What I want to do is really simple. I have a bunch of primitives at varying heights (z is up in my Houdini, so the prims are at varying P values) that I copied downwards and I want to delete the prims that are below the origin/ground level. How do I do this?

Thanks in advance!

- use Delete SOP set to Primitives, Delete By Expression: $TY<0

- or Attrib Wrangle set to Primitives (Primitive Wrangle) with code like: if (@P.y<0) removeprim(0, @primnum, 1);

- or you can use Blast SOP with group set like: @P.y<0
but that would work only on points (you'd need to set Group Type to Points as well )
if you want to use that for primitives you'd have to have primitive attrib representing position like primP, then @primP.y<0 would work

Great, thanks!