An InRoads user asked for Mark's help with this problem she was having :

Zen Dude,

Is there an easy way to determine the intersection point between two InRoads surfaces? The cross section below illustrates my problem. I have a pre-construction surface (red) and an as-built surface (purple). We need to determine final earthwork quantities from the surfaces, but the as-built surface extends above the pre-construction surfaces in many locations. This is an 8 mile roadway project, so we're looking for a quick way to correct this problem over the 8 miles. Is there a way to force the as-built surface to truncate at the pre-construction surface or to create a surface boundary for the as-built surface based on where it intersects the pre-construction surface?

Rachel

Here is the Zen Dude's response:

Hi Rachel!

...What a fun problem!

My first direction in solving this if it were me, would be to create an Isopach surface which basically compares two surfaces and determines the delta's between them. Have you ever created or used an Isopach surface before?

Once the Isopach surface is created you could view the contours. The 'zero' contour defines the intersection of the two surfaces. Then I would take that 'zero' contour line (or lines) and develop an Exterior Boundary from it. That edge would be loaded back into one of your surfaces to define its outer limits to be used for the earthwork.

Since I don't know what your skill level is with InRoads these days, I'm not sure if you're tracking with my explanation above. If you felt like sending me the DTMs and a drawing file I could help explain it further with some graphical snapshots of what you could expect to see.

Let me know if you need some additional direction and I'll do what I can (I might have some different thoughts if I saw the project data and thought about it for a bit).

Are you still working in AutoCAD?

Civilly yours,

- zen

Zen Dude,

Thanks. I'm an intermediate InRoads user so I follow what you're saying. I haven't done much with manipulating surfaces, so I've never created an Isopach surface. I have created exterior boundaries. Would there be a problem with the boundary being at a zero elevation? Would I need to drape the boundary line before defining it as an exterior boundary?

I'll give your solution a try. I've attached the two surfaces so you can view them and see if there's another way to handle my problem.

Thanks for the quick response!

Rachel

Rachel,

I'll check out the attachments ...but yes, you would need to drape the edge on the surface that you are putting it into. Zero elevation Exterior Boundaries are too unpredictable and I never use them.

Civilly yours,

- zen

Rachel,

I started looking at this briefly and the first thing I noticed was the 30' Max Triangle length. That length is creating some oddities in your Perimeter that need to be looked at. If you want to use that 30', I would do a thorough review of the results to make sure that it's giving you what you expect.

Civilly yours,

- zen

Thanks Zen Dude. I'll look into it.

I tried the Isopach surface that you suggested and it will eliminate about 75% of the work we thought we'd have to do. I appreciate the help.

Rachel

 Okay, cool Rachel, 

I won't look at this data anymore but I would really watch that 30' triangle length because I just found another area where the 30' wasn't quite enough to remove an inappropriate triangle. ...and personally, I rarely use the Maximum Length setting in situations with data like you have here, even though it might seem very convenient.

I'm glad this helped.

Civilly yours,

- zen