Mesh volume calculation accuracy?

[R Muller]

I am having inconsistent results in calculating volumes of cut and fill, as compared to the "2D" way of doing it by measuring areas outlined by "before" and "after" contour lines. I am beginning to suspect that the mesh tool itself may be inaccurate, and that I shouldn't try to rely on SEO between existing and new meshes to give me sufficiently accurate quantities of cut/ fill.

As an experiment, I created a mesh with fairly straight contours, defined by just a few points. When I added additional points to the existing contours (no change in elevation, just put another point on the existing line) the volume of the mesh changed. It changed even more when I added a "valley" line that cross-connects the contour lines along the valley that I am creating with my grading.

Is anyone else having an issue with the accuracy of cut/ fill calculations based on meshes?

[David Maudlin]

R wrote:
Yes, David, I have noticed that ArchiCAD seems to prefer the interpretation in your left example to the one on the right, which is why I have learned I need to add ridge and valley lines in these situations. Do you suppose there is a preference for flat ridges over sloped ones?

What I am wondering is if there is any sort of systematic bias, that would tend to show less cut and more fill, for example.

Rather than adding lines to the mesh, I have added points to break the triangles into more realistic grading shapes. I don't have any feel for how ArchiCAD triangulates the site, but I have seen many instances where the triangulation yields elongated triangles (2 very long sides and 1 short side) rather than triangles with 3 roughly equal sides. I add points to break the elongated triangles into more even shapes.

[As an example of lack of control, when I created the meshes in my earlier post, I created the first one and set the heights, then copied it while wondering how I was going to force it to triangulate on the opposite axis. The copy divided the other way on its own, go figure.]

I have been creating meshes for visualization only, not for volume calculations, so I don't know about the methods used for cut & fill balance. I think you will need to understand the calculation method used by your consultant and be able to relate that to the ways the mesh is formed in ArchiCAD to see a way of editing the mesh so the two methods are closer.

HTH

David

[Karl Ottenstein]

R Muller,

Interesting project and issue. I have never modeled a terrain mesh as a series of plateaus as you have done, but as a smooth (fit to user ridges) mass that attempts to interpolate to reality. By stepping your terrain, the value error could be significant.

The only other thing I have to wonder about is the fact that you have 22 meshes and are doing SEOps on them. Typically, I'll have two - the complete 'before' mesh (smooth) and complete 'after'.

Nice to see planning work like yours being done in AC. 🙂

Karl