@TIG
Thank you for your suggestion. The hard part then is how to make the skirt identical. My contour lines were made in Autocad with the same identical property line and then moved to the actual Z value of each contour. The contour lines were imported to Sketchup and transformed to surface by Sandbox. I could not control the boundary afterwards, but they looked almost identical. After the surfaces were made I then used your plugin to make the skirts for each terrain. That was what the result came out. Any suggestion please. Thank you.

@pillarpom said:

@tig said:

Because the two 'solid' groups are subtracted to get the two cut and fill volumes in the Z/blue direction, then the XY/red-green dimensions and outline of the 'skirt' need to be identical and overlaid exactly.
The 'bounding-box' of both groups should appear the same and at the same point at bounds.min...
It see your Red axis but where is the green/blue ?
It's wise to always model near the origin 0,0,0 (0;0;0 if you use ',' as your decimal separator)
To see the 'origin' of the groups switch on Model > Components > Show axes.
You can always explode and regroup each of these groups IN TURN to ensure they coincide.
To Move exactly to the origin, preselect a group and click a bottom minimal corner on the 'skirt' as the start point, then type [0,0,0] ( OR [0;0;0] ) plus <enter> - repeat the Move for the other group, starting with the exact same point on the 'skirt'...
They should now be overlaid exactly.

Thank you very much for your comment. I'll try again.

Still can't get it done,used v1.2 . I've also attached the skp. file for you to review. Please check it out, thank you in advance.

Test_cut_fill.zip

@tig said:

Because the two 'solid' groups are subtracted to get the two cut and fill volumes in the Z/blue direction, then the XY/red-green dimensions and outline of the 'skirt' need to be identical and overlaid exactly.
The 'bounding-box' of both groups should appear the same and at the same point at bounds.min...
It see your Red axis but where is the green/blue ?
It's wise to always model near the origin 0,0,0 (0;0;0 if you use ',' as your decimal separator)
To see the 'origin' of the groups switch on Model > Components > Show axes.
You can always explode and regroup each of these groups IN TURN to ensure they coincide.
To Move exactly to the origin, preselect a group and click a bottom minimal corner on the 'skirt' as the start point, then type [0,0,0] ( OR [0;0;0] ) plus <enter> - repeat the Move for the other group, starting with the exact same point on the 'skirt'...
They should now be overlaid exactly.

Thank you very much for your comment. I'll try again.

This is the very useful plugins for me. Unfortunately I can't get it done for me. It keeps showing that I have to align the corner of the two groups although I've done so.
Can someone tells me what I was wrong.