Stacked feature lines

[Anonymous]

I am currently creating essentially a two part site grading plan for a tank that I am sinking.  I create a rectangle that I convert to feature line and call it "bottom of excavation" and set an elevation say -10 feet.  I don't erase the existing rectangle but click on the newly formed feature line and send it back exposing the new poly line.  I click on it and create a new feature line for "top of access port" and this time remove the entity (rectangle polyline) and assign elevation of +10.  I then send it back to go back to the "bottom of excavation".  I click on it and in my ribbon click on grading tools.  Go through the steps and grade it.  When I am done I turn off my contours (doing cut and fill obviously) and send the feature line to the back.  I then can click on "top of access port"; however, even though I assigned the elevation, the elevation that shows up is the "bottom of excavation".  When I make my changes in elevation editor, it changes but it also changes the "bottom of excavation" to the same elevation.  Both feature lines are separate but are part of the same site but  I would not think that it would have a difference.  Any ideas on what I may be doing wrong?  Is sending the object back not the correct way?  Thank you in advance.  Civil 3d 2010 is what I am using.   

[Sinc]

The reason you are seeing the elevations change in both polylines is because the polylines are directly on top of each other, and they are on the same site.  Feature Lines that are in the same site will affect each other, and edits in one will affect the other wherever they are touching.  Since your Feature Lines are on top of each other and in the same Site, any edits to one are affecting the other.

Another thing to keep in mind is that you can't have true vertical faces in a TIN.  If you want to create a Surface using these Feature Lines, your top and bottom would have to be offset slightly from each other.

[Jeff_M]

Featurelines in the same site will react to each other where they cross/overlap. You should offset one of them 0.01 (since surfaces can't be vertical you need to do this anyway if generating a surface) then they will not cross.

[Jeff_M]

Heh, my turn to be a slow typist...

[Anonymous]

Sinc and Jeff -

Thanks for the reply.  I am trying to visualize what exactly you mean that a tin cannot be vertical and it would seem that I am not quite following. Are you assuming that I am taking a volume from one feature line (bottom of excavation) to (top of excavation)?  In this exercise, I am taking the bottom to existing grade to get my cut and then my top of  access riser to existing grade to get final grade.  How would you model say a rectangular box as a tank buried when the faces are vertical on the box itself to give you a net cut and fill?  I am  assuming the box would have to face vertical faces or would you have to skew that too? 

In terms of the stacked feature lines, when you mean to offset one feature line by 0.01 (which I think I ran  across in a tutorial somewhere this was done but did not understand why) then I could keep the two feature lines under the same site.  If I did not want to offset it, could I create another site to represent each feature line independently and still stack them and would act independent of each other?

Thanks again for the reply, especially so late.  

[Jeff_M]

Right, different sites would allow you to have the 2 featurelines on top of each other. But, if you intend to use both to define a surface then they will need to be offset ever so slightly. TIN surfaces do not allow vertical faces, nor are overhangs allowed so make sure you offset in the right direction (a hole will have the bottom slightly smaller than the top, where a raised box would have a larger bottom than top).

[Anonymous]

So when you create the box, like I am trying to later on (right now focusing on  showing construction grading versus final grading), you would stack offseting and making the top rectangle slightly smaller than the bottom and project the grading from the top feature line to the bottom feature line?  Is that what is typically done when sinking an object into a ground?  Is there any other options to show an object's volume as a "subtracted" value from the overall net cut/fill?   

[Neilw_05]

darkman,

One rule for Triangulated Irregular Networks (TIN's) is that any point in the network can have only one elevation. Therefore you cannot model a perfectly vertical face in a TIN since it would result in 2 coincident elevations.

Civil 3D's featurelines are designed to obey rules (called topology rules) that prevent them from creating 2 elevations at a common point. Thus any time featurelines touch each other, C3D will force them to have a common elevation at those points. If there is a physical vertex on the featurelines where they meet, the elevation of the older featureline is adjusted to equal the vertex of the newer, the order of precedence being determined by which featureline was most recently edited. In a case where there is no physical vertex where they meet, such as when featurelines cross each other, a pseudo vertex is automatically created at the point where they cross and they will be forced to share a common elevation at that point. These pseudo vertices will appear as white triangles when looking at them in the elevation editor. You cannot edit the elevation of a pseudo vertex since it's value is dictated by the newest of the featurelines.

Since there may be scenarios where you do not want the featurelines to interact with each other, C3D has provided a means to isolate them topologically called "sites". The types of objects that can interact with each other are featurelines, gradings, alignments and parcels. By assigning these objects to separate sites they will not be forced to interact with each other by the topology rules.

For a simple way to see how this works, draw 2 featurelines that cross each other at different elevations. In the featureline creation dialogue make sure they are both in a the same site. Open each in the elevation editor and note that a new vertex is created where they cross. It appears as a white triangle in the editor. Next, access the properties of one of them and assign it to a new site and note the white triangle vertex will no longer be generated. Also, try creating 2 featurelines that share a common vertex. When they are in the same site, any edits to the vertex of one will cause the other to update to match. Again put them in separate sites and the behavior stops.

So with regard to your tank, the reason your featurelines are influencing each other are twofold, 1) they are exactly coincident with each other and 2) they are in the same site. Accordingly they cannot have different elevations. Editing one will cause the other to update to match. The solution is to either put them in separate sites or to offset them very slightly so they don't touch each other. However for both options you will need to offset them before using them as breaklines. Otherwise you will violate the TIN topology rule (one elevation at any point) and C3D will report an error.

To model your tank you should offset the top featureline to the OUTSIDE of the bottom. If you make the bottom of the tank larger than the top you will violate the TIN topology rule since the top surface triangles would overlap the bottom.

To get volumes you would first create a surface object for the tank and add the bottom and top featurelines as breaklines to create 3D pit with open top. C3D will create the triangles between the top and bottom automatically. Next you would create a volume surface and use the existing terrain as the base surface and tank as the reference surface. Of course if the top of the tank is above the existing ground you will need to transition the terrain to meet the tank, either by using gradings or featurelines and adding them to the tank surface object. Then you will get both cut and fill volumes for the tank.