I have issues when doing stockpile measurements where the pile is partially inside a bunker. I know that ground is level. what I need to be able to do is insert a “wall” to trim the 3d model and have the calculation include the whole volume. This would be applicable to quarries where the overburden is piled up against the hill. We need to be able to manually divide the areas where it is below ground (of course we would know where the bunkers are). I have tried dragging the manual tie points down below ground but it is a bit hit and miss. even if we could edit the tie point altitude (like a checkpoint) the model would automatically include the area above as it does now.
I hope this makes sense 
Dear Steven,
You can find the procedure to lower one of the vertices of a volume object below ground on this webpage.
https://support.pix4d.com/hc/en-us/articles/202560279
You can also edit the point cloud as explained here
https://support.pix4d.com/hc/en-us/articles/202560499
Best regards,
Great … I thought it should be possible. I’ll give it a try.