[Therion] Fwd: Re: How to calculate the volume of a cave

[Stacho Mudrak]

Hi,

vtk that is used by loch has a volume calculation filter. I created
Tools->Survey statistics dialog in the latest commit, where volume for LRUD
envelope and scrap walls is calculated separately.

Hopefully, it will give some reasonable numbers. There is still a problem,
that if there is 3D data interpolated from splay shots - they self
intersect a lot. They are exported as scraps, so with these data, it will
probably give unrealistic large  numbers.

S.

On Sun, 30 Jan 2022 at 00:43, Tarquin Wilton-Jones via Therion <
therion at speleo.sk> wrote:

> > In this case the cave is very small, only 50 meters, and there are only
> 14 survey stations.  Retyping it into a Compass data file did not take long.
> >
> > Compass and MeshLab do very different calculations to come up with
> volume.  For my cave Compass gives about 2900 cubic feet and MeshLab gives
> over 20,000 cubic feet (about 585 cubic meters).  Compass is doing a simple
> calculation based on polygons and using only centerline plus LRUD.  MeshLab
> is a much more complex calculation based on a 3D model of triangles.
> Ultimately its calculation can be traces back to centerline, LRUD, walls,
> passage height objects and cross sections.  Therion uses all of that to
> build a .lox file which, through translations, becomes the MeshLab file.
>
> If it's that short, I would be tempted to do it manually, for a kind of
> sanity check. Pretend it's a cave made up from cuboids. Take the cross
> sectional area for each station (passage height * width), calculate the
> cuboid volume for each leg. See if it closer to one of the other.
>
> The MeshLab version seems like it might be doubling (or more) up,
> because of the way Therion generates cave tubes and passages from
> multiple overlapping 3D shapes, not a single "tube". One tube for the
> leg, then extra 3D shapes layered into the same 3D space for the wall
> drawings. If MeshLab is treating those as separate entities, rather than
> just the overall outer volume, then it will get a vastly larger
> measurement.
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