Volume surface elevation auto shifted

[bruceauyeungAK9S9]

Dear All,

when I create volume surface it fly up sometimes, am I did something wrong?

Many Thanks,

Bruce

[Pointdump]

Hi Bruce,
"...I did something wrong?"
Almost certainly. Can you show some screenshots of what you're seeing?
Dave

[bruceauyeungAK9S9]

Here is the photo, many thanks 

[bruceauyeungAK9S9]

CAD file is here:
https://drive.google.com/file/d/1dTN4jygZ2sRJphGeNurK1912Zowfo_E6/view?usp=sharing

Many Thanks

[pperales2]

What is it are you trying to do? I see some surfaces and +3. For example, if you are just trying to get the volume of a detention pond (from the trapezoidal shape) the nuse the stage storage tools.

Analyze tab > design ribbon drop-down > stage storage.

[bruceauyeungAK9S9]

Dear pperales2,
Thank you for your reply. Your answer is useful to some case. Many Thanks.
But this surface is about refill, so I still would like to know what is happening on the volume surface.