignment, it simply isn't practical from a survey setout and construction point of sense. I'd stick with just a couple of tangential curves only, which still gives only small offsets to your points anyway.

If you did want to do it, then you could use the 'create alignment of best fit' tool (which uses simple linear regression) to approximate in the alignment to start with and then you could manually improve the geometry from there. Another option is to visually look at the data, find groups of adjacent points that seem aligned together and create a line of best fit for those. Then you can manually add in tangential curves between these as appropriate. Your line lengths will still be short though, making it a pain for whoever has to try to set it out and build it.

Hello all

I am using Autocad Civils 3d and for the last 3 days I have been struggling to create a transition between 2 arcs.

Arc 1 has a radius of 1260387.4276 & Arc 2 has a radius of 1004689.5343.

What I need to do is create a transition (any type of transition, clothoid, cubic parabola etc) between arc 1 and arc 2 such that the transition is connected between both arcs tangentially.

I have attached the AutoCAD file that contains the 2 arcs, can anyone have a crack at it and explain how you did it.

Thank you.

Help With Transition.dwg

You can simply draw a connecting arc between your other two arcs.

Select the arc command with start point, end point and radius. Set your object snap to tangential and pick the two arcs. When prompted for the radius enter a value bigger than both of the radii of your existing arcs.

Because your arcs are so close together and their radii so large you could also connect your two arcs with a straight line. Why does it have to be an arc?

Would drawing a circle between the two arcs using the TTR option then trimming away what isn't needed work? Unfortunately I am not on my CAD computer so I can't open and look at your drawing.

That would work too ReMark.

If a straight forward arc or line will not do then check your Civil 3D Help searching for "Adding a Free Spiral Between Two Curves". It explains how to create a clothoid between two arcs,

Perhaps draw a spline between the ends of the arcs. As they are arcs, you know the tangent direction of each of the ends, so you can align the ends of the spline curve.

This is of course only using vanilla AutoCAD.

If it is to be a proper transition curve, you would need all sorts of other information, like the length, so that the proper lateral acceleration is applied.

Hi all

I am using Autocad Civils 3D.

I have a series of points that represents an existing alignment.

I have created (via arc of best fit) two arcs (shown in pink & green) that go through some of my points (see attached autocad file).

I need to create another arc that connects the two existing arcs tangentially; also the new arc should be as close to the points as possible ideally

More than 1 arc can be used to connect the existing arcs and the existing arcs can be trimmed but the above restriction still applies, needs to join to the other arcs tangentially and the distance between the points and the new arcs ideally be

Can anyone help; I have been trying for over a week with little success. Please can you be detailed in your response and if possible can someone have an attempt.

I have attached the AutoCAD file containing the existing arcs and points and a PDF diagram to illustrate what I am trying to achieve.

Any help will be greatly appreciated (I am getting desperate)

Many Thanks

Alignment Pic.pdf

Arc Problem.dwg

Hasn't this question already been asked?

Why not avoid confusion and continue original discussion?

Parametric software reveals that there is no solution with an arc - a spline will be needed to connect the existing arcs with a tangent curve.

Oh wait a minute - looking at your new information in the pdf you changed the problem.

Let me check for a solution with the new information.

In your original problem it I thought you were stating the ends of the magenta and green arcs were known and you were trying to connect these endpoints with a tangent arc - there is no solution.

In your pdf you show a red convex arc tangent to the magenta and green wherever it is tangent.

There is a solution for this, but the solution radius puts it significantly larger than your plotted points.

There is another solution with a concave arc that is much closer to your plotted points.

The solution can be found by using Parameters in later versions of AutoCAD (not sure if r2011 had Parameters).

I will let others try and then post my solution tomorrow.

Can anyone help; I have been trying for over a week with little success. Please can you be detailed in your response and if possible can someone have an attempt.

 

Any help will be greatly appreciated (I am getting desperate)

Hasn't this question already been asked?

Why not avoid confusion and continue original discussion?

I have a solution, but it is not an arc. It is the tangent between the two arcs.

If the magenta arc is extended some 7000 units, then it is possible to draw a line with the Tangent Osnap between the two arcs. Then the arcs can be trimmed and the green arc is shortened some 5000 units.

To help me to arrive at this solution, I changed the Point to be a cross with a circle, and setting the point size as 80 absolute units, you can see visually whether the line passes within 40 units of the point. I think they all do.

Draw a short line section at the position you need the arc to pass through the points. using draw circle tan, tan, tan, pick appropriate poits on the existing arcs and the line, then trim back the resultant circle. Does that do what you need?

I have combined your threads. Stop creating NEW threads on the same topic, it causes confusion.

It might actually HELP, if you would respond to already offered suggestions.

I have also moved your thread to the Civil 3D & LDD forum, please post all Civil 3D related topics in the Civil 3D & LDD forum.