And only for fairly localized queries, or we have again memory problems. Usually if we go beyond level 10, we ARE interested in very small areas on the sphere, and the performance for that is excellent. Use BitLists for large-area queries on indices up to level 8, and use only Lists afterward. It is enough to set keeplevel to 2. Don't store too many levels.
Level is the effective level of the index as it is seen by the user.
Keeplevel is the level actually kept in memory. Testing is done on a Sun UltraSparc 5 and on a Dec Alpha. What takes a long time after level 12 is to get the bits out of the bitlist. 6.46641 134M at the 13th level it does not take more than 50 seconds to find the bits in the resulting full and partial BitLists. This placeholder is favorable for small queries.