I was lucky enough that the LP tank i cut open had a backing strip inside of it (not sure if this is standard or not) I used the backing strip as a straight edge to rest my plasma cutter against
![Very Happy :D](./images/smilies/icon_e_biggrin.gif)
. It did make each half slightly smaller than a true hemisphere so the resulting ball is ever so slightly egg shaped but not enough to tell a difference....tack weld the two pieces together until ready to cut (no use welding the seam I'm about to cut out). Since I had a straight center line I used it as reference to drop the "ground level" for the trees about 3" below the weld line.. I had a small scrap 3" wide I rest the soap stone on the bottom of the scrap and follow the center line around the ball with the top of the scrap giving me a line below and parallel to the center line. After it was on the base I tossed in a marble to find the low point and cut a hole for drainage. There are also holes in my base to let the water out.