Drape lines command

Does anyone use the drape lines on surface command regularly? I feel like this has some great potential for design work.

One thing I do not like about it though, is that it chords the horizontal linework, shown below. I draped a line and then tried and compared it to the reference surface and there were discrepancies. Small albeit, but enough to create issues down the road.

If it would keep the horizontal identical and just apply verticals nodes, I feel like this could be a kick butt tool.

It chords it because if e.g. you have an arc that falls on an inclined plane then you cannot just elevate the points where the arc crosses the triangle sides you also need to compute points in between and elevate those as well to get the correct vertical shape of the line and those are only accurate where you computed them - at some point something is chorded because you are trying to draw 3D arcs on inclined planes and to not approximate you would need many many points in those areas - so you approximate with a tolerance to get the right answer - the original line is still a part of the package and that holds the original unchorded shape so it can recompute accurately if you pick up and move the object that is being draped etc.

With vertical nodes these would all have to be deleted and recreated if anything changes because their position along the line would change - that is why the chord points are created as 3D nodes not VPIs as those are more efficient computationally.

Can you provide an example where non chording for Design purposes would benefit ?


Thanks for the video - I use Change Elevation because that does create VPIs at the locations where the curve crosses the TIN sides and doesn’t try to add extra points into the curves to account for the curve on the slope which drape is doing which is why you see the wavy chorded line when you use drape - it however is not a dependent object so would not update based on the underlying vertical design surface - but if you can live with the non dependency then it is likely a better solution. we can look at Drape Line adding VPIs rather than 3D nodes at some point - I will talk to Gary this week on that as it may be something he can do - my guess is that Drape Line was written before linestrings had separated VPIs maybe and that is why it is as it is today.

Because a Drape Line is not a linestring it may need the VPI capability added to the object type


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That makes sense. Thank you.

I also use the change elevation but I think this could be handy in a few situations.

If he is looking at it, something else that might be useful is the option to apply an offset. This may be too complex but worth a shot. This would be great for utilities; water, force main, etc from a design stand point.

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I am looking to see if we can create a dependent drape object that uses vpis that could do this

We are looking at priorities for Q1 currently


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I also would like this solved. I had to resolve to a quick Side Slope to elevate my lines.
I normally use change elevation by surface. I do not like the drape line as like Alan mentioned its dependant of the surface.

Change elevation also allows you to do the vertical offset for a Water Line etc - it is just not a dependent object.


Alan I also use the “change elevation” command and then “surface elevation” and wish there was a way only elevate the line based on where each VPI on the line is. there is a Drape only at vertices option but I haven’t had any success with it.