After several months of using and tweaking your new software you've decided you need an upgrade. For planning purposes you often add docks to the dictionary you decide against actually building on or above the planet's surface. The extra unused docks are cluttering your database, so you will add the ability do delete these unused records from the B+ tree.
You've also decided to experiment with a new PR Quadtree structure to try and improve performance for range searches on the planet. Since the PR Quadtree is only a 2-d structure you've decided to test it only on the docks on the planets surface. If successful you may expand it to handle the higher level docks at a later time.