ALIGNMENT FROM OBJECTS

[trumankiko]

question! why does my alignment made from a polyline always have tangency error. its always not tangent with the next element. how to remove this error?

[rl_jackson]

Hard to say if the line curve it tangent to the incoming and outgoing tangent.

Can you post your alignment?

[jroot]

Were the curves on the polyline made by the FILLET command? That for sure would make everything tangent.

[v-silvestre]

Jeff Bartels has a great video on this topic:  https://youtu.be/SmOlQLXDYoo?si=8TyFX0bZZnXTY-Ra

Particularly about solving tangency at PI's as well as the difference between creating alignments "from objects" versus "by layout", at timestamp 5:33.

[trumankiko]

its made from the usual polylines for straight line and arcs for curves

[trumankiko]

my alignment is made from the usual pline command for the line and arcs for the curves

[Joe-Bouza]

Where does the poly line come from

[trumankiko]

sorry but wdym where does it come from? i made it right in the model space of civil 3d then used it as an object to create alignment

[rl_jackson]

Can you post a drawing of your alignment? I would think that when drawn it was not defined as tangential, if you don't want to see the warning these can be turned off, but I would not recommend this if your intent was for all and future curves to be tangent.

[Joe-Bouza]

That's why I ask. If it came from out of office or some foreign data set I would not be surprised.

If you are drawing it in your own space, I suggest drawing the alignment directly with the layout tools. it works exactly as drawing a pline and your done, without the mishaps 

[tcorey · Accepted solution]

One mistake I see people make when drawing a polyline and wanting the segments to be tangent.

1. Draw a line segment

2. Press A for Arc segment and draw an arc segment. This is automatically tangent to the line

3. Here's where a mistake can be made. You press L to go back to Line segment, but you also need to input another L, meaning Length, which forces the line segment to be tangent to the arc. If you omit this further step of inputting another L, then your segment is not necessarily tangent.

Command: PL
PLINE
Specify start point:
Current line-width is 0.0000
Specify next point or [Arc/Halfwidth/Length/Undo/Width]:
Specify next point or [Arc/Close/Halfwidth/Length/Undo/Width]: a

Specify endpoint of arc (hold Ctrl to switch direction) or
[Angle/CEnter/CLose/Direction/Halfwidth/Line/Radius/Second pt/Undo/Width]:
Specify endpoint of arc (hold Ctrl to switch direction) or
[Angle/CEnter/CLose/Direction/Halfwidth/Line/Radius/Second pt/Undo/Width]: l
Specify next point or [Arc/Close/Halfwidth/Length/Undo/Width]: l           The critical input
Specify length of line:

[AllenJessup]

@tcorey 

Good info Tim. I never knew that. I was wondering why the OP would think the outbound line would be tangent.

I alway defaulted to using the line and arc commands. Forcing them to be tangent by hitting Enter when asked for the next First Point.

[AllenJessup]

If you're working with high coordinate values. Sometimes the program will find something way out in the decimal places that will result in that kind of error. If you check the angle between the radial and tangent and it's 90° within a tenth or hundredth of a second. I'd consider it close enough to tangent for road construction.

[trumankiko]

ahhh that's it! thank you so much! i can now create a perfect alignment with curves while also following my assigned design criteria. ❤️

[Joe-Bouza]

Hi Allen, @trumankiko @tcorey @rl_jackson @v-silvestre 

Is there a benefit to drawing the polyline to convert to alignment over just drawing the alignment?

[rl_jackson]

I almost always draw an alignment and then convert it using objects. This is mostly because I'm referring to existing documents or plans (DOT or others). I can also say that I'm a little OCD in that I prefer to draw the incoming tangent, turn the 90 or -90 to the radius point, turn the delta and turn the 90 or -90 for the outgoing tangent as a check to the documents, trust me I've found a lot of mistakes in curves in my years where the curve was intended to be tangent and did not compute as tangent.

As for my years doing design work, I would say that most of the time the alignment came from objects as well. This was due to zoning and a host of other constraints that once the layout was complete it was generally easier to create the alignment from the objects.

[v-silvestre]

Hi Joe, I think a benefit would be just how quickly & easily you can create an alignment from an object is.  I primarily use the create alignment by Layout option.  If I do use the "by objects" objects option, I don't convert any arc segments and manually add them in after the fact.  I noticed that converting an arc segment into an alignment that it doesn't give you the projected PI, and when grip editing it will not maintain the arc tangency.

[Joe-Bouza]

I'll by that, if using existing line work. I'm struggling with drawing the poly lines to convert to an alignment

[AllenJessup]

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@Joe-Bouza wrote:

Is there a benefit to drawing the polyline to convert to alignment over just drawing the alignment?

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Only in the way you work/think. I've been doing it that way since the wayback. If I use the Alignment Layout Tools I always have to stop and think about how I'm doing it rather than just doing it. If you know what I mean.

[trumankiko]

totally agree!