Item storage facilities such as warehouses and distribution centers are designed to store inventory items.  To this end, such facilities may include shelving units, such as vertical shelving units, where the inventory items are held until they are needed (e.g., to fulfill a customer order).  A combination of automated and human operators may be required to place and remove inventory items from the shelving units.  For instance, a human operator may be instructed to remove an item from a conveyor belt and place it on a particular shelf where the item will be stored for some period of time.  When an order for the item is received, the human operator can navigate to a location in the warehouse where the shelf is located, and there retrieve the item to fulfill the order.