Agisoft Metashape
Agisoft Metashape => General => Topic started by: DriesS on January 02, 2019, 01:24:01 PM

I am working with volume measurements of heaps of sand, gravel, etc..:
Based on the terrain profile, we estimate an average custom ground level (as often mounds are next to each other, rendering the 'mean level' option useless)
We then use this to calculate the volume; I've always been using the 'volume above', but my colleague uses 'total volume'
I was using the volume above, because I assume that by using a mean level, you cut of some volume below the level of the actual mound, but add that volume by taking the ground below the surface, above the chosen (mean) surface level, hence ignoring the volume below.
To clarify this I made a sketch:
Brown line = mound of which I need to know the volume.
Black line = surface
orange line = 'custom (mean) ground level'
Purple = ground volume
green = volume below
So am I correct to assume that:
'volume above' = purple + volume of brown line above the orange line.
=
volume of the entire mound, above the black surface line (assuming the mean ground level is chosen correctly)
I am still wondering why 'total volume' is then calculated by subtracting the volume below from the volume above.
Thanks in advance

So uh, can the devs at least confirm that this is the way agisoft measures the volumes?

Hello DriesS,
The volume is calculated in the polygonal shape boundaries and is performed based on the available DEM. So Metashape cannot distinguish between black and brown lines on your picture. DEM will represent the single "surface line" for them.
"Volume above" is calculated for the surface that is above the custom level (so in your case it is the "purple" volume and the volume of the mound part that is above the custom level line), "volume below" is calculated when the surface goes below the custom level  on practice is happens if you have quarries, pits and similar structures. So "volume below" is usually used to calculate the "negative" volume. I'm also attaching the sketch that should clarify the difference. On your sketch the "green" part is not a "volume below", because there's no information about the ground surface (black line)  the volume below on your image will be calculated to the rightmost part, where the surface goes under the custom level line.

makes total sense! And also, my assumptions for measuring the volume of a mound on a slope hold true. ::)
Thank you for the reply Alexey! I hope I'm not annoying you with all my questions lately :X
Dries

Hello Dries,
Yes, I believe it is reasonable to use Volume above option, just to avoid the subtraction of the "holes" volume that are below the custom level.

Dear Ms/Mr Alexey Pasumansky,
I am wondering that is the orange line in your picture is the mean sea level if I use WGS 84: EPSG: 4326 ?
Thank you

Hello Cuong Le,
It is a Custom Level plane, parallel to XY. If you set the custom level value as zero, then it would be related to the zero altitude above WGS84 ellipsoid.

Dear Ms/Mr Alexey Pasumansky,
Thank you for your information on the mean sea level. It is helpful for me to know that.
Have a nice day,
Cuong Le

Hello Cuong Le,
Mean sea level doesn't correspond to zero altitude of WGS84 ellipsoid. You may need to use WGS84 + EGM96 compound coordinate system, if you need to have altitude=0 corresponding to the mean sea level.