Site model contours taking shortcut

[kufuffin]

I'd like to use the 2D view of the contours in this site model as a grading plan. However the contour at 505' is generated incorrectly when I create the site model.

Instead of bulbing out to create a level area (see original contour in grey) it chooses the shortest line between the two points where the bulb-out begins and cuts across.

I've tried changing it with proposed pads and contours and it doesn't work. I've also tried creating a pad in the source data to no avail.

Has anyone found a way around this issue? Thank you

link to image: http://imgur.com/Qj8czzW

[Art V]

These two discussions contain solutions/tips for this.

"Terrain Model Precision"

"Contours are very choppy"

It's a very annoying issue and I've come across things that look even worse than your case, it's something that should definitely be fixed by Vectorworks.

[kufuffin]

Thanks! I tried searching but everyone is using different terms, 'bridging' 'truncate', so nothing turned up when I searched. Seems thought that this only happens when you have a concave shape like a valley. Why does this happen only for concave shapes?

[benboggs]

I assume this is the survey base information? One thing that was pointed out to me was to use the CAD DTM (if it is an autocad file) for importing the survey info. This gave me almost 100% accurate survey contour lines. The one thing I did notice is that this created an unusually large file. As in 125 meg site model file. When I just imported the CAD survey file for use as the site model info the file was about 15 megs. So I am not sure why the file size is 10 times larger - as it makes it almost impossible for me to work in.

[Art V]

The issue is with the DTM itself. When generating a DTM you can have it show the contours in 3D view. These contours are not the original ones used for creating the DTM but re-generated contours by the DTM. The latter contours have a tendency to get clipped/truncated/merged in ways you don't want.

When you edit the DTM data then the original contours do show up.

See attached examples, the first one shows the contours as generated by the DTM, the second one is with an overlay (in blue) of the original contours. You can see that the DTM contours are cut of or merging areas as being one elevation while that is actually not the case.

[Art V]

Thanks! I tried searching but everyone is using different terms, 'bridging' 'truncate', so nothing turned up when I searched. Seems thought that this only happens when you have a concave shape like a valley. Why does this happen only for concave shapes?

An explanation that I have been given is that the DTM triangulates the terrain and sometimes the triangles are of such a shape that Vectorworks has trouble connecting the line(s) across the points and then "jumps" to the next best point.

I'd have to check previous DTM's to see if it only happens with concave areas or not. If it does maybe that could give the software engineers something to work with.

When you look at the terrain itself it pretty much follows the original contour lines, but the regeneration of the contour lines by the DTM is going wrong. So the only option for now is to disable the showing of contours as per DTM and overlay your original contours for top and top/plan views by putting them in a separate layer above your DTM.

This contour issue is something that should be fixed by Vectorworks as it may make people wonder how accurate the DTM actually is, especially when it comes to e.g. cut and fill calculations in areas where the contours are cut off.

[kufuffin]

benboggs, I'm not sure what you mean when you say to use the CAD DTM.

These contours came in a dwg file from an architect using archicad, so it's not the survey base file. Although usually I am working from the survey base file.

[Rossford]

An explanation that I have been given is that the DTM triangulates the terrain and sometimes the triangles are of such a shape that Vectorworks has trouble connecting the line(s) across the points and then "jumps" to the next best point.

Yeah, I would love a technical explanation of why that happens. A 3D DTM point is a point, no? Why would some be harder for the computer to recognize than others?

BTW, my chops usually come in the valleys, too.