# Thread: Incorporating 3D polyline into an existing surface

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## Incorporating 3D polyline into an existing surface

Hi - I am trying to project a new Base Flood Elevation on an existing ground surface. What I have is the eg surface and 4 points of known BFE. What I need is an interpolated 3D line using these 4 points to show where the BFE line would fall on the existing ground. I keep thinking the answer should be right in front of me, but all I've been doing is banging my head against the wall. Any help would be appreciated. (LDD, Civil, Survey 2005)

Thanks - Judy

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## Re: Incorporating 3D polyline into an existing surface

Terrain > 3d poly > by elevation >

From point (or Entity): *select first point*
Starting elevation (or Dtm) <0>: D
*select surface*

You should be able to figure it out from there.

Hope this helps.
Have one on me.

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## Re: Incorporating 3D polyline into an existing surface

That still only gives me a line between the 4 known points - I want to interpolate btween those points based upon the existing ground dtm - in other words, it's not a straight shot between the points - the BFE line (which varies in elevation) meanders around based on the elevation of the existing ground. This shouldn't be this difficult, so I'm either making it much harder than it is or...

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## Re: Incorporating 3D polyline into an existing surface

Points >
Create Points - Surface >
Along Polyline/Contour >
Distance between points <15>: (enter desired distance)

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## Re: Incorporating 3D polyline into an existing surface

Create a surface with those four points, then create a volume surface using your BFE surface and existing surface, then create your daylight line between the two.

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## Re: Incorporating 3D polyline into an existing surface

Originally Posted by jrosenbaum
That still only gives me a line between the 4 known points - I want to interpolate btween those points based upon the existing ground dtm - in other words, it's not a straight shot between the points - the BFE line (which varies in elevation) meanders around based on the elevation of the existing ground. This shouldn't be this difficult, so I'm either making it much harder than it is or...
If I am understanding you correctly, you want to create a line on the EG, similar to a contour that identifies where the BFE is.

If you create a 3D polyline as described above for the four points, you should be able to use your grading menu to create a "daylight." I don't have a lot of experience with this, but I'm sure there are those that do, if you need further instruction.

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## Re: Incorporating 3D polyline into an existing surface

The quickest way that I have found to create the Daylight PLINE is to follow the steps in my previous post to create your Volume surface, then set that surface current, and create contours for that surface, only you want to change the contour interval to 20 or 30 (Make this number larger than the variations in your elevations for the area in question) for your Major and only draw the Major contours. You should get one continuous polyline that defines your BFE and maybe a couple little ones inside your area. You can erase those and you have your Flood Area defined.

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## Re: Incorporating 3D polyline into an existing surface

Thanks for the help - i haven't had a chance to work any further on it; I've been swamped trting to finish other projects up before I leave on vacation (wa-hoo!). I'll give it a go when I return. Thanks again - i never would have figured this one out.

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## Re: Incorporating 3D polyline into an existing surface

In Civil 3D. Have your original surface. Then use the cross-section lines of the BFE and set them to the BFE elevation. Making sure the lines go all the way accross the stream and past the BFE at that location. Use these cross-section lines that are set to the BFE to create a surface. Now create a Volume surface from these two. Once you do this display the volume surface with contours. Contour 0 will be your BFE line. Then simply go under suface-Utilites-Extract Object from Surface and click on the contour 0 line. Now you have a polyline that is your BFE line. MAGIC!

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