[MDAL-Developer] Volume intrepretation

[Martin Dobias]

Hi Paul

Probably the best description how the vertical levels in meshes are meant
to work is this QGIS enhancement proposal:
https://github.com/qgis/QGIS-Enhancement-Proposals/issues/158

In short, within one face, if you have N vertical levels, there will be N-1
volumes in between those levels for that particular face. There are two
important considerations that may not be obvious:
1. for each face, the vertical levels may be at different elevations/depths
2. for each face, there could be a different number of vertical levels
MDAL_D_volumesCount() returns _total_ number of volumes, not per-face
number of volumes. Based on all this, the total number of vertical level
entries therefore is "number of volumes" + "number of faces" because for
each face we need that one extra vertical level.

Regards
Martin


On Mon, Aug 23, 2021 at 9:23 PM Paul Harwood <runette at gmail.com> wrote:

> Hello all
>
> Another question for you. I am looking at an app that converts from
> MDAL DataOnVolume dataset to a voxel-based representation.
>
> That should not be particularly difficult. However, it has made apparent
> that it is not particularly clear what the "Vertical Levels" data in
> MDAL actually means.
>
> My first understanding - that since the data values are volumetric the
> "vertical levels" represent the centroid of the volume being measured - is
> slightly undermined by the fact that there are (face count + volume count)
> values in the array which is too many for that interpretation.
>
> The size of the array suggests that the interpretation should be one of
> layered meshes and the volumes are the spaces between the layers. That is
> fair enough but there is only one value per extruded face cell - when it is
> clear that the extruded face cell is not necessarily parallel to anything
> else. Is this vale then the vertical level of the centroid extruded face
> cell?
>
> Does anyone have a definitive answer?
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