instead it is desirable to transfer just that information related to changes made to the existing database. a major difficulty is that any change made to the content of a map database generally causes changes to all assigned entity ids and all assigned indices during the compilation process. these new ids and indices permeate the entire compiled database so that any collection of increments will likely constitute most of the database. to overcome this difficulty, three approaches have been taken, which are briefly 1) onboard compiler 2) look-aside store 3) geographical tiles.