  Many Ore Reserves are harvested by a combination of open pit and underground
mining methods. In these cases there is often material that could be mined by
either method, and a choice has to be made. The area containing this material
is referred to as the transition zone. Deciding where to finish the open pit
and start the underground is referred to as the transition problem and it has
received some attention in the literature since the 1980s.
  In this paper we provide a review of existing approaches to the transition
problem encompassing: graph-theory based optimisation employing an opportunity
cost approach; heuristics and integer programming. We also present a novel
opportunity cost approach, allowing it to take into account a crown pillar, and
show how the new approach can be best applied through the unconventional
application of an existing mine optimisation tool.
